Atomic TheoryAtomic Theory

• 2.1.1 State the position of protons, electrons and neutrons in the atom

• 2.1.2 State the relative masses and relative charges of protons, neutrons and electrons

• 2.1.3 Define the terms mass number (A), atomic number (Z) and isotopes of an element

• 2.1.4 Deduce the symbol for an isotope given its mass number and atomic number

• 2.1.5 Calculate the number of protons, neutrons and electrons in atoms and ions from the mass number, atomic number and charge.

• 2.1.6 Compare the properties of the isotopes of an element• 2.1.7 Discuss the uses of radioisotopes

History of the atomHistory of the atom• Democritus (400 BC) suggested that the material Democritus (400 BC) suggested that the material

world was made up of tiny, indivisible particlesworld was made up of tiny, indivisible particles• atomosatomos, Greek for “uncuttable”, Greek for “uncuttable”

• Aristotle believed that all matter was made up of Aristotle believed that all matter was made up of 4 elements, combined in different proportions4 elements, combined in different proportions

• Fire - HotFire - Hot• Earth - Cool, heavyEarth - Cool, heavy• Water - WetWater - Wet• Air - LightAir - Light

• The “atomic” view of matter faded for centuries, The “atomic” view of matter faded for centuries, until early scientists attempted to explain the until early scientists attempted to explain the properties of gasesproperties of gases

Re-emergence of Atomic Re-emergence of Atomic TheoryTheory

John Dalton postulated John Dalton postulated that:that:1.1. All matter is All matter is

composed of composed of extremely small, extremely small, indivisible particles indivisible particles called atomscalled atoms

2.2. All atoms of a given All atoms of a given element are identical element are identical (same properties); the (same properties); the atoms of different atoms of different elements are differentelements are different

3. Atoms are neither created 3. Atoms are neither created nor destroyed in chemical nor destroyed in chemical reactions, only rearrangedreactions, only rearranged

4. Compounds are formed 4. Compounds are formed when atoms of more than when atoms of more than one element combineone element combine• A given compound always A given compound always has the same relative has the same relative number and kind of atomsnumber and kind of atoms

Atoms are divisible!Atoms are divisible!

• By the 1850s, By the 1850s, scientists began to scientists began to realize that the atom realize that the atom was made up of was made up of subatomic particlessubatomic particles

• Thought to be Thought to be positive and negativepositive and negative

• How would we How would we knowknow this if we can’t see it this if we can’t see it or touch it?or touch it?

Cathode Rays and Cathode Rays and ElectronsElectrons

• Mid-1800’s scientists began to study Mid-1800’s scientists began to study electrical discharge through cathode-ray electrical discharge through cathode-ray tubes. Ex: neon signstubes. Ex: neon signs• Partially evacuated tube in which a current Partially evacuated tube in which a current

passes throughpasses through• Forms a beam of electrons which move from Forms a beam of electrons which move from

cathode to anodecathode to anode• Electrons themselves can’t be seen, but certain Electrons themselves can’t be seen, but certain

materials fluoresce (give off light) when materials fluoresce (give off light) when energisedenergised

Oh there you are!Oh there you are!

• JJ Thompson observed that when a JJ Thompson observed that when a magnetic or electric field are magnetic or electric field are placed near the electron beam, placed near the electron beam, they influence the direction of flowthey influence the direction of flow• opposite charges attract each opposite charges attract each

other, and like charges repel.other, and like charges repel.•The beam is negatively charged The beam is negatively charged

so it was repelled by the so it was repelled by the negative end of the magnetnegative end of the magnet

• http://www.chem.uiuc.edu/clcwebsite/video/Cath.mov

• Magnetic field forces the beam Magnetic field forces the beam to bend depending on to bend depending on orientationorientation

• Thompson concluded that:Thompson concluded that:• Cathode rays consist of beams of Cathode rays consist of beams of

particlesparticles• The particles have a negative chargeThe particles have a negative charge

• Thompson understood that all matter was Thompson understood that all matter was inherently neutral, so there must be a inherently neutral, so there must be a countercounter• A positively charged particle, but where to put A positively charged particle, but where to put

itit

• It was suggested that the negative charges It was suggested that the negative charges were balanced by a positive umbrella-were balanced by a positive umbrella-chargecharge• “ “Plum pudding model” “chocolate chip cookie Plum pudding model” “chocolate chip cookie

model”model”

Rutherford and the Nucleus

• This theory was This theory was replaced with replaced with another, more another, more modern onemodern one

• Ernest Ernest Rutherford (1910) Rutherford (1910) studied angles at studied angles at which which particles particles (nucleus of helium) (nucleus of helium) were scattered as were scattered as they passed through they passed through a thin gold foila thin gold foil

• http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Rutherford expected …• Rutherford believed that the mass and positive charge was evenly distributed throughout the atom, allowing the particles to pass through unhindered

particles

Rutherford explained …

+

• Atom is mostly empty space• Small, dense, and positive at the center• Alpha particles were deflected if they got close enough

particles

• NucleusNucleus: : Containing Containing protons and neutrons, it protons and neutrons, it is the bulk of the atom is the bulk of the atom and has a positive and has a positive charge associated with itcharge associated with it

• Electron cloud: Electron cloud: Responsible for the Responsible for the majority of the volume of majority of the volume of the atom, it is here that the atom, it is here that the electrons can be the electrons can be found orbiting the found orbiting the nucleus (extranuclear)nucleus (extranuclear)

The modern atom is composed of two The modern atom is composed of two regions:regions:

Major Subatomic Major Subatomic ParticlesParticles

• Atoms are measured in picometers, 10Atoms are measured in picometers, 10-12-12 meters meters• Hydrogen atom, 32 pm radiusHydrogen atom, 32 pm radius

• Nucleus tiny compared to atomNucleus tiny compared to atom• If the atom were a stadium, the nucleus would be a If the atom were a stadium, the nucleus would be a

marblemarble

• Radius of the nucleus is on the order of 10Radius of the nucleus is on the order of 10-15 -15 mm• Density within the atom is near 10Density within the atom is near 101414 g/cm g/cm33

Name Symbol Charge Relative Mass (amu)

Actual Mass (g)

Electron e- -1 1/1840 9.11x10-28

Proton p+ +1 1 1.67x10-24

Neutron no 0 1 1.67x10-24

Elemental Elemental ClassificationClassification

• Atomic Number (Z) = number of protons Atomic Number (Z) = number of protons (p(p++) in the nucleus) in the nucleus• Determines the type of atomDetermines the type of atom

• Li atoms always have 3 protons in the nucleus, Hg Li atoms always have 3 protons in the nucleus, Hg always 80always 80

• Mass Number (A) = number of protons + Mass Number (A) = number of protons + neutrons [Sum of pneutrons [Sum of p++ and n and nº]º]• Electrons have a negligible contribution to Electrons have a negligible contribution to

overall massoverall mass

• In a neutral atom there is the same In a neutral atom there is the same number of electrons (enumber of electrons (e--) and protons ) and protons (atomic number)(atomic number)

Nuclear SymbolsNuclear Symbols• Every element is given a corresponding Every element is given a corresponding

symbol which is composed of 1 or 2 letters symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as (first letter upper case, second lower), as well as the mass number and atomic well as the mass number and atomic numbernumber

E A

Z

elemental elemental symbolsymbol

mass numbermass number

atomic atomic numbernumber

• Find the Find the • number of protonsnumber of protons• number of neutronsnumber of neutrons• number of electronsnumber of electrons• atomic numberatomic number• mass numbermass number

W184 74

F199 Br80

35

IonsIons

• CationCation is a positively charged is a positively charged particle. Electrons have been particle. Electrons have been removed from the element to form removed from the element to form the + charge.the + charge. ex: Na has 11 e-, Naex: Na has 11 e-, Na++ has 10 e- has 10 e-

• AnionAnion is a negatively charged is a negatively charged particle. Electrons have been added particle. Electrons have been added to the atom to form the – charge.to the atom to form the – charge.

ex: F has 9 e-, Fex: F has 9 e-, F-- has 10 e- has 10 e-

IsotopesIsotopes• Atoms of the same element can have Atoms of the same element can have

different numbers of neutrons and different numbers of neutrons and therefore have different mass numberstherefore have different mass numbers

• The atoms of the same element that differ The atoms of the same element that differ in the number of neutrons are called in the number of neutrons are called isotopesisotopes of that element of that element

• When naming, write the mass number after the When naming, write the mass number after the name of the elementname of the element

H11Hydrogen-1

H21

Hydrogen-2

H31Hydrogen-3

How heavy is an atom of How heavy is an atom of oxygen?oxygen?

• There are different kinds of oxygen atoms There are different kinds of oxygen atoms (different isotopes)(different isotopes)• 1616O, O, 1717O, O, 1818OO

• We are more concerned with We are more concerned with average average atomic massesatomic masses, rather than exact ones, rather than exact ones• Based on abundance of each isotope found in Based on abundance of each isotope found in

naturenature

• We can’t use grams as the unit of measure We can’t use grams as the unit of measure because the numbers would be too smallbecause the numbers would be too small• Instead we use Atomic Mass Units (u)Instead we use Atomic Mass Units (u)

• Standard u is 1/12 the mass of a carbon-12 atomStandard u is 1/12 the mass of a carbon-12 atom

• Each isotope has its own atomic massEach isotope has its own atomic mass

Calculating AveragesCalculating Averages– Average = (% as decimal) x (massAverage = (% as decimal) x (mass11) + ) +

(% as decimal) x (% as decimal) x (mass(mass22) + ) + (% (% as decimal) x (massas decimal) x (mass33) + …) + …

– Problem:Problem:– Silver has two naturally occurring isotopes, Silver has two naturally occurring isotopes,

107107Ag with a mass of 106.90509 u and Ag with a mass of 106.90509 u and abundance of 51.84 % ,and abundance of 51.84 % ,and 109109Ag with a mass Ag with a mass of 108.90476 u and abundance of 48.16 % of 108.90476 u and abundance of 48.16 % What is the average atomic mass? What is the average atomic mass?

– AverageAverage = (0.5184)(106.90509 u) + (0.4816) = (0.5184)(106.90509 u) + (0.4816)(108.90476 u)(108.90476 u)

= = 107.87 amu107.87 amu

• If not told otherwise, the mass of the If not told otherwise, the mass of the isotope is the mass number in amuisotope is the mass number in amu

• The average atomic masses are not whole The average atomic masses are not whole numbers because they are an average mass numbers because they are an average mass valuevalue

• Remember, the atomic masses are the Remember, the atomic masses are the decimal numbers on the periodic tabledecimal numbers on the periodic table

Average Atomic Masses

Properties of IsotopesProperties of Isotopes• Chemical properties are primarily determined Chemical properties are primarily determined

by the by the number of electronsnumber of electrons• All isotopes has the same number of electrons, All isotopes has the same number of electrons,

so they have so they have nearly identicalnearly identical chemical chemical properties even though they have different properties even though they have different masses.masses.

• Physical propertiesPhysical properties often often depend on the depend on the massmass of the particle, so among isotopes they of the particle, so among isotopes they will have slightly different physical properties will have slightly different physical properties such as density, rate of diffusion, boiling point…such as density, rate of diffusion, boiling point…

• The isotopes of an element with fewer neutrons will The isotopes of an element with fewer neutrons will have:have:

• Lower massesLower masses • faster rate of diffusion• faster rate of diffusion• Lower densitiesLower densities • lower melting and boiling points• lower melting and boiling points

• Calculate the atomic mass of copper if Calculate the atomic mass of copper if copper has two isotopescopper has two isotopes• 69.1% has a mass of 62.93 amu69.1% has a mass of 62.93 amu• The rest (30.9%) has a mass of 64.93 amuThe rest (30.9%) has a mass of 64.93 amu

• Magnesium has three isotopesMagnesium has three isotopes• 78.99% magnesium 24 with a mass of 23.9850 78.99% magnesium 24 with a mass of 23.9850

amuamu• 10.00% magnesium 25 with a mass of 24.9858 10.00% magnesium 25 with a mass of 24.9858

amuamu• The rest magnesium 26 with a mass of 25.9826 The rest magnesium 26 with a mass of 25.9826

amuamu• What is the atomic mass of magnesium?What is the atomic mass of magnesium?

More Practice Calculating Averages

RadioisotopesRadioisotopes

• Isotopes of atoms that have had an Isotopes of atoms that have had an extra neutron attached to their extra neutron attached to their nucleus.nucleus.

• Carbon-14Carbon-14 radioactive decay is used radioactive decay is used to measures the date of objects.to measures the date of objects.– After 5700 years the amount of After 5700 years the amount of 1414C will C will

be half its original value.be half its original value.• Iodine-125 or 131Iodine-125 or 131 is used to monitor is used to monitor

the activity of the thyroid gland (b/c the activity of the thyroid gland (b/c the thyroid tends to absorb iodine)the thyroid tends to absorb iodine)

• Cobalt-60Cobalt-60 produces gamma rays produces gamma rays (intense radioactivity) and is used in (intense radioactivity) and is used in radiation treatment of cancer.radiation treatment of cancer.

• NoteNote: gamma rays are the shortest : gamma rays are the shortest wavelength on the electromagnetic wavelength on the electromagnetic spectrum. They are the most spectrum. They are the most dangerous and difficult to shield dangerous and difficult to shield from.from.

2.2 The Mass Spectrometer

Mass SpectrometerMass Spectrometer

• The mass spectrometer is an instrument used:The mass spectrometer is an instrument used:• To measure the relative masses of isotopesTo measure the relative masses of isotopes• To find the relative abundance of the To find the relative abundance of the

isotopes in a sample of an elementisotopes in a sample of an element

When charged particles pass through a magnetic field, the particles are deflected by the magnetic field, and the amount of deflection depends upon the mass/charge ratio of the charged particle.

Mass Spectrometer – Mass Spectrometer – 5 5 StagesStages

• Once the sample of an element has been Once the sample of an element has been placed in the mass spectrometer, it placed in the mass spectrometer, it undergoes five stages.undergoes five stages.

• VaporisationVaporisation – the sample has to be in – the sample has to be in gaseous form. If the sample is a solid or gaseous form. If the sample is a solid or liquid, a heater is used to vaporise some liquid, a heater is used to vaporise some of the sample.of the sample.

X (s) X (g)

or X (l) X (g)

Mass Spectrometer – Mass Spectrometer – 5 5 StagesStages

• IonizationIonization – sample is bombarded – sample is bombarded by a stream of high-energy by a stream of high-energy electrons from an electron gun, electrons from an electron gun, which ‘knock’ an electron from an which ‘knock’ an electron from an atom. This produces a positive atom. This produces a positive ion:ion: X (g) X + (g) + e-

Acceleration – an electric field is used to accelerate the positive ions towards the magnetic field. The accelerated ions are focused and passed through a slit: this produces a narrow beam of ions.

Mass Spectrometer – Mass Spectrometer – 5 5 StagesStages

• DeflectionDeflection – – The accelerated ions are deflected into the magnetic field. The amount of deflection is greater when:

• the mass of the positive ion is less• the charge on the positive ion is greater• the velocity of the positive ion is less• the strength of the magnetic field is greater

Mass SpectrometerMass Spectrometer

• If all the ions are travelling at the same If all the ions are travelling at the same velocity and carry the same charge, the velocity and carry the same charge, the amount of deflection in a given magnetic amount of deflection in a given magnetic field depends upon the mass of the ion.field depends upon the mass of the ion.

• For a given magnetic field, only ions with For a given magnetic field, only ions with a particular a particular relative relative mass (mass (mm)) to to charge charge ((zz)) ration – the ration – the m/z valuem/z value – are deflected – are deflected sufficiently to reach the detector.sufficiently to reach the detector.

Mass SpectrometerMass Spectrometer

• DetectionDetection – ions that reach the detector – ions that reach the detector cause electrons to be released in an cause electrons to be released in an ion-ion-current detectorcurrent detector

• The number of electrons released, hence The number of electrons released, hence the current produced is proportional to the current produced is proportional to the number of ions striking the detector.the number of ions striking the detector.

• The detector is linked to an amplifier and The detector is linked to an amplifier and then to a recorder: this converts the then to a recorder: this converts the current into a current into a peakpeak which is shown in the which is shown in the mass spectrum.mass spectrum.

•

Atomic Structure – Mass Spectrometer

Isotopes of boron

m/z value 11 10

Relative abundance %

18.7 81.3

Ar of boron = (11 x 18.7) + (10 x 81.3) (18.7 + 81.3)

= 205.7 + 813 100

= 1018.7 = 10.2 100

Mass Spectrometer – Mass Spectrometer – QuestionsQuestions

• A mass spec chart for a sample of A mass spec chart for a sample of neon shows that it contains:neon shows that it contains:– 90.9% 90.9% 2020NeNe– 0.17% 0.17% 2121NeNe– 8.93% 8.93% 2222NeNe

Calculate the relative atomic mass of Calculate the relative atomic mass of neonneon

You must show all your work!You must show all your work!

Mass Spectrometer – Mass Spectrometer – QuestionsQuestions

– 90.9% 90.9% 2020NeNe– 0.17% 0.17% 2121NeNe– 8.93% 8.93% 2222NeNe

(90.9 x 20u) + (0.17 x 21u) + (8.93 x

22u)100

Ar= 20.18u

Mass Spectrometer – Mass Spectrometer – QuestionsQuestions

Calculate the Calculate the relative relative atomic mass atomic mass of leadof lead

You must show You must show all your work!all your work!m/e204 206 207 208

52.3

23.622.61.5

Mass Spectrometer – Mass Spectrometer – QuestionsQuestions

– 1.5% 1.5% 204204PbPb– 23.6% 23.6% 206206PbPb– 22.6% 22.6% 207207PbPb– 52.3% 52.3% 208208PbPb

(1.5 x 204) + (23.6 x 206) + (22.6 x 207)+(52.3 x 208)100

306 + 4861.6 + 4678.2 + 10878.4100

20724.2100

Ar= 207.24

=

2.3 Electron 2.3 Electron ArrangementArrangement2.3.1 Describe the electromagnetic 2.3.1 Describe the electromagnetic

spectrumspectrum2.3.2 Distinguish between a 2.3.2 Distinguish between a

continuous spectrum and a line continuous spectrum and a line spectrumspectrum

2.3.3 Explain how the lines in the 2.3.3 Explain how the lines in the emission spectrum of hydrogen are emission spectrum of hydrogen are related to electron energy levelsrelated to electron energy levels

2.3.4 Deduce the electron 2.3.4 Deduce the electron arrangement for atoms and ions up arrangement for atoms and ions up to Z=20to Z=20

Electromagnetic Electromagnetic radiationradiation..

Electromagnetic Electromagnetic RadiationRadiation

Electromagnetic Electromagnetic RadiationRadiation

• Most subatomic particles Most subatomic particles behave as PARTICLES and obey behave as PARTICLES and obey the physics of waves. the physics of waves.

wavelengthVisible light

wavelength

Ultaviolet radiation

Amplitude

Node

Electromagnetic Electromagnetic RadiationRadiation

Electromagnetic Electromagnetic RadiationRadiation

Wavelengths and energyWavelengths and energy

• Understand that different Understand that different wavelengths of electromagnetic wavelengths of electromagnetic radiation have different energies. radiation have different energies.

• Waves have a frequencyWaves have a frequency• c=c=ννλλ

– c=velocity of wave (2.998 x 10c=velocity of wave (2.998 x 1088 m/s) m/s)– νν=(nu) frequency of wave, =(nu) frequency of wave, units are units are

“cycles per sec”“cycles per sec”– λλ=(lambda) wavelength =(lambda) wavelength

Electromagnetic SpectrumElectromagnetic Spectrum Electromagnetic SpectrumElectromagnetic Spectrum

In increasing energyIn increasing energy, R, ROOYY GG BBIIVV

Electromagnetic Electromagnetic SpectrumSpectrum

Electromagnetic Electromagnetic SpectrumSpectrum

Long wavelength --> small frequencyLong wavelength --> small frequency

Short wavelength --> high frequencyShort wavelength --> high frequency

increasing increasing frequencyfrequency

increasing increasing wavelengthwavelength

Bohr’s ModelBohr’s Model

• Why don’t the electrons fall into Why don’t the electrons fall into the nucleus?the nucleus?

• Move like planets around the sun.Move like planets around the sun.• In circular orbits at different In circular orbits at different

levels.levels.• Amounts of energy separate one Amounts of energy separate one

level from another.level from another.

Bohr postulated that:Bohr postulated that:

• Fixed energy related to the orbitFixed energy related to the orbit• Electrons cannot exist between orbitsElectrons cannot exist between orbits• The higher the energy level, the The higher the energy level, the

further it is away from the nucleusfurther it is away from the nucleus• An atom with maximum number of An atom with maximum number of

electrons in the outermost orbital electrons in the outermost orbital energy level is stable (unreactive)energy level is stable (unreactive)

• Think of Noble gasesThink of Noble gases

Those who are not Those who are not shocked when they first shocked when they first come across quantum come across quantum theory cannot possibly theory cannot possibly

have understood it.have understood it.

(Niels Bohr on Quantum (Niels Bohr on Quantum Physics)Physics)

Atomic Line Emission Atomic Line Emission Spectra and Niels BohrSpectra and Niels BohrAtomic Line Emission Atomic Line Emission

Spectra and Niels BohrSpectra and Niels BohrBohr’s greatest contribution to Bohr’s greatest contribution to science was in building a science was in building a simple model of the atom. It simple model of the atom. It was based on an understanding was based on an understanding of theof the LINE EMISSION LINE EMISSION SPECTRASPECTRA of excited atoms.of excited atoms.•Problem is that the model only Problem is that the model only works for Hydrogenworks for HydrogenNiels BohrNiels Bohr

(1885-(1885-1962)1962)

How did he develop his How did he develop his theory?theory?

• He used mathematics to explain the visible He used mathematics to explain the visible spectrum of hydrogen gasspectrum of hydrogen gas

• Lines are associated with the fall of an Lines are associated with the fall of an excited electron back down to its ground excited electron back down to its ground state energy level.state energy level.

• http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/linesp16.swf

Spectrum of White Spectrum of White LightLight

The line spectrum The line spectrum

• electricity passed electricity passed through a gaseous through a gaseous element emits light element emits light at a certain at a certain wavelengthwavelength

• Can be seen when Can be seen when passed through a passed through a prismprism

• Every gas has a Every gas has a unique pattern unique pattern (color)(color)

Line Emission Line Emission Spectra Spectra

of Excited Atomsof Excited Atoms

Line Emission Line Emission Spectra Spectra

of Excited Atomsof Excited Atoms• Excited atoms emit light of only Excited atoms emit light of only certain wavelengthscertain wavelengths

• The wavelengths of emitted light The wavelengths of emitted light depend on the element.depend on the element.

Spectrum of Spectrum of Excited Hydrogen GasExcited Hydrogen Gas

Line Spectra of Other Line Spectra of Other ElementsElements

Line spectrumLine spectrum

Continuous line spectrum

• Bohr also postulated that an Bohr also postulated that an atom would not emit radiation atom would not emit radiation while it was in one of its stable while it was in one of its stable states but rather only when it states but rather only when it made a transition between made a transition between states. states.

• The frequency of the radiation The frequency of the radiation emitted would be equal to the emitted would be equal to the difference in energy between difference in energy between those states divided by Planck's those states divided by Planck's constant. constant.

EEhighhigh-E-Elowlow= h= hνν = hc/ = hc/λλ

h=h=6.63 × 106.63 × 10–34–34 J s J s = Planck’s constant= Planck’s constant

E= energy of the emitted light (photon)E= energy of the emitted light (photon)

νν = frequency of the photon of light = frequency of the photon of lightλλ = is usually stated in nm, but for calculations = is usually stated in nm, but for calculations

use m.use m.

• This results in a unique emission spectra for This results in a unique emission spectra for each element, like a fingerprint.each element, like a fingerprint.

• electron could "jump" from one allowed energy electron could "jump" from one allowed energy state to another by absorbing/emitting photons state to another by absorbing/emitting photons of radiant energy of certain specific of radiant energy of certain specific frequencies. frequencies.

• Energy must then be absorbed in order to Energy must then be absorbed in order to "jump" to another energy state, and similarly, "jump" to another energy state, and similarly, energy must be emitted to "jump" to a lower energy must be emitted to "jump" to a lower state. state.

• The frequency, The frequency, νν, of this radiant energy , of this radiant energy corresponds exactly to the energy difference corresponds exactly to the energy difference between the two states.between the two states.

• In order for the emitted energy to be In order for the emitted energy to be seen as light the wavelength of the seen as light the wavelength of the energy must be in between 380 nm to energy must be in between 380 nm to 750 nm750 nm

Bohr’s TriumphBohr’s Triumph

• His theory helped to explain periodic His theory helped to explain periodic law (the trends from the periodic law (the trends from the periodic table)table)

• Halogens (gp.17 or group VII) are so Halogens (gp.17 or group VII) are so reactive because it has one e- less reactive because it has one e- less than a full outer orbitalthan a full outer orbital

• Alkali metals (gp. 1 or group I) are Alkali metals (gp. 1 or group I) are also reactive because they have only also reactive because they have only one e- in outer orbitalone e- in outer orbital

DrawbackDrawback

• Bohr’s theory did Bohr’s theory did not explain or not explain or show the shape or show the shape or the path traveled the path traveled by the electrons.by the electrons.

• His theory could His theory could only explain only explain hydrogen and not hydrogen and not the more complex the more complex atomsatoms

Energy level populationsEnergy level populations

• Electrons found per energy level of the atom.Electrons found per energy level of the atom.• The first energy level holds 2 electronsThe first energy level holds 2 electrons• The second energy level holds 8 electrons (2 The second energy level holds 8 electrons (2

in s and 6 in p)in s and 6 in p)• The third energy level holds 18 electrons (2 The third energy level holds 18 electrons (2

in s, 6 in p and 10 in d) There is overlapping in s, 6 in p and 10 in d) There is overlapping here, so when we do the populations there here, so when we do the populations there will be some changes.will be some changes.

That is as far as this course requires us to go!That is as far as this course requires us to go!

Examples for group 1Examples for group 1

• Li Li 2.12.1 • Na Na 2.8.12.8.1 • K K 2.8.8.12.8.8.1

The Quantum Mechanical The Quantum Mechanical ModelModel

• Energy is quantized. It comes in chunks.Energy is quantized. It comes in chunks.• A quanta is the amount of energy needed A quanta is the amount of energy needed

to move from one energy level to another.to move from one energy level to another.• Since the energy of an atom is never “in Since the energy of an atom is never “in

between” there must be a quantum leap between” there must be a quantum leap in energy.in energy.

• Schrödinger derived an equation that Schrödinger derived an equation that described the energy and position of the described the energy and position of the electrons in an atomelectrons in an atom

Schrodinger applied idea of eSchrodinger applied idea of e-- behaving as a wave to the problem behaving as a wave to the problem of electrons in atoms.of electrons in atoms.

He developed the He developed the WAVE WAVE EQUATIONEQUATION

Solution gives set of math Solution gives set of math expressions called expressions called WAVE WAVE

FUNCTIONS, FUNCTIONS, Each describes an allowed energy Each describes an allowed energy

state of an estate of an e--

E. SchrodingerE. Schrodinger1887-19611887-1961

Quantum or Wave Quantum or Wave MechanicsMechanics

Quantum or Wave Quantum or Wave MechanicsMechanics

Heisenberg Heisenberg Uncertainty PrincipleUncertainty Principle

• The problem of defining nature of electrons in atoms was solved by W. Heisenberg.

• He observed that one cannot simultaneously define the position and momentum (= m•v) of an electron.

• If we define the energy exactly of an electron precisely we must accept limitation that we do not know exact position.

• The problem of defining nature of electrons in atoms was solved by W. Heisenberg.

• He observed that one cannot simultaneously define the position and momentum (= m•v) of an electron.

• If we define the energy exactly of an electron precisely we must accept limitation that we do not know exact position.

W. HeisenbergW. Heisenberg1901-19761901-1976

A good site:A good site:http://www.chemguide.co.uk/basicorg/bonding/orbitals.html

Electron Electron ConfigurationConfiguration

HL onlyHL only12.1.3 State the relative energies of s, p, d, and f 12.1.3 State the relative energies of s, p, d, and f orbitals in a single energy levelorbitals in a single energy level12.1.4 State the maximum number of orbitals in a 12.1.4 State the maximum number of orbitals in a given energy level.given energy level.12.1.5 Draw the shape of an s orbital and the 12.1.5 Draw the shape of an s orbital and the shapes of px, py and pz orbitalsshapes of px, py and pz orbitals12.1.6 Apply the Aufbau principle, Hund’s rule 12.1.6 Apply the Aufbau principle, Hund’s rule and the Pauli exclusion principle to write electron and the Pauli exclusion principle to write electron configurations for atoms and ions up to Z=54.configurations for atoms and ions up to Z=54.

S orbitalsS orbitals

• 1 s orbital for1 s orbital for

every energy levelevery energy level

1s 2s1s 2s 3s 3s• Spherical shapedSpherical shaped• Each s orbital can hold 2 electronsEach s orbital can hold 2 electrons• Called the 1s, 2s, 3s, etc.. orbitalsCalled the 1s, 2s, 3s, etc.. orbitals

P orbitalsP orbitals

• Start at the second energy level Start at the second energy level • 3 different directions3 different directions• 3 different shapes3 different shapes• Each orbital can hold 2 electronsEach orbital can hold 2 electrons

The D sublevel contains 5 D The D sublevel contains 5 D orbitalsorbitals• The D sublevel starts in the 3The D sublevel starts in the 3rdrd energy level energy level

• 5 different shapes (orbitals)5 different shapes (orbitals)• Each orbital can hold 2 electronsEach orbital can hold 2 electrons

The F sublevel has 7 F orbitalsThe F sublevel has 7 F orbitals• The F sublevel starts in the fourth energy levelThe F sublevel starts in the fourth energy level• The F sublevel has seven different shapes The F sublevel has seven different shapes

(orbitals)(orbitals)• 2 electrons per orbital2 electrons per orbital

SummarySummary

s

p

d

f

# of shapes (orbitals)

Max # of electrons

1 2 1

3 6 2

5 10 3

7 14 4

Sublevel

Starts at energy level

Electron ConfigurationsElectron Configurations

• The way electrons are arranged in atoms.The way electrons are arranged in atoms.• Aufbau principleAufbau principle-- electrons enter the electrons enter the

lowest energy first.lowest energy first.• This causes difficulties because of the This causes difficulties because of the

overlap of orbitals of different energies.overlap of orbitals of different energies.• Pauli Exclusion PrinciplePauli Exclusion Principle-- at most 2 at most 2

electrons per orbital - different spinselectrons per orbital - different spins• Hund’s RuleHund’s Rule-- When electrons occupy When electrons occupy

orbitals of equal energy they don’t pair up orbitals of equal energy they don’t pair up until they have to .until they have to .

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• Phosphorous, 15 e- to Phosphorous, 15 e- to placeplace

• The first to electrons The first to electrons go into the 1s orbitalgo into the 1s orbital

• Notice the opposite Notice the opposite spinsspins

• only 13 moreonly 13 more

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next electrons The next electrons go into the 2s go into the 2s orbitalorbital

• only 11 moreonly 11 more

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next electrons go into the 2p orbital

• only 5 more

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next electrons go into the 3s orbital

• only 3 more

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

Incr

easi

ng e

nerg

y

1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The last three electrons go into the 3p orbitals.

• They each go into separate shapes

• 3 unpaired electrons

• 1s22s22p63s23p3

Orbitals fill in order Orbitals fill in order • Lowest energy to higher energy.Lowest energy to higher energy.• Adding electrons can change the Adding electrons can change the

energy of the orbital.energy of the orbital.• Half filled orbitals have a lower Half filled orbitals have a lower

energy.energy.• Makes them more stable.Makes them more stable.• Changes the filling orderChanges the filling order

Write these electron Write these electron configurationsconfigurations

• Titanium - 22 electronsTitanium - 22 electrons

• 1s1s222s2s222p2p663s3s223p3p664s4s223d3d22

• Vanadium - 23 electrons Vanadium - 23 electrons

• 1s1s222s2s222p2p663s3s223p3p664s4s223d3d33

• Chromium - 24 electronsChromium - 24 electrons• 1s1s222s2s222p2p663s3s223p3p664s4s223d3d4 4 is expectedis expected

• But this is wrong!!But this is wrong!!

Chromium is actuallyChromium is actually• 1s1s222s2s222p2p663s3s223p3p664s4s113d3d55

• Why?Why?• This gives us two half filled orbitals.This gives us two half filled orbitals.• Slightly lower in energy.Slightly lower in energy.• The same principal applies to The same principal applies to

copper.copper.

Copper’s electron Copper’s electron configurationconfiguration

• Copper has 29 electrons so we Copper has 29 electrons so we expectexpect

• 1s1s222s2s222p2p663s3s223p3p664s4s223d3d99

• But the actual configuration isBut the actual configuration is• 1s1s222s2s222p2p663s3s223p3p664s4s113d3d1010

• This gives one filled orbital and one This gives one filled orbital and one half filled orbital.half filled orbital.

• Remember these exceptionsRemember these exceptions

Electronic StructureElectronic Structure – of – of

transition metalstransition metals • With the transition metals it is the With the transition metals it is the

4s4s electrons that are lost first when electrons that are lost first when they form ions:they form ions:– Titanium (Ti) - loss of 2 eTitanium (Ti) - loss of 2 e--

1s2 2s2 2p6 3s2 3p6 4s2 3d2 Ti atom Ti2+ ion

Cr atom

Cr3+ ion

1s2 2s2 2p6 3s2 3p6 3d2

1s2 2s2 2p6 3s2 3p6 4s1 3d5 1s2 2s2 2p6 3s2 3p6 3d3

Chromium (Cr) - loss of 3 e-

Electronic Structure Electronic Structure - - QuestionsQuestions

• Copy and complete the following Copy and complete the following table:table:

Atomic no.

Mass no.

No. of protons

No. of neutrons

No. of electrons

Electronic structure

Mg 121s2 2s2 2p6

3s2

Al3+ 27 10

S2- 16 16

Sc3+ 21 45

Ni2+ 30 26

Ionization EnergyIonization Energy12.1.112.1.1 Explain how evidence from first Explain how evidence from first

ionization energies ionization energies across periods accounts for across periods accounts for the existence of main the existence of main energy levels and energy levels and sub-levels in atomssub-levels in atoms12.1.2 Explain how successive ionization 12.1.2 Explain how successive ionization

energy data is energy data is related to the electron configuration of related to the electron configuration of

an atom an atom

Ionization EnergyIonization Energy• The amount of energy required to The amount of energy required to

completely remove an electron from a completely remove an electron from a gaseous atom.gaseous atom.

• An atom's 'desire' to grab another An atom's 'desire' to grab another atom's electrons. atom's electrons.

• Removing one electron makes a +1 ion.Removing one electron makes a +1 ion.• The energy required is called the first The energy required is called the first

ionization energy.ionization energy.

XX(g) (g) + energy+ energy →X →X++ + e- + e-

Ionization EnergyIonization EnergyThe second and third ionization The second and third ionization

energies can be represented as energies can be represented as follows:follows:

•   XX++ (g)(g) + energy + energy X X2+ 2+ (g)(g) + e- + e-

• XX2+2+ (g)(g) + energy + energy X X3+ 3+ (g)(g) + e- + e-

• More energy required to remove 2More energy required to remove 2ndnd electron, and still more energy electron, and still more energy required to remove 3required to remove 3rdrd electron electron

Group trendsGroup trends• Ionization energy decreases down the Ionization energy decreases down the

group.group.

Going from Be to Mg, IE decreases Going from Be to Mg, IE decreases because:because:– Mg outer electron is in the 3s sub-shell Mg outer electron is in the 3s sub-shell

rather than the 2s. This is higher in energyrather than the 2s. This is higher in energy– The 3s electron is further from the nucleus The 3s electron is further from the nucleus

and shielded by the inner electronsand shielded by the inner electrons– So the 3s electron is more easily removedSo the 3s electron is more easily removed

• A similar decrease occurs in every A similar decrease occurs in every group in the periodic table.group in the periodic table.

Notice any trends? Any Notice any trends? Any surprises?surprises?

• General trendGeneral trend: Increasing I.E. as we go : Increasing I.E. as we go across a periodacross a period

• Look at the peak at Mg and the plateau Look at the peak at Mg and the plateau between P and S. Can you explain why?between P and S. Can you explain why?

Why is there a fall from Why is there a fall from Mg to Al?Mg to Al?

•   Al has configuration 1sAl has configuration 1s22 2s 2s22 2p 2p66 3s 3s22 3p3p11, its outer electron is in a p , its outer electron is in a p sublevelsublevel

•    Mg has electronic configuration Mg has electronic configuration 1s1s222s2s222p2p663s3s22..

•    The p level is higher in energy and The p level is higher in energy and with Mg the s sub level is full – with Mg the s sub level is full – this this gives it a slight stability advantagegives it a slight stability advantage

Why is there a fall from P to Why is there a fall from P to S?S?

•    This can be explained in terms of electron This can be explained in terms of electron pairing.pairing.

•    As the p sublevel fills up, electrons fill up As the p sublevel fills up, electrons fill up the vacant sub levels and are unpaired.the vacant sub levels and are unpaired.

• This configuration is more energetically This configuration is more energetically stable than S as stable than S as all the electrons are all the electrons are unpaired.unpaired. It requires more energy to pair up It requires more energy to pair up the electrons in S so it has a lower the electrons in S so it has a lower Ionization energy.Ionization energy.

• There is some repulsion between the paired There is some repulsion between the paired electrons which lessens their attraction to electrons which lessens their attraction to the nucleus.the nucleus.

•    It becomes easier to remove!It becomes easier to remove!

Driving ForceDriving Force• Full Energy Levels are very low Full Energy Levels are very low

energy.energy.• Noble Gases have full energy levels.Noble Gases have full energy levels.• Atoms behave in ways to achieve Atoms behave in ways to achieve

noble gas configuration.noble gas configuration.

22ndnd Ionization Energy Ionization Energy• For elements that reach a filled or For elements that reach a filled or

half filled sublevel by removing 2 half filled sublevel by removing 2 electrons 2electrons 2ndnd IE is lower than IE is lower than expected.expected.

• Makes it easier to achieve a full Makes it easier to achieve a full outer shellouter shell

• True for sTrue for s2 2

• Alkaline earth metals form +2 ions.Alkaline earth metals form +2 ions.

33rdrd IE IE• Using the same logic sUsing the same logic s22pp11

atoms atoms have an low 3have an low 3rdrd IE. IE.

• Atoms in the aluminum family form Atoms in the aluminum family form +3 ions.+3 ions.

•22ndnd IE and 3 IE and 3rdrd IE are IE are always higher than 1always higher than 1stst IE!!!IE!!!