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Atomic TheoryAtomic Theory
Defining the AtomDefining the Atom The Greek philosopher The Greek philosopher DemocritusDemocritus (460 B.C. – 370 (460 B.C. – 370
B.C.) was among the first to suggest the existence B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)of atoms (from the Greek word “atomos”) He believed that atoms were He believed that atoms were indivisibleindivisible and and
indestructibleindestructible His ideas did agree with later scientific theory, but His ideas did agree with later scientific theory, but
did not explain chemical behavior, and was did not explain chemical behavior, and was not not based on the scientific methodbased on the scientific method – but just – but just philosophyphilosophy
Dalton’s Atomic TheoryDalton’s Atomic Theory (experiment based!) (experiment based!)
3) Atoms of different elements combine in simple whole-number ratios to form chemical compounds
4) In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.
1) All elements are composed of tiny indivisible particles called atoms
2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element.
John Dalton(1766 – 1844)
Sizing up the AtomSizing up the Atom Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element
If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm longDespite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes
Structure of the Nuclear AtomStructure of the Nuclear Atom
One change to Dalton’s atomic theory is that One change to Dalton’s atomic theory is that atoms are divisibleatoms are divisible into subatomic particles: into subatomic particles: Electrons, protons, and neutronsElectrons, protons, and neutrons are are
examples of these fundamental particlesexamples of these fundamental particles There are many other types of particles, There are many other types of particles,
but we will study these threebut we will study these three
Discovery of the ElectronDiscovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron
Modern Modern CCathode athode RRay ay TTubesubes
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.
Television Computer Monitor
Mass of the ElectronMass of the Electron
1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge
The oil drop apparatus
Mass of the electron is 9.11 x 10-28 g
Conclusions from the Study Conclusions from the Study of the Electron:of the Electron:
a) Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
b) Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
c) Electrons have so little mass that atoms must contain other particles that account for most of the mass
Conclusions from the Study Conclusions from the Study of the Electron:of the Electron:
Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)
1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton
Subatomic ParticlesSubatomic Particles
ParticleParticle ChargeCharge Mass (g)Mass (g) LocationLocation
ElectronElectron
(e(e--)) -1-1 9.11 x 109.11 x 10-28-28 Electron Electron cloudcloud
ProtonProton (p(p++)) +1+1 1.67 x 101.67 x 10-24-24 NucleusNucleus
NeutronNeutron
(n(noo)) 00 1.67 x 101.67 x 10-24-24 NucleusNucleus
Thomson’s Atomic ModelThomson’s Atomic Model
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
J. J. Thomson
Ernest Rutherford’sErnest Rutherford’sGold Foil Experiment - 1911Gold Foil Experiment - 1911
Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded
Rutherford’s problem:Rutherford’s problem:In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?
Target #1
Target #2
The Answers:The Answers:
Target #1 Target #2
Rutherford’s FindingsRutherford’s Findings
a) The nucleus is smallb) The nucleus is densec) The nucleus is positively
charged
Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The Rutherford Atomic ModelThe Rutherford Atomic Model Based on his experimental evidence:Based on his experimental evidence:
The atom is mostly empty spaceThe atom is mostly empty space All the positive charge, and almost all the mass is All the positive charge, and almost all the mass is
concentrated in a small area in the center. He concentrated in a small area in the center. He called this a “called this a “nucleusnucleus””
The nucleus is composed of protons and neutrons The nucleus is composed of protons and neutrons (they (they makemake the nucleus!) the nucleus!)
The electrons distributed around the nucleus, and The electrons distributed around the nucleus, and occupy most of the volumeoccupy most of the volume
His model was called a “His model was called a “nuclear modelnuclear model””
Bohr ModelBohr Model In 1913, the Danish In 1913, the Danish
scientist Niels Bohr scientist Niels Bohr proposed an proposed an improvement. In his improvement. In his model, he placed model, he placed each electron in a each electron in a specificspecific energy level. energy level.
Bohr ModelBohr Model According to Bohr’s According to Bohr’s
atomic model, atomic model, electrons move in electrons move in definite definite orbitsorbits around around the nucleus, much the nucleus, much like planets circle the like planets circle the sun. These orbits, or sun. These orbits, or energy energy levelslevels, are , are located at certain located at certain distances from the distances from the nucleus.nucleus.
Wave Model
The Wave ModelThe Wave Model Today’s atomic model Today’s atomic model
is based on the is based on the principles of principles of wavewave mechanicsmechanics..
According to the According to the theory of wave theory of wave mechanics, electrons mechanics, electrons do not movedo not move about an about an atom in a atom in a definite definite path,path, like the planets like the planets around the sun.around the sun.
The Wave ModelThe Wave Model
In fact, it is In fact, it is impossibleimpossible to determine the exact to determine the exact location of an electron. The location of an electron. The probableprobable location of location of an electron is based on how much an electron is based on how much energyenergy the the electron has.electron has.
According to the modern atomic model, at atom According to the modern atomic model, at atom has a has a small positively charged nucleussmall positively charged nucleus surrounded by a large region in which there are surrounded by a large region in which there are enough electrons to make an atom neutral.enough electrons to make an atom neutral.
Electron CloudElectron Cloud A space in which A space in which
electrons are likely to electrons are likely to be found.be found.
Electrons Electrons whirlwhirl about about the nucleus billions of the nucleus billions of times in one secondtimes in one second
They are not moving They are not moving around in around in randomrandom patterns.patterns.
Location of electrons Location of electrons depends upon how depends upon how much much energyenergy the the electron has.electron has.
Electron CloudElectron Cloud
Depending on their energy they are locked Depending on their energy they are locked into a certain area in the cloud.into a certain area in the cloud.
Electrons with the Electrons with the lowestlowest energy are found energy are found in the energy level in the energy level closestclosest to the nucleus to the nucleus
Electrons with the Electrons with the highesthighest energy are energy are found in the found in the outermostoutermost energy levels, energy levels, farther from the nucleus.farther from the nucleus.
IndivisibleIndivisible ElectronElectron NucleusNucleus OrbitOrbit Electron Electron CloudCloud
GreekGreek XX
DaltonDalton XX
ThomsonThomson XX
RutherfordRutherford XX XX
BohrBohr XX XX XX
WaveWave XX XX XX
Atomic StructureAtomic Structure
An atom consists of:An atom consists of: nucleusnucleus – contains protons and neutrons – contains protons and neutrons electronselectrons in space around the nucleus. in space around the nucleus. The atom is mostly empty spaceThe atom is mostly empty space
The The AtomAtom
NucleusNucleus
Electron cloudElectron cloud
ATOMIC COMPOSITIONATOMIC COMPOSITION Protons (pProtons (p++))
+ electrical charge+ electrical charge mass = 1.672623 x 10mass = 1.672623 x 10-24-24 g g relative mass = 1.007 atomic mass units relative mass = 1.007 atomic mass units
(amu) (amu) but we can round to 1but we can round to 1 Electrons (eElectrons (e--))
negative electrical chargenegative electrical charge relative mass = 0.0005 amu relative mass = 0.0005 amu
but we can round to 0but we can round to 0 Neutrons (nNeutrons (noo))
no electrical chargeno electrical charge mass = 1.009 amu mass = 1.009 amu but we can round to 1but we can round to 1
Atomic Number, ZAtomic Number, ZAll atoms of the same element have All atoms of the same element have
the same number of protons in the the same number of protons in the nucleus, nucleus, ZZ
1313
AlAl
26.98126.981
Atomic numberAtomic number
Atom symbolAtom symbol
AVERAGE Atomic MassAVERAGE Atomic Mass
Atomic NumberAtomic Number
Atoms are composed of Atoms are composed of identicalidentical protons, protons, neutrons, and electronsneutrons, and electrons How then are atoms of one element different How then are atoms of one element different
from another element?from another element? Elements are different because they contain Elements are different because they contain
different numbers of different numbers of PROTONSPROTONS The “The “atomic numberatomic number” of an element is the ” of an element is the number number
of protonsof protons in the nucleus in the nucleus # protons in an atom = # electrons# protons in an atom = # electrons
Atomic NumberAtomic Number
ElementElement # of protons# of protons Atomic # (Z)Atomic # (Z)
CarbonCarbon 66 66
PhosphorusPhosphorus 1515 1515
GoldGold 7979 7979
Mass Number, AMass Number, A A carbon atom with 6 protons and 6 A carbon atom with 6 protons and 6
neutrons is the mass standard = 12 neutrons is the mass standard = 12 atomic mass unitsatomic mass units
Mass NumberMass Number (A) (A)= # protons + # neutrons= # protons + # neutrons
NOT on the periodic table…(Round the NOT on the periodic table…(Round the AVERAGE atomic mass on the table)AVERAGE atomic mass on the table)
A boron atom can have A boron atom can have A = 5 p + 5 n = 10 amuA = 5 p + 5 n = 10 amu
A
Z
10
5B
A
Z
10
5B
Mass NumberMass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0
NuclideNuclide pp++ nn00 ee-- Mass #Mass #
Oxygen Oxygen - - 1010
- - 3333 4242
- - 3131 1515
8 8 1818
Arsenic 75 33 75
Phosphorus 15 3116
Complete SymbolsComplete Symbols
Contain the symbol of the element, Contain the symbol of the element, the mass number and the atomic the mass number and the atomic number.number.
X Massnumber
Atomicnumber
Subscript →
Superscript →
SymbolsSymbols Find each of these: Find each of these:
a)a) number of protonsnumber of protons
b)b) number of number of neutronsneutrons
c)c) number of number of electronselectrons
d)d) Atomic numberAtomic number
e)e) Mass NumberMass Number
Br80 35
SymbolsSymbols If an element has an atomic If an element has an atomic
number of 34 and a mass number of 34 and a mass number of 78, what is the: number of 78, what is the:
a)a) number of protonsnumber of protons
b)b) number of neutronsnumber of neutrons
c)c) number of electronsnumber of electrons
d)d) complete symbolcomplete symbol
SymbolsSymbols If an element has 91 If an element has 91
protons and 140 neutrons protons and 140 neutrons what is the what is the
a)a) Atomic numberAtomic number
b)b) Mass numberMass number
c)c) number of electronsnumber of electrons
d)d) complete symbolcomplete symbol
SymbolsSymbols If an element has 78 If an element has 78
electrons and 117 neutrons electrons and 117 neutrons what is the what is the
a)a) Atomic numberAtomic number
b)b) Mass numberMass number
c)c) number of protonsnumber of protons
d)d) complete symbolcomplete symbol
IsotopesIsotopes
IsotopesIsotopes
Dalton was wrong about all Dalton was wrong about all elements of the same type elements of the same type being identicalbeing identical
Atoms of the same element Atoms of the same element cancan have different numbers of have different numbers of neutronsneutrons..
Thus, different mass numbers.Thus, different mass numbers.These are called These are called isotopesisotopes..
IsotopesIsotopes
Frederick SoddyFrederick Soddy (1877-1956) proposed the (1877-1956) proposed the idea of isotopes in 1912idea of isotopes in 1912
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Soddy won the Nobel Prize in Chemistry in Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and 1921 for his work with isotopes and radioactive materials.radioactive materials.
Naming IsotopesNaming Isotopes
We can also put the mass We can also put the mass number number afterafter the name of the the name of the element:element:carbon-12carbon-12carbon-14carbon-14uranium-235uranium-235
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
IsotopeIsotope ProtonsProtons ElectronsElectrons NeutronsNeutrons NucleusNucleus
Hydrogen–1Hydrogen–1
(protium)(protium) 11 11 00
Hydrogen-2Hydrogen-2
(deuterium)(deuterium) 11 11 11
Hydrogen-3Hydrogen-3
(tritium)(tritium)
11 11 22
IsotopesIsotopesElements occur in nature as mixtures of isotopes.
Isotopes are atoms of the same element that differ in the number of neutrons.
Isotopes Isotopes & Their & Their
UsesUsesBone scans with Bone scans with radioactive radioactive technetium-99. technetium-99.
Isotopes & Their Isotopes & Their UsesUses
The The tritiumtritium content of ground water is content of ground water is used to discover the source of the used to discover the source of the water, for example, in municipal water water, for example, in municipal water or the source of the steam from a or the source of the steam from a volcano. volcano.
Atomic MassAtomic Mass How heavy is an atom of oxygen?How heavy is an atom of oxygen?
It depends, because there are different It depends, because there are different kindskinds of oxygen atoms. of oxygen atoms.
We are more concerned with the We are more concerned with the average average atomic mass.atomic mass.
This is based on the abundance This is based on the abundance (percentage) of each variety of that (percentage) of each variety of that element in nature.element in nature. We don’t use grams for this mass because We don’t use grams for this mass because
the numbers would be too small.the numbers would be too small.
Measuring Atomic MassMeasuring Atomic Mass Instead of grams, the unit we use is the Instead of grams, the unit we use is the Atomic Atomic
Mass UnitMass Unit (amu)(amu) It is defined as one-twelfth the mass of a carbon-12 It is defined as one-twelfth the mass of a carbon-12
atom.atom. Carbon-12 chosen because of its Carbon-12 chosen because of its isotope purityisotope purity. .
Each isotope has its own atomic mass, thus we Each isotope has its own atomic mass, thus we determine the average from percent abundance.determine the average from percent abundance.
To calculate the average:To calculate the average: Multiply the atomic mass of each isotope Multiply the atomic mass of each isotope
by it’s abundance (expressed as a by it’s abundance (expressed as a decimal), then add the results.decimal), then add the results.
If not told otherwise, the mass of the If not told otherwise, the mass of the isotope is expressed in isotope is expressed in atomic mass unitsatomic mass units (amu)(amu)
Atomic MassesAtomic Masses
IsotopeIsotope SymbolSymbol Composition of Composition of the nucleusthe nucleus
% in nature% in nature
Carbon-12Carbon-12 1212CC 6 protons6 protons
6 neutrons6 neutrons
98.89%98.89%
Carbon-13Carbon-13 1313CC 6 protons6 protons
7 neutrons7 neutrons
1.11%1.11%
Carbon-14Carbon-14 1414CC 6 protons6 protons
8 neutrons8 neutrons
<0.01%<0.01%
Atomic mass is the average of all the naturally occurring isotopes of that element.
Carbon = 12.011
Average Atomic MassAverage Atomic Mass
weighted average of all isotopesweighted average of all isotopes on the Periodic Tableon the Periodic Table round to 2 decimal placesround to 2 decimal places
100
(%)(mass(mass)(%) )
Avg.AtomicMass
Avg.AtomicMass
D. Average Atomic MassD. Average Atomic Mass
EXEX: Calculate the avg. atomic mass of oxygen if : Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% its abundance in nature is 99.76% 1616O, 0.04% O, 0.04% 1717O, and 0.20% O, and 0.20% 1818O.O.
100
(18)(0.20)(17)(0.04))(16)(99.76 16.00amu
Avg.AtomicMass
D. Average Atomic MassD. Average Atomic Mass
EXEX: Find chlorine’s average atomic mass if : Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.35 and 2 are chlorine-37.
10
(37)(2)(35)(8)35.40 amu