The evolution of Atomic Theory The evolution of Atomic Theory Dalton’s Atomic Theory Development of structure of the atom Nucleus Dimitri Mendeleev(Periodicity)

The evolution of Atomic TheoryThe evolution of Atomic Theory

Dalton’s Atomic Theory

Development of structure of the atom

Nucleus

Dimitri Mendeleev(Periodicity)

Modern periodic table

New model of the atom

Chemistry Timeline #1Chemistry Timeline #1B.C. 400 B.C. Demokritos and Leucippos use the term "atomos”

1500's Georg Bauer: systematic metallurgy Paracelsus: medicinal application of minerals

1600'sRobert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of elements

1700s'Georg Stahl: Phlogiston Theory Joseph Priestly: Discovery of oxygen Antoine Lavoisier: The role of oxygen in combustion, law of conservation of mass, first modern chemistry textbook

2000 years of Alchemy

Chemistry Timeline #2Chemistry Timeline #2

1800's Joseph Proust: The law of definite proportion (composition) John Dalton: The Atomic Theory, The law of multiple proportions Joseph Gay-Lussac: Combining volumes of gases, existence of diatomic molecules Amadeo Avogadro: Molar volumes of gases Jons Jakob Berzelius: Relative atomic masses, modern symbols for the elements Dmitri Mendeleyev: The periodic table J.J. Thomson: discovery of the electron Henri Becquerel: Discovery of radioactivity

1900's Robert Millikan: Charge and mass of the electron Ernest Rutherford: Existence of the nucleus, and its relative size Meitner & Fermi: Sustained nuclear fission Ernest Lawrence: The cyclotron and trans-uranium elements

Dalton’s Atomic Theory Dalton’s Atomic Theory (1808)(1808)

Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged

All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

John Dalton

Modern Atomic TheoryModern Atomic TheorySeveral changes have been made to Dalton’s theory.

Dalton said:

Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties

Modern theory states:Atoms of an element have a

characteristic average mass which is unique to that element.

Modern Atomic Theory #2Modern Atomic Theory #2

Dalton said:

Modern theory states:

Atoms cannot be subdivided, created, or destroyed

Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

Adding Electrons to the ModelAdding Electrons to the Model

1) Dalton’s “Billiard ball” model (1800-1900)Atoms are solid and indivisible.

2) Thompson “Plum pudding” model (1900)Negative electrons in a positive framework.

3) The Rutherford model (around 1910)Atoms are mostly empty space.Negative electrons orbit a positive nucleus.

Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube (demos). These rays have a small mass and are negative.Thompson noted that these negative subatomic particles were a fundamental part of all atoms.

Law of conservation of matterLaw of conservation of matter

When a chemical change (reaction) takes place, matter is neither created nor destroyed.

Antoine Lavoisier (1775)

Law of definite ProportionsLaw of definite Proportions

When two elements react to form a compound, the total amount(mass) of the compound formed is determined by the composition of the compound and not by the masses of the elements used.

ExampleExample

If 50.0 g of water is decomposed into

5.6 g of hydrogen gas and 44. 4g of

oxygen gas. What is the percent by

mass of these elements?

Law of multiple proportionsLaw of multiple proportions When two elements (a and b) can combine to

form more than one compound, then for a fixed weight of a, the weights of b in two different compounds always form a ratio that is expressible in small whole numbers.

Development of the structure of Development of the structure of the atomthe atom

Electron (e-) Thomson,1897 present in all atoms, negative charge(-1), 1/1836 mass of H atom

Proton (p+) Thomson and Goldstein,1907

Present in all atoms, about the same mass of H, charge (+1)

Neutron (no) Chadwick, 1932 Same mass as proton , 0 charge

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.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Thomson’s ExperimentThomson’s Experiment

A stream of charged particles flows from cathode to the anode

Voltage source

- +

Vacuum tube

Metal Disks

Thomson ExperimentThomson Experiment

Voltage source

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

- +


Voltage source

+

-

By adding an electric field By adding an electric field


Voltage source

+

-By adding an electric field he found that By adding an electric field he found that the moving pieces were negative the moving pieces were negative

Thomson’s Atomic Thomson’s Atomic ModelModel

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

What did Thomson conclude from What did Thomson conclude from his experiment?his experiment?

He concluded that a cathode ray is composed of particles with a negative charge. These particles are electrons.

Mass of the ElectronMass of the Electron

1909 – Robert Millikan determines the mass of the electron.

The oil drop apparatus Mass of the

electron is 9.109 x 10-31 kg

Conclusions from the Study Conclusions from the Study of the Electronof the Electron

Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

Atomic ParticlesAtomic ParticlesParticle Charge Mass (kg) Location

Electron

-1 9.109 x 10-31 Electron cloud

Proton +1 1.673 x 10-27 Nucleus

Neutron

0 1.675 x 10-27 Nucleus

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Nucleus: holds protons and Nucleus: holds protons and neutrons.neutrons.

Rutherford’s gold foil experiment

“It was… as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”

Try it Yourself!Try it Yourself!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?

The AnswersThe Answers

Target #1 Target #2

The Atomic The Atomic ScaleScale

Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space

“q” is a particle called a “quark”

About Quarks…About Quarks…Protons and neutrons are NOT fundamental particles.Protons are made of two “up” quarks and one “down” quark.Neutrons are made of one “up” quark and two “down” quarks.

Quarks are held together by “gluons”

Rutherford’s FindingsRutherford’s Findings

The nucleus is small The nucleus is dense The nucleus is positively charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

Conclusions:

IsotopesIsotopes Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.Isotope Proto

nsElectron

sNeutron

sNucleus

Hydrogen–1

(protium)

1 1 0

Hydrogen-2

(deuterium)

1 1 1

Hydrogen-3

(tritium)

1 1 2

Atomic Atomic MassesMasses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12

12C 6 protons 6 neutrons

98.89%

Carbon-13


1.11%

Carbon-14


<0.01%

Atomic mass is the average of all the naturally isotopes of that element.Carbon = 12.011

The structure of the atom

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

Learning CheckLearning Check

An atom has 14 protons and 20 neutrons.A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is1) 14 2) 16 3) 34

C. The element is1) Si 2) Ca 3) Se

D. Another isotope of this element is

1) 34X 2) 34X 3) 36X 16 14 14

Law of MendeleevLaw of Mendeleev

Properties of the elements recur in regular cycles (periodically) when the elements are arranged in order of increasing atomic weight

Mendeleev’s Periodic TableMendeleev’s Periodic Table

Dmitri Mendeleev

Modern Periodic TableModern Periodic Table

Vertical columns (groups)Horizontal rows (periods)Group IA-VIIIA representative or

main group of elementsGroup IB-VIIIB are transition metalsGreen rows at the bottom: (rare earth

metals) Lanthanides and Actinides

The Periodic TableThe Periodic Table

Period

Group or family

PeriodGroup or Family

Easily lose valence electron (Reducing agents)

React violently with water Large hydration energy React with halogens (group

viiA) to form salts

The Properties of a Group: The Properties of a Group:

the Alkali Metalsthe Alkali Metals

Properties of MetalsProperties of Metals

Metals are good conductors of heat and electricity

Metals are malleable

Metals are ductile

Metals have luster

PropertiesProperties of Nonmetalsof Nonmetals

Carbon, the graphite in “pencil lead” is a great example of a nonmetallic element. • Nonmetals are poor conductors of heat and electricity • Nonmetals tend to be brittle • Many nonmetals are gases at room temperature

Properties of MetalloidsProperties of Metalloids

Metalloids straddle the border between metals and nonmetals on the periodic table.

They have properties of both metals and nonmetals. Metalloids are more brittle than metals, less brittle than most nonmetallic solids Metalloids are semiconductors of electricity Some metalloids possess metallic luster

Silicon, Si – A MetalloidSilicon, Si – A Metalloid

Silicon has metallic luster Silicon is brittle like a nonmetal Silicon is a semiconductor of electricity

Other metalloids include:

Boron, B Germanium, Ge Arsenic, As Antimony, Sb Tellurium, Te

Atomic sizeAtomic size Atomic radius(measured by

experimental techniques) measured in angstroms(10-10 meter), nanometers(nm, 10-9) or picometers(pm, 10-12)

In a group, as you move down the atoms get larger(there are exceptions)

Across a period from left to right the atoms get smaller

Half of the distance between nuclei in covalently bonded diatomic molecule

"covalent atomic radii"

Periodic Trends in Atomic Radius

Radius decreases across a period

Radius increases down a group

Determination of Atomic Radius:Determination of Atomic Radius:

Ionic RadiiIonic Radii

Cations (+)

Positively charged ions formed when an atom of a metal loses one or more electrons Smaller than the corresponding atom

Anions (-)

Negatively charged ions formed when nonmetallic atoms gain one or more electrons Larger than the corresponding atom (the extra repulsion produced by the

incoming electron causes the atom to expand)

Forming Cations & Forming Cations & AnionsAnionsForming Cations & Forming Cations & AnionsAnions

A A CATIONCATION forms forms when an when an atom atom losesloses one or one or more electrons.more electrons.

An An ANIONANION forms forms when an when an atom atom gainsgains one or one or more electronsmore electrons

Mg --> Mg2+ + 2 e- F + e- --> F-

Predicting Ionic ChargesPredicting Ionic Charges

Group 1Group 1::Lose 1 electron to form Lose 1 electron to form 1+1+ ions ions

HH++ LiLi++ NaNa++ KK++

Predicting Ionic ChargesPredicting Ionic Charges

Group 2Group 2::Loses 2 electrons to form Loses 2 electrons to form 2+2+ ions ions

BeBe2+2+ MgMg2+2+ CaCa2+2+ SrSr2+2+ BaBa2+2+

Predicting Ionic ChargesPredicting Ionic ChargesGroup 13Group 13:: Loses 3 Loses 3 electrons to form electrons to form 3+3+ ions ions

BB3+3+ AlAl3+3+ GaGa3+3+

Predicting Ionic ChargesPredicting Ionic ChargesGroup 14Group 14:: Loses 4 Loses 4 electrons or electrons or gains gains 4 electrons4 electrons

Caution! Caution! CC222-2- and C and C4-4-

are both called are both called carbidecarbide

Predicting Ionic ChargesPredicting Ionic ChargesGroup 15Group 15:: Gains 3 Gains 3 electrons to form electrons to form 3-3- ions ions

NN3-3-

PP3-3-

AsAs3-3-

Nitride

Phosphide

Arsenide

Predicting Ionic ChargesPredicting Ionic ChargesGroup 16Group 16:: Gains 2 Gains 2 electrons to form electrons to form 2-2- ions ions

OO2-2-

SS2-2-

SeSe2-2-

Oxide

Sulfide

Selenide

Predicting Ionic ChargesPredicting Ionic ChargesGroup 17Group 17:: Gains 1 Gains 1 electron to form electron to form 1-1- ions ions

FF1-1-

ClCl1-1-

BrBr1-1-Fluoride

Chloride

Bromide

II1-1- Iodide

Predicting Ionic ChargesPredicting Ionic ChargesGroup 18Group 18:: Stable Stable Noble gases Noble gases do do notnot form ions! form ions!

Ionization EnergyIonization Energy

Minimum amount of energy required to remove the outermost electron from an atom (IE)

IE decreases as you go down a group

IE increases as you go from left to right in a period

ElectronegativityElectronegativity

A measure of the ability of an atom in a chemical compound to attract electrons

Electronegativities tend to increase across a period

Electronegativities tend to decrease down a group or remain the same

Summation of Periodic TrendsSummation of Periodic Trends

New model of the atomNew model of the atom

New model came from closer examinations of properties of light given off by gaseous atoms

Light could be defined as particles or pockets of energy called photons or as waves of energy.

As a wave it is characterized by lambda(λ)

Lambda= distance between identical adjacent points on the wave(distance between one crest or trough)

Visible light ranges from Visible light ranges from 380-750nm380-750nm

The visible and invisible wavelengths are called electromagnetic radiation

The entire spectrum=electromagnetic spectrum

The white light dispersed by a prism into different wavelengths gives the continuous spectrum.

Documents

The evolution of Atomic Theory The evolution of Atomic Theory Dalton’s Atomic Theory Development of structure of the atom Nucleus Dimitri Mendeleev(Periodicity)