The evolution of Atomic Theory

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The evolution of Atomic Theory

• Daltons Atomic Theory
• Development of structure of the atom
• Nucleus
• Dimitri Mendeleev(Periodicity)
• Modern periodic table
• New model of the atom

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Chemistry Timeline 1
B.C. 400 B.C. Demokritos and Leucippos use the
term "atomos
? 2000 years of Alchemy

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

1600's Robert BoyleThe 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

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Chemistry 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

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Daltons Atomic Theory (1808)

• 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

• 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

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Modern Atomic Theory
Several changes have been made to Daltons 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.
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Modern Atomic Theory 2
Dalton said
Atoms cannot be subdivided, created, or destroyed

Modern theory states
Atoms cannot be subdivided, created, or destroyed
in ordinary chemical reactions. However, these
changes CAN occur in nuclear reactions!
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Adding Electrons to the Model
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.

• Daltons Billiard ball model (1800-1900)
• Atoms are solid and indivisible.

• Thompson Plum pudding model (1900)
• Negative electrons in a positive framework.

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

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Law of conservation of matter

• When a chemical change (reaction) takes place,
  matter is neither created nor destroyed.
• Antoine Lavoisier (1775)

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Law 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.

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Example

• 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?

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Law 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.

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Development of the structure of the 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

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Discovery of the Electron
In 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.
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Thomsons Experiment

-
Vacuum tube
Metal Disks
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Thomson Experiment

-

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

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Thomsons Experiment

-

• By adding an electric field

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Thomsons Experiment

-

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

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Thomsons Atomic Model
Thomson believed that the electrons were like
plums embedded in a positively charged pudding,
thus it was called the plum pudding model.
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What did Thomson conclude from his experiment?

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

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Mass 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
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Conclusions from the Study of 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

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Atomic Particles
Particle 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
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Rutherfords Gold Foil Experiment

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

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Nucleus holds protons and neutrons.

• Rutherfords 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

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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?
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The Answers
Target 1
Target 2
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The Atomic Scale

• 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
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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
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Rutherfords Findings

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

Conclusions

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

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Isotopes
Isotopes are atoms of the same element having
different masses due to varying numbers of
neutrons.
Isotope Protons Electrons Neutrons Nucleus
Hydrogen1 (protium) 1 1 0
Hydrogen-2 (deuterium) 1 1 1
Hydrogen-3 (tritium) 1 1 2
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Atomic Masses
Atomic mass is the average of all the naturally
isotopes of that element.
Carbon 12.011
Isotope Symbol Composition of the nucleus in nature
Carbon-12 12C 6 protons 6 neutrons 98.89
Carbon-13 13C 6 protons 7 neutrons 1.11
Carbon-14 14C 6 protons 8 neutrons lt0.01
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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
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Learning Check

• An atom has 14 protons and 20 neutrons.
• A. Its atomic number is
• 1) 14 2) 16 3) 34
• B. Its mass number is
• 1) 14 2) 16 3) 34
• C. The element is
• 1) Si 2) Ca 3) Se
• D. Another isotope of this element is
• 1) 34X 2) 34X 3) 36X
• 16 14
  14

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Law of Mendeleev

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

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Mendeleevs Periodic Table
Dmitri Mendeleev
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Modern Periodic Table

• Vertical columns (groups)
• Horizontal rows (periods)
• Group IA-VIIIA representative or main group of
  elements
• Group IB-VIIIB are transition metals
• Green rows at the bottom (rare earth metals)
• Lanthanides and Actinides

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Period
The Periodic Table
Group or Family
Group or family
Period
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The Properties of a Group the Alkali Metals

• Easily lose valence electron
• (Reducing agents)
• React violently with water
• Large hydration energy
• React with halogens (group viiA) to form salts

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Properties of Metals

• Metals are good conductors of heat and
  electricity
• Metals are malleable
• Metals are ductile
• Metals have luster

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Properties of 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

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Properties 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

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Silicon, 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

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Atomic 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

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Determination of Atomic Radius
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

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Ionic Radii

•   Positively charged ions formed when
• an atom of a metal loses one or
• more electrons

Cations ()

•   Smaller than the corresponding
• atom

• Negatively charged ions formed
• when nonmetallic atoms gain one
• or more electrons

Anions (-)

• Larger than the corresponding
• atom (the extra repulsion produced by the
  incoming electron causes the atom to expand)

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Forming Cations Anions
A CATION forms when an atom loses one or more
electrons.
An ANION forms when an atom gains one or more
electrons
F e- --gt F-
Mg --gt Mg2 2 e-
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Predicting Ionic Charges
Group 1
Lose 1 electron to form 1 ions
H
Li
Na
K
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Predicting Ionic Charges
Group 2
Loses 2 electrons to form 2 ions
Be2
Mg2
Ca2
Ba2
Sr2
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Predicting Ionic Charges
Group 13
Loses 3 electrons to form 3 ions
B3
Al3
Ga3
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Predicting Ionic Charges
Group 14
Loses 4 electrons or gains 4
electrons
Caution! C22- and C4- are both called carbide
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Predicting Ionic Charges
Nitride
N3-
Group 15
Gains 3 electrons to form 3- ions
P3-
Phosphide
As3-
Arsenide
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Predicting Ionic Charges
Oxide
O2-
Group 16
Gains 2 electrons to form 2- ions
S2-
Sulfide
Se2-
Selenide
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Predicting Ionic Charges
F1-
Br1-
Fluoride
Bromide
Group 17
Gains 1 electron to form 1- ions
Cl1-
Chloride
I1-
Iodide
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Predicting Ionic Charges
Group 18
Stable Noble gases do not form ions!
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Ionization 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

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Electronegativity
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

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Summation of Periodic Trends
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New 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(?)
• Lambda distance between identical adjacent
  points on the wave(distance between one crest or
  trough)

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Visible light ranges from 380-750nm

• The visible and invisible wavelengths are called
  electromagnetic radiation
• The entire spectrumelectromagnetic spectrum
• The white light dispersed by a prism into
  different wavelengths gives the continuous
  spectrum.