Atomic Structure

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Atomic Structure
Image courtesy of www.lab-initio.com
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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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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 Atomic Model
J.J. Thomson
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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Mass of the Electron
1909 Robert Millikan determines the mass of the
electron.
Mass of the electron is 9.109 x 10-31 kg
The oil drop apparatus
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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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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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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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Rutherfords Findings

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

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Atomic Particles
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The Atomic Scale
Helium-4

• 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

Image User Yzmo Wikimedia Commons.
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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
Images Arpad Horvath, Wikimedia Commons.
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Isotopes
Isotopes are atoms of the same element having
different masses due to varying numbers of
neutrons.
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Atomic Masses
Atomic mass is the weighted average of all the
naturally isotopes of that element.
Carbon 12.011
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Atomic Number
Atomic number (Z) of an element is the number of
protons in the nucleus of each atom of that
element.
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Mass Number
Mass number is the number of protons and neutrons
in the nucleus of an isotope.
Mass p n0
8
8
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Arsenic
75
33
75
Phosphorus
15
31
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