The Development of Atomic Theory

1
The Development of Atomic Theory

• Nothing exists except atoms and empty space all
  else is opinion.
• -Demokritos of Abdura

2
Early Atomic Theorists

• 450 BC Leukippos of Miletus and Demokritos
  of Abdura
• 300 BC Epikouros of Samos

Matter cannot be subdivied without limit Atoms
are tiny, separate and indestructible units of
matter Atoms are in constant motion through
empty space Atoms have mass Size and shape of
an atom determine all material properties

• 300BC Aristotle argued extensively against
  atomism

3
18th Century Atomic Theory

• 1789 Antoine Lavoisier
• The Law of Conservation of Mass

In a chemical reaction the mass of the reactants
must equal the mass of the products

• 1797 Joseph Proust
• The Law of Definite Proportions

A compound is always made of the same elements
in the same proportions by mass
4
Daltons Atomic Theory (1803)

• Elements are composed of atoms with
  characteristic mass.
• Compounds are made up of atoms combined in small,
  whole number ratios. (The Law of Multiple
  Proportions)
• Chemical reactions rearrange connections between
  atoms.

John Dalton
Daltons book, A New System of Chemical
Philosophy published in 1808, is considered the
beginning of modern atomic theory.
5
J.J. Thomson

• Discovered the electron in 1897
• Calculated the charge to mass ratio for the
  electron
• Plum Pudding Atomic Theory
• Nobel Prize in 1906

Joseph John Thomson
6
Ernest Rutherford

• Discovered the atomic Nucleus by alpha ray
  scattering (1912)
• Atom mostly empty space with a small, dense
  center
• Nobel Prize in Chemistry 1908

Ernest Rutherford
7
Alpha Ray Scattering Experiment
8
Basic Atomic Information
9
Some Important Definitions
Atomic Number (Z) The number of protons in the
nucleus of an atom.
Nucleon A particle found in the nucleus i.e. a
proton or a neutron.
Mass Number (A) The number of nucleons in an
atom.
Isotope Two (or more) atoms with the same atomic
number but a different mass number.
10
The Isotopes of Hydrogen
11
Calculating the Atomic Mass
12
Basic Radioactive Particles
13
Radioactive Decay
alpha decay

238U
4He 2
234Th
92
2
90
Alpha particle
Parent nuclide
daughter nuclide
beta decay
e -
234Th

234Pa
90
91
beta particle
Parent nuclide
daughter nuclide
14
Beta Decay Process

e -
1n
beta decay
1H
0
1
beta particle
proton
neutron
beta decay
e -
234Pa

234U
91
92
beta particle
Parent nuclide
daughter nuclide