Atomic Theory

1
Atomic Theory

• The development of the scientific model of the
  atom.

2
Earliest Atomic Theory

• 400 B.C. Democritus, a Greek philosopher,
  developed the first atomic theory.
• He believed that matter was made up of tiny
  particles called atoms.
• He also believed that matter could not be
  created, destroyed, or further divided.
• His theory was met with criticism from other
  influential philosophers such as Aristotle.
• His theory was eventually rejected because it was
  not supported by experimental evidence.

3
Democritus Model of the Atom
4
1808-Dalton, an English school teacher, proposed
an atomic theory. His theory stated

• All matter is composed of extremely small
  particles called atoms.
• All atoms of a given element are identical.
  Atoms of a specific element are different from
  those of any other element.
• Atoms cannot be created, destroyed, or divided
  into smaller particles.
• Different atoms combine in simple whole-number
  ratios to form compounds.
• In a chemical reaction, atoms are separated,
  combined, or rearranged.

5
Daltons Model of the Atom
6
Daltons Symbols for the Elements
7
Daltons theory was supported by experimental
evidence which led to the general acceptance of
his theory. His theory has had to be revised,
however, as additional information has been
learned.
8
Late 1890s- Thomson, an English physicist,
discovered the existence of a small negatively
charged particle (the electron) through his
cathode ray tube experiment.
9
1909- Millikan, an American physicist,
determined the charge of an electron through his
Oil Drop Experiment. The value he determined is
still the accepted value used today.
10
As a result of these discoveries, Thomson
proposed a model of the atom that became known as
the Plum Pudding model (or Chocolate Chip Cookie
model).
11
1911- Rutherford, a New Zealand chemist,
conducted the Gold-Foil Experiment.
12
He recorded the following observations during his
experiment

• Most of the alpha particles passed through the
  gold foil undeflected.
• A smaller percentage of the particles were
  slightly deflected.
• A very small number of particles were deflected
  straight back towards their source.

13
From these observations, he concluded the
following

• The atom is mostly empty space.
• There is a tiny, dense central core in the atom.
  He called this the nucleus.
• The nucleus is positively charged.

14
From this experiment he concluded that the Plum
Pudding model was incorrect.
15
1919- Goldstein concluded that the nucleus
contained positively charged particles called
protons. This particle carries a charge equal
but opposite to that of an electron.
16
1932- Chadwick, an English physicist, showed that
the nucleus also contains another subatomic
particle. This neutral particle was called
the neutron. Its mass is nearly equal to that of
a proton, but it carries no charge.
17
As a result of this knowledge, a new model of the
atom was developed by Rutherford and Bohr, a
Danish physicist. This model is sometimes called
the Planetary model because it describes the
atoms as being similar to the solar system. The
sun represents the nucleus and the planets
represent the electrons.
18
Summary of the Subatomic Particles
Particle Symbol Location Relative Charge Relative Mass
Proton p Nucleus 1 1
Neutron no Nucleus 0 1
Electron e- In the space surrounding the nucleus 1- 1/1840
19
Isotopes

• Atoms of the same type of element that have
  DIFFERENT numbers of neutrons.
• Carbon 14 6 p, 6 e, 8 n
• Carbon 15 6 p, 6 e, 9 n
• Carbon 16 6 p, 6 e, 10 n