Atomic Models: Developing the Structure of the Atom
Transcript Atomic Models: Developing the Structure of the Atom
Developing the Structure
of the Atom
The Greek Model
Around 440 BC, Leucippus originated the
atom concept. He and his pupil, Democritus,
refined and extended it in future years.
Five major points to their atomic idea.
– All matter is composed of atoms, bits of matter too
small to be seen. These atoms CANNOT be
– There is a void (empty space) between atoms
– Atoms are completely solid
– Atoms of different materials are different in size,
shape, and weight
The Dalton Model (1803)
John Dalton of England introduced the
atomic idea to “Modern” chemistry
The basic ideas in Dalton's atomic theory:
– Chemical elements are made of atoms
– The atoms of an element are identical
– Atoms of different elements have different
– Atoms only combine in small, whole number
ratios such as 1:1, 1:2, 2:3 and so on.
Dalton’s Atom, 1807
The atom is a solid indivisible sphere
that can undergo chemical reactions
but is unchanged by the reaction.
The Thomson Model,1903
In 1897, J.J. Thomson discovered
the first subatomic particle, the
electron, using a cathode ray tube.
First model showing an atom that
is divisible (has smaller parts).
Plum Pudding Model
The Gold Foil Experiment
The nucleus was
The Rutherford Model
1. The atom contains a tiny dense center:
– the nucleus is about 1/10 trillionth the volume of
2. The nucleus contains almost the entire mass of the atom.
3. The nucleus is positively charged.
4. The electrons move around in the empty
space (the void?) surrounding the nucleus
The Bohr Model (1913)
Niels Bohr applies
mathematics to Rutherford's
model. This eventually led
to the idea of energy levels
for the electrons.
Led scientists to the
understanding that valence
electrons, those in the
highest energy level,
Hypothesized that the mass of the nucleus
could not be explained by the masses of
Be foil experiment discovered neutrons,
particles that had mass but no charge.
moving around the
outside of the nucleus
Chemical versus Nuclear Reactions
Stable atoms (Not radioactive)
Electrons exchanged between
Radioactive nuclei = unstable,
Nucleus ejects/absorbs neutrons,
Atoms are conserved (unchanged
Atoms change into new
Atomic # or
mass # changes
Reactions are affected by
temperature and pressure
Reactions NOT affected by
temperature or pressure.