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A brief history of atomic structure
Advanced Higher Chemistry
Unit 1
Atomic Theory of matter
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British Chemist John Dalton (1766-1844) laid the foundations of
modern atomic theory by proposing the idea of atoms.

At the end of the 19th century chemists had discovered that atoms
themselves were made up of smaller particles.
Discovery of electrons
ANODE
VACCUM
CATHODE
• British physicist J. J. Thompson began experiments on the discharge
of electricity through gases and discovered the electron.
Discovery of the nucleus
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In 1911 British Physicist Ernst Rutherford
performed the alpha particle scattering
experiment
Gold foil was bombarded with alpha radiation
from a radioactive substance
Concluded mass of an atom was concentrated
in a centre, which was also positively charged
An early structure for the atom
Quantization of energy

When heated, solids emit light.

Max Planck discovered that the atoms of a
solid vibrate with energy of a definite
frequency, f, depending on the solid.

The energy of this light is given by E = nhf,
where h is Planck’s constant, 6.63 x 10-34
and n = 1, 2, 3,...

This means that the energies of a vibrating
atom must take the form hf, 2hf, 3hf etc.

In this way thee energy said to be quantised
– it cannot change smoothly from one value
to another, only in whole units at a time.

In 1900, Albert Einstein had
proposed that instead of being
thought of as a continuous wave,
light could also be thought of as
particles – “packets” of light –
PHOTONS.

Based on the theories of Einstein
and Planck, Danish scientist
Niels Bohr proposed a model of
electronic structure to explain the
results of atomic spectra.

Hot gases emit light, in the form of a line
spectrum
Link to Periodic Table of Spectra
Hydrogen
Bohr’s model of the atom
4
3
1
2
3
2
E
n
e
r
g
y
1
ENERGY LEVELS (SHELLS)
1.
2.
An electron can only have specific energy values in an atom, ENERGY
LEVELS
An electron can change energy only by going from one energy level to
another
Explaining emission spectra
One photon, hf
E1
E2
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An electron in an excited state can fall down an
energy level, emitting a photon
Energy of emitted photon = hf = Ei - Ef
This will produce an emission spectrum
Transitions in a Hydrogen Atom