1. A brief history of atomic structure
Advanced Higher Chemistry
Unit 1

2. Atomic Theory of matter
British Chemist John Dalton ( ) laid the foundations of modern atomic theory by proposing the idea of atoms.
Dalton’s theory had 4 postulates:
All matter is composed of indivisible atoms. These retain their identity during chemical reactions;
An element is a type of matter composed of only one atom;
A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. These occur as simple as ratios;
A chemical reaction consists of the rearrangement of the atoms present in the reacting substance to give a new chemical combination. Atoms cannot be created, destroyed or broken into into smaller particles in any chemical reaction
Dalton thought atoms were solid entities
At the end of the 19th century chemists had discovered that atoms themselves were made up of smaller particles.

3. Discovery of electrons
Tube has been evacuated
High-voltage current passed through
Beam is formed by cathode rays passed through a small hole
A television works by the same principle of deflecting cathode rays in a magnetic field
Thompson was awarded the Noble prize in physics in 1906
ANODE
CATHODE
VACCUM
British physicist J. J. Thompson began experiments on the discharge of electricity through gases and discovered the electron.

4. Discovery of the nucleus
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
The experiments on which Rutherford based his theories were actually carried out by Geiger and Marsden

5. An early structure for the atom

6. 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 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.
Solids glow red at 750, white at 1200 degrees celcius. More blue and yellow light is emitted as the solid gets towards 1200.
If an atom of energy 3hv changes to 2hv, then hv of energy must be released, ie it is quantised
Einstein used the idea of photons to explain the photoelectric effect

7. 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.
In Rutherford’s model, an electron would be constantly loosing energy and would spiral into the nucleus

8. Link to Periodic Table of Spectra
Hot gases emit light, in the form of a line spectrum
Hydrogen
Link to Periodic Table of Spectra

9. Bohr’s model of the atom4
Ene r gy 3 1 2 3 2 1
ENERGY LEVELS (SHELLS)
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

10. Explaining emission spectra
One photon, hf
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

11. Transitions in a Hydrogen Atom