A brief history of atomic structureA brief history of atomic structure

Advanced Higher ChemistryAdvanced Higher Chemistry

Unit 1Unit 1

Atomic Theory of matterAtomic Theory of matter 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 electronsDiscovery of electrons

• British physicist J. J. Thompson began experiments on the discharge of electricity through gases and discovered the electron.

ANODE CATHODEVACCUM

Discovery of the nucleusDiscovery of the nucleus In 1911 British Physicist In 1911 British Physicist Ernst RutherfordErnst Rutherford

performed the alpha particle scattering performed the alpha particle scattering experimentexperiment

Gold foil was bombarded with alpha radiation Gold foil was bombarded with alpha radiation from a radioactive substancefrom a radioactive substance

Concluded mass of an atom was concentrated Concluded mass of an atom was concentrated in a centre, which was also positively chargedin a centre, which was also positively charged

An early structure for the atomAn early structure for the atom

Quantization Quantization of energyof 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 Based on the theories of Einstein and Planck, Danish scientist and Planck, Danish scientist Niels BohrNiels Bohr proposed a model of proposed a model of electronic structure to explain the electronic structure to explain the results of results of atomic spectraatomic spectra..

Hot gases emit light, in the form of a line Hot gases emit light, in the form of a line spectrumspectrum

HydrogenLink to Periodic Table of Spectra

ENERGY LEVELS (SHELLS)

1 2 3

1

2

34

Ene r gy

1.1. An electron can only have specific energy values in an atom, An electron can only have specific energy values in an atom, ENERGY ENERGY LEVELSLEVELS

2.2. An electron can change energy only by going from one energy level to An electron can change energy only by going from one energy level to anotheranother

Bohr’sBohr’s model of the atom model of the atom

Explaining emission spectraExplaining emission spectra

E1

E2

An electron in an excited state can fall down an energy An electron in an excited state can fall down an energy level, emitting a photonlevel, emitting a photon

Energy of emitted photon = hEnergy of emitted photon = hff = E = Eii - E - Eff

This will produce an emission spectrumThis will produce an emission spectrum

One photon, hf

Transitions in a Hydrogen AtomTransitions in a Hydrogen Atom