A brief history of atomic structure Advanced Higher

A brief history of atomic structure Advanced Higher Chemistry Unit 1

Atomic Theory of matter l British Chemist John Dalton (1766 -1844) laid the foundations of modern atomic theory by proposing the idea of atoms. l At the end of the 19 th 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 l l l 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 l When heated, solids emit light. l Max Planck discovered that the atoms of a solid vibrate with energy of a definite frequency, f, depending on the solid. l 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, . . . l This means that the energies of a vibrating atom must take the form hf, 2 hf, 3 hf etc. l 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.

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

l 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 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 E n e r g y

Explaining emission spectra One photon, hf E 1 E 2 l l l 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