A brief history of atomic structure Advanced Higher Chemistry Unit 1

Atomic Theory of matter British Chemist John Dalton (1766-1844) 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.

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.

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

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. 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

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

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

Bohr’s model of the atom 4 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

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

Transitions in a Hydrogen Atom