Atoms

An atom is the smallest particle of an element that has the properties of that element.

There have been various models that have been proposed to describe the structure of the atom.

Billiard ball model - John Dalton
Plum pudding model - J.J. Thomson
Nuclear model - E. Rutherford
Shell model of the atom - N. Bohr
The quantum model of the atom - various scientists.

The atom consists of three subatomic particles. The proton, the neutron and the electron.

In a chemical reaction all the action takes place in the outer or valence shell of electrons.

Molecules

What is a molecule? A molecule is a group of atoms that joined together by a covalent bond.

Models of the atom

MODELS OF THE ATOM - SUMMARY

Scientists	*Model*	Description	Evidence

		The atom is the smallest particle of an element. The atom is a solid, indestructible unit. Atoms of different elements have different masses.	Law of Constant Proportions. (% mass composition) Law of Multiple Proportions. E.g. Two compounds of nitrogen oxide. The ratio of oxygen between both compounds is 2:3.
J. Dalton	*The Billiard Ball model (1808)*

		The ‘pudding’ is the positive material of an atom. The embedded ‘raisins’ are negative electrons.	Cathode rays (negative particles or electrons) Canal rays (positive particles with large masses)
J. J. Thomson	*The Plum Pudding model (1903)*

		The mass and positive charge of an atom is concentrated in a small core called the nucleus. Negative electrons orbit the nucleus. The atom is mostly made up of empty space. (Ratio of nucleus diameter to outer electron orbit = 1 : 50,000)	The scattering of alpha particles through thin gold foil. A small percentage of particles were deflected at large angles, some even returning. ‘it is like a cannonball rebounding off a piece of paper’
Ernest Rutherford	*The Nuclear model (1911)*	Protons & later neutrons are identified.

		Electrons are arranged around the nucleus in discrete energy levels or shells.	Explains emission spectra (flame test) Explains patterns in the successive ionisation energies of an element.
Neils Bohr	*The Shell model (1923)*

s-orbital	p-orbitals	Electrons exist in orbitals. i.e. an area surrounding the nucleus that has a 90% probability of containing an electron.	Better explains anomalies in successive ionisation energies and emission spectra. E.g. double yellow lines for sodium.
	d-orbitals	Orbitals differ in shape(s, p, d, & f orbitals) and size(dependent upon energy level). No orbital may contain more than two electrons. The electrons in an orbital spin in opposite directions.
Various Scientists Heisenburg, Schrodinger, de Broglie, Pauli	*The Quantum model (1935)*