Atoms are the smallest particle into which a sample of an element can be divided without losing the chemical properties of an element.

Molecules are a group of atoms of the same/different elements joined by covalent bonds.

Ions are a group of atoms which carries a permanent electric charge.

Isotopes are atoms of the same element with different numbers of neutrons. Thus they differ in mass, physical properties (M.P., B.P. and rate of diffusion). They do not differ chemically.

Mass Number, A (Nucleon Number), is the total number of protons and neutrons in the nucleus of an atom of the element.

Proton Number, Z (Atomic Number), is the number of protons of an atom of an element. Since atoms are electrically neutral, the proton number is also equal to the number of electrons of the atom of the element.

Radioactivity These substances have atoms with unstable nuclei. When an unstable nucleus splits up (disintegrates) it emits radiation. A different atom with a different number of protons is formed.

Allotropes are different structural forms of the same element in the same physical state.
e.g. 0₂ and 0₃ (Ozone) are allotropes of oxygen.

Electrolysis is the chemical change (decomposition) brought about by an electric current.

Exothermic is the term given to a reaction which gives of heat.

Endothermic is the term given to a reaction which takes in heat energy from its surroundings.

Oxidation Is Loss of electrons and the gain of oxygen. (OIL)

Reduction Is Gain of electrons and the loss of oxygen. (RIG)

Amphoteric is the term given when something reacts with both Acids and Alkalis to make salts.

e.g. Al(OH)₃ , Al₂O₃

Precipitate is the term given to a solid formed by the reaction of two solution.

Solvent is the term given to a liquid which will dissolve in a range of solids or solutes.

A Catalyst alters the rate of a chemical reaction (almost always speeds it up) without being consumed (used up). It can be recovered unchanged chemically and in mass at the end of the reaction. Physical change (e.g. particle size) may occur.

'D H is negative' means that the HEAT change for the reaction is negative : the reaction gives out heat to the surroundings : more energy is given out than is needed to start the reaction by breaking bonds.

A Continuous Process is a process where the unused reactants are recirculated and reused.

An Ore is a naturally occurring concentration of metal compounds in the earth form which it is economical to extract the metal. Often the ore is a metal oxide or a compound which can easily be changed into a metal oxide.

An Alloy is a mixture of metals or of 1 or more metals with the non-metal carbon, which has properties suitable for a particular purpose.

Electric Current is the movement of electrons in a metal or of ions in an electrolyte

Electrolysis is chemical change - often decomposition to the elements, brought about by electric current.