An atom is the smallest component of an element. An element is a substance that is made entirely of one type of atom e.g. Lithium (Li). Elements have chemical symbols which is a one/two letter code for an element. A compound is a substance made up of two or more atoms which are chemically bonded together e.g. water (H20). Compounds also have chemical formulas made up from the chemical symbols of the elements involved.

the atom consists of a central nucleus, which is tiny compared to the atom as a whole. The nucleus contains protons and neutrons. Electrons orbit the nucleus in energy levels, also known as shells. The nuclear radius is around 1 x 10-14 m whilst the radius of the atom is about 1 x 10-10 m so the nucleus is less than 1/10,000 of the size of the atom.

The relative electronic charge of a proton is +1, neutron is 0 and electron is -1. The relative mass of a proton is 1, neutron is 1 and electron is approx. 1/1840.