Atomic Structure and the Periodic Table

All matter is made up of tiny particles known as atoms.
An atom is composed of subatomic particles: protons, neutrons, and electrons.
Protons and neutrons are in the nucleus at the centre of the atom while electrons orbit this nucleus in energy levels.
Protons have a positive charge, electrons have a negative charge and neutrons have no charge.
Atomic number is the number of protons in the nucleus of an atom. It identifies the element.
Mass number is the total number of protons and neutrons in the nucleus.
Isotopes are atoms of the same element with different numbers of neutrons.
Relative Atomic Mass (Ar) is the weighted mean mass of an atom of an element compared to 1/12 of the mass of an atom of carbon-12.

The Periodic Table organises all known elements in ascending order of atomic number.
A period on the Periodic Table represents a new energy level being used to hold the outer electrons.
A group in this table relates to the number of electrons in the outer shell of an atom. Elements in the same group have similar chemical properties.
Groups 1-2 and Groups 13-18 make up the main groups, while Groups 3-12 are the transition metals.
Metals are generally found on the left of the table, and non-metals on the right.
Noble (Inert) gases are located in Group 18, they are very stable and do not easily react.
Elements located along the staircase line separating metals and non-metals are called metalloids, they show properties of both metals and non-metals.

Elements are substances consisting of one type of atom.
A chemical symbol is used to identify an element. It is a one or two-letter abbreviation of the element’s name.
A compound is a substance made of two or more elements chemically bonded together.
A chemical formula, made up of the chemical symbols of the elements present, represents a compound.
When elements combine, they do so in a fixed ratio to form a compound. This principle is known as the law of constant composition.
Chemical reactions involve the rearrangement of atoms; they can either be exothermic (releasing energy) or endothermic (absorbing energy).
The representing of a chemical reaction with the reactants and products is known as a chemical equation. These should always be balanced to obey the law of conservation of mass.