This section draws together ideas from many sections of the syllabus and develops further the Periodic Table to give an indication of the ways chemists use theories to rationalise their knowledge. Ideas are developed about atoms, molecules and ions by consideration of their electronic structure.

An atom has a core made up of neutrons and protons, which is called the nucleus. Surrounding the nucleus are shells which hold electrons. The first shell can hold a maximum of two electrons. The second and third shells can hold a maximum of eight electrons. The number of shells an atom has depends on the number of electrons it holds (usually the same as the number of protons in the nucleus) as each shell can only hold a limited number of electrons. Neutrons have no charge, but protons have a positive charge and electrons have a negative charge. Thus, when the number of protons and electrons is the same the atom is neutral.

The periodic table is an organized display of all the known elements. The position of each atom in the table is determined by the number of protons (which is usually also the number of electrons) they hold. 

As a result of this organization of the elements, certain features have developed in the table. Firstly, the columns in the table are called groups. Which group each element belongs to determines the number of electrons it holds in the outer shell. Elements in the same group generally have similar properties, for example the noble gases which belong to group 8.  The noble gases have a full outer shell and so they are completely stable. Another example is alkali metals, which belongs to group 1. Alkali metals have one extra electron in their outer shells, so they are reactive.  For metals, their reactivity increases as you go down the group. For non-metals, their reactivity increases as you go up the group.

The rows of the periodic table signify periods. This tells us about the element how many electron shells each atom has. For example, period 1 has one shell (eg. hydrogen and helium) and period 2 has two shells (eg. lithium, magnesium and carbon).

To form molecules atoms have to bond together. One type of bond is called a covalent bond. When atoms share a covalent bond, they share one or more electrons. This happens between non-metals when atoms only need one or two electrons to achieve a full outer shell. 

Another type of bonding is called ionic bonding. This bonding is between metals and non-metals.Ther There is a strong bond between them. When this bonding occurs, the atoms transfer electrons to obtain a full outer shell, for example sodium tries to lose two electrons and chlorine tries to gain two electrons to obtain a full outer shell. When ionic bonding takes place, there would be a reaction.  When an element obtained a full outer shell it becomes an ion.

 Isotopes are different forms of the same element. They have the same number of protons and electrons but different numbers of neutrons.