An atom consists of Protons, Neutrons and Electrons. The Protons (+ve charged particles) and Neutrons form the nucleus of the atom around which the Electrons (- ve charged particles) rotate. The number of protons in the nucleus is referred as its Atomic number. The Mass number is the number of protons and neutrons of an atom. The atom is the smallest particle of an element that can enter into a chemical combination with another element. A molecule (group of atoms) is the smallest particle of an element or compound that can exist independently.
Atoms always maintain a balanced electrical charge. Hence, the number of negatively charged electrons surrounding the nucleus is equal to the number of protons. It is also known that electrons are present with different energies and it is convenient to consider these electrons surrounding the nucleus in energy “shells.” The maximum number of electrons that are in any shell is determined by the number of shells. The first shell which is closest to the nucleus marked ‘K’, the second ‘L’, third ‘M’, and so on. The maximum number of electrons that can be in a shell is 2 in K, 8 in L, 18 in M, 32 in N, 18 in O and 8 in P.
All chemical bonds involve electrons. Atoms will stay close together if they have a shared interest in one or more electrons. Atoms are at their most stable when they have no partially-filled electron shells. If an atom has only a few electrons in a shell, it will tend to lose them to empty the shell. These elements are metals. When metal atoms bond, a metallic bond occurs. When an atom has a nearly full electron shell, it will try to find electrons from another atom so that it can fill its outer shell. These elements are usually described as non metals. For example, potassium, with an atomic number of 19, has two electrons in the inner shell, eight in the second and third shell, one in the outer shell. Potassium (2,8,8,1) is an unstable atom due to the extra electron in its outer shell and will try to lose this electron. Therefore, it commonly bonds with Chlorine (2,8,7) which has 7 electrons in its outer shell to form KCl.
The bond between two non metal atoms is usually a covalent bond. Where metal and non metal atom comes together, an ionic bond occurs. There are also other, less common, types of bond but the details are beyond the scope of this material. On the next few pages, different types of bonding in the solids will be discussed.