Chemical bonding in covalent molecules

You will remember that the elements are arranged in a systematic way in the periodic table according to their atomic numbers (i.e. number of positively charged protons on the nucleus). When atoms react together and share electrons to form a covalent molecule, they try to obtain a stable electronic structure. They achieve this by forming a similar stable outer electron arrangement to that found in the elements on the right-hand side of the table. These elements are knownas the ‘noble gases’ (helium, neon, argon, etc.).
They have full outer electron shells and are unreactive stable elements.
Helium has two electrons in its outer shell. This inner electron shell is smaller than the rest and is full when it contains two electrons. Neon, with a larger outer electron shell, is full when there are eight electrons in its outer shell, i.e. Ne 2.8, and Argon has two a full outer electron shells, i.e. Ar 2.8.8, etc.

Covalent bonding usually occurs between the elements in the centre of the table, e.g. carbon, and hydrogen or those elements on the right-hand side of the table, e.g. oxygen, nitrogen or chlorine. In the following examples each element achieves the stable outer electron structure of helium, neon or argon.
The most important set of compounds for us to consider is that of carbon. The most simple carbon–hydrogen compound is methane. Carbon (in group 4) and hydrogen will form methane (CH4) by a covalent sharing arrangement.
Carbon has the electronic structure C 2.4 and hydrogen H 1. In its outer shell carbon needs four electrons to achieve the same electronic structure as neon. Hydrogen needs one electron to achieve the same electron arrangement as helium. So if four hydrogen atoms share their electrons with the carbon atom, both atoms will get what they want, namely a full, stable outer electron shell. Methane then becomes a stable molecule (Figure 2.2). (We only need to consider the outside electron shell when electrons form chemical bonds with another atom. It is the outer electrons that ‘bang into’ each other first and are rearranged when a chemical reaction occurs.) Each electron shell now has the stable arrangement of a group 8 element. Simplistically we say it is easier for the carbon to share its electrons with another element, like hydrogen, than to try to give away four electrons and become C4þ ion, or grab four electrons from another element to become a C4 ion.
Shared pairs of electrons (one electron from each atom) are usually shown as a simple straight line. There is no such thing as this stick-like formation, but it is a convenient way of representing a pair of electrons between atoms, one from each atom. Now look at another familiar compound – carbon dioxide. We write carbon dioxide in its abbreviated form as O C O or CO2. There are two pairs of electrons between each carbon and oxygen atom. We call this a double bond.