Dr. Ramadevi Pulipaka

Suppose A-B are two atoms bonded to each other by a bond. Now there are mainly two different ways of forming this bond:-

1. Ionic bond: either A gives away/donates its valence electrons (outermost electrons) to B, or vice versa to attain stable electronic configuration- this will result in forming an ionic bond between A and B.
2. Covalent bond: A or B can neither donate nor accept electrons to attain a stable configuration. Another option is that they both share their valence electrons. This sharing of electrons forms a covalent bond. 

Now in the case of Carbon:- it has 4 electrons in its valence shell. It needs four more electrons to complete its octet. It may lose these 4 electrons or gain four more electrons to get a stable electronic configuration. But both these processes are high energy processes. Hence C would instead share its 4 electrons with four other atoms forming four covalent bonds. In doing so, C has now 8 electrons in its valence shell (4 are its own and four from sharing with four other atoms), attaining a stable octet.