Final answer:
Atoms of main-group elements form bonds so that the number of electrons in the outermost energy levels of each atom is determined by their need to reach an octet. Group 14 elements, like carbon, require four more electrons to reach an octet, so they form four covalent bonds.
Step-by-step explanation:
The number of bonds that an atom can form can often be predicted from the number of electrons needed to reach an octet (eight valence electrons); this is especially true of the nonmetals of the second period of the periodic table (C, N, O, and F). Each atom of a group 14 element (such as carbon) has four electrons in its outermost shell and therefore requires four more electrons to reach an octet. These four electrons can be gained by forming four covalent bonds.