Main group elements form covalent bonds by sharing electron pairs. Unshared or lone pairs around the central atom contribute to the octet of electrons. Table 7.3 summarizes the trends for the elements in the second row.
You may wonder why elements in group 5A do not form five bonds, since the elements in group 3A form three bonds and the elements in group 4A form four bonds. Let's consider ammonia, NH3. Nitrogen has five valence electrons,
and the ammonia molecule has one non-bonding pair and three bonds.
Thus, ammonia has an octet of electrons by forming three bonds. Elements in Group 6 and 7 similarly achieve an octet configuration by sharing pairs and lone pairs.