An atom's tendency is to become stable. what does this mean?

This means an atom is always searching to fill its outer shell of electrons. For example, oxygen has 8 electrons, 2 in the first shell, and 6 in the next. Oxygen wants to gain 2 more electrons in order to become stable. It can do this through ionic or covalent bonding. An atom that is stable has a full outer shell. Noble gases are stable as they have full outer shells and this therefore means they are unreactive because they don't need to make bonds to become stable :)