Hi,
The answer is very simple and of basic nature, let us get into the basics.
We have studied since very long that atoms form chemical bonds with other atoms because they want to get stable. They can get stable through octet or duplet rule that means that atoms needs to have two (in the shell nearest to nucleus) and eight electrons in the outer most shell.
So, if an atom already has enough number of electrons that it is obeying octet or duplet rule, it means that the atom is stable. It will tend not to form a chemical bond with other atom by sharing, losing or gaining electrons. Now, the electrons in outer most shell are called valence electrons and they are distributed in sub-energy levels, s and p respectively. S orbital can accommodate only two electrons, while P can accommodate 6 electrons, d can accommodate 10 electrons while f orbital can accommodate 14 electrons. So, we can say that the bond formation is the most important property of an atom and it is strongly effected by number of electrons in an energy level.
In addition, number of electrons in outer most shell also effect many other properties of an atom like shielding or screening effect, metallic or non-metallic nature that also effects the chemical bonding of atoms. Since, metallic nature of an atom is due to its ability to lose outermost shell's electrons. Therefore, the whole phenomenon of chemical bonding is linked with number of electrons in outer most shell.
Hope it helps!
