Here's why:

* Valence electrons are the electrons in the outermost energy level of an atom. They are the ones farthest from the nucleus and therefore have the weakest attraction to the atom's core.

* Chemical bonds form when atoms share or transfer valence electrons to achieve a stable electron configuration. This stability arises from having a full outer shell of electrons, like the noble gases.

Examples:

* Covalent bonding: Atoms share valence electrons to form a stable molecule. For example, in water (H₂O), hydrogen atoms share their single valence electron with oxygen, which has six valence electrons. This sharing results in a stable molecule with two shared pairs of electrons.

* Ionic bonding: One atom transfers valence electrons to another atom, creating ions with opposite charges that attract each other. For example, in sodium chloride (NaCl), sodium (Na) loses its single valence electron to chlorine (Cl), becoming a positively charged ion (Na+). Chlorine gains the electron, becoming a negatively charged ion (Cl-). The opposite charges attract and form an ionic bond.

In summary, it's the valence electrons, those in the outermost energy level, that play a key role in forming chemical bonds.