Here's why:
* Electrons occupy the outermost shell of an atom (valence shell). These electrons are the ones that interact with other atoms.
* Chemical bonds are formed by the sharing or transfer of electrons between atoms. This sharing or transfer is driven by the desire of atoms to achieve a stable electron configuration, usually by having a full outer shell.
* The number of valence electrons determines an atom's reactivity. Atoms with nearly full or nearly empty outer shells are more reactive, while atoms with a full outer shell are less reactive.
While protons and neutrons are important for defining the identity of an atom (its atomic number and mass), they are located in the nucleus and don't directly participate in chemical bonding.
