The part of the atom that allows one atom to react with another is its outermost electrons , called valence electrons.

These valence electrons are the ones that:

  • Get shared between atoms in covalent bonds.
  • Get transferred from one atom to another in ionic bonds.
  • Move freely in metals, forming a “sea of electrons” in metallic bonding.

Because chemical reactions are really about making and breaking chemical bonds, and bonds involve these outer electrons, valence electrons are the key part of the atom that controls how, and how strongly, atoms react with each other.