What part of an atom is involved in a chemical reaction?

Valence electrons are involved in a chemical reaction.

Explanation:

A chemical reaction takes place when there is an involvement of valence electrons. Bond formation involves either sharing of valence electrons or transfer of valence electrons. Bond breaking involves the separation of valence electrons.

When valence electrons are shared between the atoms, it leads to the formation of covalent bond.

When transfer of electrons takes place from one atom to another atom, it leads to the formation of ionic bond.

Hence, valence electrons are involved in a chemical reaction.

increased stability in the completion of fluorine's outer shell.