If the total mass of the products of a reaction is less than the mass of the reactants, what must be true?
a nuclear reaction has taken place.
a chemical reaction has taken place.
nuclear fusion has occurred.
nuclear fission has occurred.

My pick would be nuclear fission has occurred.

For chemical reactions, mass will always be conserved.

Nuclear reactions encompass both nuclear fission and nuclear fusion.

For nuclear fission, products of it will be approximately half of the original reactants' mass. The nuclei would be split into a lighter nuclei.

Hence, if mass of reactants > mass of products, I would go for nuclear fission has occurred.

They are all full and don't need any more electrons in there outer shell.