Question

Give another name for the smallest particle of an element capable of taking part in a chemical reaction.

Answer 1

atom is the smallest particle of an element which is capable of taking part in a reaction

Answer 2

According to modern atomic theory , an atom is the smallest particle that can take part in a chemical reaction. But during the formation of hydronium ion ,H+ ion reacts with H2O to form H3O+. Here, the H+ion which is smaller than the hydrogen ion takes part in the chemical reaction.
But , generally, atoms can be called as the smallest particle of an element capable of taking part in a chemical reaction.

Hope that helps you.....