Answer : Electrons.
Explanation : The subatomic particle which is responsible for combining the atoms together to form a new substance is ELECTRON.
Electrons are negatively charged tiny particles which are found outside the nucleus and is one of the three subatomic particles. These are responsible for formation of new substance through transfer or sharing of electrons between the atoms of the elements. Mainly the outermost revolving electrons are the ones which undergoes transformation and forms new substance. It influences many physical and chemical properties of the substances.