When determining an element’s chemical properties, the most important subatomic particle to look at is

Electrons

Explanation:

When determining the chemical properties of elements, the most important subatomic particle to consider is the electrons.

Electrons are negatively charged particles that orbits an atom. They occupy the extranuclear space of an atom.

These electrons determines the chemical properties of each atom. Most especially, the valence ones. All elements wants to resemble the noble gases, so they alter their number of electrons in order to attain this stable state.