Final answer:
The electron is the subatomic particle that determines an element's chemical properties, as it is involved in chemical reactions and bonding.
Step-by-step explanation:
The subatomic particle that determines an element's chemical properties is the electron. Electrons are negatively charged subatomic particles that reside in the orbitals around the nucleus of an atom. The number of protons in the nucleus gives an atom its identity and is the atomic number, but it is the electrons, and particularly the electrons in the outermost shell, that are involved in chemical reactions and bonding, thus determining the chemical properties of the element.
While nucleons, which include both protons and neutrons, contribute to the mass number of an atom, they do not directly determine the chemical properties. Every atom has a unique number of protons, known as the atomic number (denoted by the letter Z), which is responsible for the identity of the element. However, it's the arrangement and number of electrons that dictate how an element will interact with others.