Concept:Chemical properties of an atom are determined by its valence electrons, which are the electrons in the outermost shell.
Explanation:An atom contains protons, neutrons, and electrons.
Chemical properties describe how an atom reacts with other atoms.
These reactions involve the sharing, gaining, or losing of electrons.
Only the electrons, especially the valence electrons, participate in chemical bonding.
Protons and neutrons are in the nucleus and do not directly affect chemical reactivity.
Therefore, the species that determines chemical properties is the electron.
Answer:Option A — Electron.