International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which subatomic particle is essential in determining the identity of an element?

1. Neutrons

2. Protons

3. Electrons

4. Positrons

The correct answer is: Protons

The identity of an element is fundamentally determined by the number of protons it has in its nucleus. Each element on the periodic table is defined by its atomic number, which corresponds to the number of protons. For example, hydrogen has one proton, making it hydrogen; helium has two protons, classifying it as helium. This relationship is crucial because even if an atom gains or loses neutrons (which creates isotopes) or electrons (which can form ions), its identity as a specific element remains unchanged as long as the proton count is constant. In contrast, neutrons and electrons do not define the elemental identity; neutrons affect the isotope but do not alter the elemental type, while electrons are involved in chemical bonding and charge but do not contribute to the overall identity of the element itself. Positrons, being the antimatter counterparts of electrons, are not relevant to the determination of an element's identity. Therefore, protons play a critical role in defining what an element is.