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

Which particle determines an atom's identity?

1. Protons

2. Electrons

3. Neutrons

4. Nuclei

The correct answer is: Protons

The particle that determines an atom's identity is protons. Each element on the periodic table is defined by the number of protons it contains in its nucleus, known as the atomic number. For instance, an atom with one proton is hydrogen, while an atom with six protons is carbon. This unique number of protons gives each element its chemical properties and distinguishes it from other elements. Electrons, although they play a crucial role in chemical bonding and the reactivity of an atom, do not define the atom's identity. Similarly, neutrons contribute to the atomic mass and stability of an atom but do not affect which element it is; isotopes of the same element can have different numbers of neutrons. The term 'nuclei' refers to the central part of an atom that contains protons and neutrons, but it does not specify which element the atom represents. Thus, protons are the definitive particles that identify an atom.