Number of protons
Explanation
The fundamental identity of a chemical element is defined by the composition of its nucleus. In modern atomic theory, specific terms are used to quantify the subatomic particles (protons, neutrons, and electrons) that constitute an atom.
Analysis of Atomic Parameters
- Atomic Number ($Z$): This is defined as the total number of protons present in the nucleus of an atom. The number of protons is unique to each element and determines its chemical identity and position in the Periodic Table. For example, all atoms with 6 protons are Carbon atoms.
- Mass Number ($A$): This represents the total number of nucleons, which is the sum of protons and neutrons ($p + n$). This value determines the atomic mass but not the specific identity of the element.
- Neutrons: The number of neutrons can vary among atoms of the same element; these variations are known as isotopes. Therefore, neutrons do not define the atomic number.
- Electrons: In a neutral atom, the number of electrons equals the number of protons. However, atoms can lose or gain electrons to form ions without changing their elemental identity. Thus, electrons are not used to define the atomic number.
Key Takeaway:
The Atomic Number ($Z$) is strictly equal to the number of protons in the nucleus. It is the fundamental property that distinguishes one element from another.