Question

Atomic number of an element is equal to the:

• A. Number of electrons
• B. Number of neutrons
• C. Number of protons
• D. Total number of protons and neutrons

Hint:

Atomic number is defined by the nucleus, specifically its positive charge.

Answer 1

The Correct Option is C

The task is to identify what atomic number actually measures.

1. Electron count changes when an atom forms a positive or negative ion, so it cannot be the fixed definition of atomic number.
2. Neutron count can differ between isotopes of the same element, for example carbon-12 and carbon-14, yet both have the same atomic number, so neutrons are not the definition.
3. Total protons plus neutrons is called the mass number, a different quantity from atomic number.
4. Only the proton count in the nucleus stays constant for a given element and uniquely identifies it on the periodic table, this is exactly what atomic number means.

So the atomic number equals the number of protons, option C.