What is the atomic number of an element?

The atomic number of an element is defined as the number of protons present in the nucleus of an atom of that element. This fundamental property not only identifies the element but also determines its position on the periodic table. For example, hydrogen, with an atomic number of 1, has one proton, while carbon has an atomic number of 6, indicating it has six protons.

The number of protons in the nucleus is crucial because it defines the element itself; changing the number of protons would change the identity of the element. Furthermore, in a neutral atom, the number of protons also equals the number of electrons, ensuring electrical balance.

Other choices mention aspects that do not specify what the atomic number is. The number of neutrons refers to a different characteristic of isotopes, while the total number of electrons only pertains to neutral atoms and can vary if the atom is ionized. The total mass of an atom involves both protons and neutrons, but it does not accurately define the atomic number, which pertains exclusively to protons.