Which one among the following statements with respect to the atomic number of an atom is correct?

The atomic number of an element is defined as the number of protons present in the nucleus of its atom. This number is unique to each element and determines its chemical identity. In a neutral atom, the atomic number also equals the number of electrons [1]. However, the number of electrons can change when an atom becomes an ion, whereas the number of protons remains constant. The sum of protons and neutrons is referred to as the mass number, not the atomic number [2]. Neutrons can vary in number among atoms of the same element, forming isotopes, which further distinguishes them from the fixed atomic number [2]. Therefore, the only statement that consistently defines the atomic number across all states of an element is that it equals the number of protons.

Sources

1. [1] https://ehss.energy.gov/ohre/roadmap/achre/intro_9_3.html
2. [2] https://www.khanacademy.org/science/biology/history-of-life-on-earth/radiometric-dating/a/atomic-number-atomic-mass-and-isotopes-article