Which one among the following most correctly determines the atomic number of an element ?

The atomic number of an element is defined as the number of protons present in the nucleus of its atom [t1][t3]. This number is a fundamental characteristic that uniquely identifies a chemical element and determines its position on the periodic table [t4]. While the atomic number also equals the number of electrons in a neutral, uncharged atom [t1][t2], this relationship does not hold for ions, where the number of electrons changes due to loss or gain [c2][c4]. For instance, a sodium cation (Na+) still possesses 11 protons (atomic number 11) but has only 10 electrons [c4]. Therefore, the number of protons is the only constant and correct determinant of the atomic number [t3]. The number of nucleons (protons plus neutrons) determines the mass number, not the atomic number [t2][t5].

Sources

1. [1] https://ehss.energy.gov/ohre/roadmap/achre/intro_9_3.html
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 25: Thunderstorm > 25.4. Lightning And Thunder > p. 348
3. [3] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > 3.3 HOW DO METALS AND NON-METALS REA ALS REACT? > p. 46
4. [4] https://www.khanacademy.org/science/biology/chemistry--of-life/elements-and-atoms/a/atomic-number-atomic-mass-and-isotopes-article