Structure of Atoms

The mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus. Electrons are NOT included because their mass is negligible.

$A=\text{protons}+\text{neutrons}=11+12=23$

The mass number is 23. Note that the number of electrons (11) is not used in this calculation.

We need the mass number, which counts protons + neutrons only.

11 is only the number of protons (the atomic number), not the mass number. This is incorrect.

12 is only the number of neutrons. This is not the mass number on its own.

22 would be protons + electrons (11 + 11), but electrons are not counted in the mass number.

23 = 11 protons + 12 neutrons, which is the correct definition of the mass number.

The atomic number (Z) uniquely identifies an element. No two elements have the same atomic number.

The atomic number equals the number of protons in the nucleus. If you change the number of protons, you change the atomic number, and therefore change the element entirely.

Protons define the element. Changing the number of neutrons only creates an isotope, and changing electrons creates an ion — neither changes the element itself.

We need the particle whose number determines which element an atom is.

Changing the number of electrons changes the charge (making an ion) but does not change which element it is — carbon with one extra electron is still carbon.

Changing the number of neutrons creates an isotope of the same element — still the same element, just with a different mass number.

Nucleon is a collective term for protons and neutrons. It is not a specific particle and is not the best answer here.

Proton is correct — the number of protons is the atomic number, which uniquely defines each element.