The nucleus of an atom is written in a short notation that uses its mass number \(A\) and atomic number \(Z\). A lot of information about the nucleus and the atom can be drawn from this notation. Consider the nucleus \({}^{A}_{Z}\mathrm{X}\). Based on this information, answer the question that follows.
What is the number of protons in this nucleus?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(Z\)
The atomic number \( Z \) gives the positive charge of the nucleus in units of \( e \). Each proton carries one such unit, so \( Z \) is also the number of protons. The mass number \( A \) counts all the nucleons, and \( A-Z \) is the number of neutrons.
NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “The number of protons in the nucleus of the atom is, therefore, exactly Z, the atomic number.”