Case: Read the passage and answer the question.
In science, there are notations and their interpretations. There is a notation to denote the nucleus of an atom in terms of its mass number A and atomic number Z. A lot of information about the nucleus and the atom can be drawn from it. Consider an element \( {}^{A}_{Z}\mathrm{X} \). Based on this information, answer the question that follows.
What is the number of neutrons in the nucleus?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( A-Z \)
In \( {}^{A}_{Z}\mathrm{X} \), the atomic number \( Z \) is the number of protons and the mass number \( A \) is the total number of protons and neutrons. With \( N \) neutrons:
\( A=Z+N \)
\( \Rightarrow\; N=A-Z \)
Therefore, the nucleus contains \( A-Z \) neutrons.
NCERT (2026-27):
Summary – “4. The atomic number Z is the number of protons in the atomic nucleus of an element. The mass number A is the total number of protons and neutrons in the atomic nucleus; A = Z+N; Here N denotes the number of neutrons in the nucleus.”