Read the following passage and answer the questions.
A nucleus is written as \({}^{A}_{Z}\mathrm{X}\), where X is the chemical symbol of the element. Here Z is the atomic number and A is the mass number. The nucleus contains Z protons and (A − Z) neutrons. Protons and neutrons are together called nucleons. For example, the aluminium nucleus \({}^{27}_{13}\mathrm{Al}\) has 13 protons and 14 neutrons. Experiments show that a nucleus of mass number A has a radius \(R=R_0A^{1/3}\), where \(R_0=1.2\ \mathrm{fm}\) and 1 fm = \(10^{-15}\ \mathrm{m}\). The protons in a nucleus are positively charged, so they repel each other. Still, the nucleus does not fly apart. The nucleons are held together by a force of a different kind, called the nuclear force. A rough graph of the potential energy of a pair of nucleons against their separation r has a minimum at a separation \(r_0\) of about 0.8 fm.
What is the charge outside the nucleus?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( -Ze \)
The nucleus of \( {}^{A}_{Z}\mathrm{X} \) holds \( Z \) protons. Since the atom is electrically neutral, it has \( Z \) electrons. All of them are outside the nucleus, and each carries a charge \( -e \).
The total charge outside the nucleus is:
\( \begin{aligned} q &= Z\times(-e) \\ &= -Ze \end{aligned} \)
The mass number \( A \) counts the protons and neutrons inside the nucleus, so it does not enter the charge of the electrons outside.
NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… All the electrons of an atom are outside the nucleus. We know that the number of these electrons outside the nucleus of the atom is Z, the atomic number. The total charge of the atomic electrons is thus (–Ze), and since the atom is neutral, the charge of the nucleus is (+Ze).”