The nucleus of gold is written as \({}^{197}_{79}\mathrm{Au}\). Taking \(e=1.6\times10^{-19}\ \mathrm{C}\), the charge on this nucleus is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(1.26\times10^{-17}\ \mathrm{C}\)
Only the protons give a nucleus its charge, because neutrons carry no charge. In the symbol \( {}^{A}_{Z}\mathrm{X} \), the lower number is the atomic number \( Z \). So the gold nucleus has 79 protons, and each proton carries a charge \( +e \).
\( \begin{aligned} q &= Ze \\ &= 79\times1.6\times10^{-19} \\ &= 1.26\times10^{-17}\ \mathrm{C} \end{aligned} \)
From the above, the nuclear charge is \( +Ze \). Hence, the gold nucleus carries a positive charge of \( 1.26\times10^{-17}\ \mathrm{C} \).
NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… The total charge of the atomic electrons is thus (–Ze), and since the atom is neutral, the charge of the nucleus is (+Ze). The number of protons in the nucleus of the atom is, therefore, exactly Z, the atomic number.”
Atomic Masses and Composition of Nucleus – “… For example, the nucleus of gold is denoted by 19779Au. It contains 197 nucleons, of which 79 are protons and the rest 118 are neutrons.”