JEE Main 2025 — 3 April, Morning Shift — Question 3

The number of atoms present in a given mass is given by

$$\text{Number \;of\; atoms} = \frac{\text{mass}}{\text{atomic \;mass}} \times N_A$$

Since, the given mass $(10^{-9}\,\text{g})$ is the same for all elements, the element having the smallest atomic mass will have the highest number of atoms.

Approximate atomic masses:

$$\mathrm{Pb} \approx 207,\quad \mathrm{Po} \approx 209,\quad \mathrm{Pt} \approx 195,\quad \mathrm{Pr} \approx 141$$

Among these, praseodymium $(\mathrm{Pr})$ has the smallest atomic mass.

Thus, the correct answer is option D.