JEE Main · 2026 · 04 Apr 2026 · Shift 2 · Atomic Structure

The Bohr radius of the $n$th orbit is $r_n=0.529\frac{n^2}{Z}\text{A˚}$, so two species share a radius only when they share the value of $n^2/Z$. For H in the first orbit $n^2/Z=1^2/1=1$; for $\mathrm{H}{\mathrm{e}}^{+}$ first orbit $1^2/2=0.5$; for $\mathrm{H}{\mathrm{e}}^{+}$ second orbit $2^2/2=2$; for $\mathrm{L}{\mathrm{i}}^{2+}$ first orbit $1^2/3=0.33$; and for $\mathrm{B}{\mathrm{e}}^{3+}$ second orbit $2^2/4=1$. The two entries with $n^2/Z=1$ are H (first orbit) and $\mathrm{B}{\mathrm{e}}^{3+}$ (second orbit), each of radius $0.529\text{A˚}$. The species having identical Bohr radii are H (first orbit) and $\mathrm{B}{\mathrm{e}}^{3+}$ (second orbit).