JEE Main · 2019 · 12 Apr 2019 · Shift 2 · Classification of Elements and Periodicity

Going down a transition group the radius usually grows, but between the second and third rows the intervening lanthanide contraction cancels that increase. Molybdenum ($4d$) and tungsten ($5d$) lie in the same group $6$, and the poor shielding by the filled $4f$ electrons leaves tungsten almost the same size as molybdenum, so $\mathrm{Mo}$ and $\mathrm{W}$ have nearly equal atomic radii.