Q1(j). Mo and W radii

Answer in brief

Lanthanide contraction nearly offsets the expected increase in radius from the 4d to the 5d series.

Key points

• Between the 4d and 5d transition series, the 4f shell is filled. The added 4f electrons shield the increasing nuclear charge relatively poorly.
• The resulting increase in effective nuclear attraction contracts the outer electron cloud across the lanthanide series.
• Tungsten therefore has almost the same radius as molybdenum despite its additional shell and electrons. Similar sizes help explain their closely related coordination chemistry.