Question

The sizes of the second and third row transition elements being almost the same. This is due to:

• A. $d$- and $f$-orbitals do not shield the nuclear charge very effectively
• B. Lanthanide contraction
• C. both (a) and (b) are true
• D. none of the above is true

Answer 1

The correct option is C both (a) and (b) are true

The size of neutral atoms of the $d$-block elements gradually decreases from left to right across a row, due to an increase in the effective nuclear charge (Zeff) with increasing atomic number. In addition, the atomic radius increases down a group, just as it does in the $s$ and $p$ blocks. Because of the lanthanide contraction, however, the increase in size between the $3d$ and $4d$metals is much greater than between the $4d$ and $5d$ metals.
Because of the lanthanide contraction, the second- and third-row transition metals are very similar in size.