Atomic radii is a useful property for determining many aspects of chemistry of elements such as physical and chemical properties. The periodic table greatly assists in finding a number of trends. The transition metals belong to group $3 - 12$ and are generally characterised by partially filled d-orbitals in the free elements or their cations. The atomic radii of transition metals show a small decrease from left to right across a period, then remain nearly constant, and finally, slight increase towards the end of the series. The trend is a result of interplay between the increasing nuclear charge and the increasing shielding effect from the filling of d-orbitals. Initially the nuclear charge dominates, leading to decrease in size, but as more d-electrons are added, their shielding becomes more effective, balancing the increased nuclear charge and causing the radii to become constant. The increase in size between $3\text{d}$ and $4\text{d}$ metals is much greater than between the $4\text{d}$ and $5\text{d}$ metals. The atomic radii of $2^{\text{nd}}$ and $3^{\text{rd}}$ series are almost the same due to lanthanoid contraction. Increase in density is also observed on moving from left to right in a period. (a) Give reasons for the following : (i) The second and third series of transition metals resemble each other much more than they resemble the first series ($3\text{d}$). (ii) Transition metals show variable oxidation states.

Year	Paper Code	Set	Position
CBSE 2026	Series 56-7	Set 1	Question 29
CBSE 2026	Series 56-7	Set 2	Question 29
CBSE 2026	Series 56-7	Set 3	Question 30

• Step 1: Stating the fundamental formula / law statement (+1 Mark)
• Step 2: Intermediate mathematical substitution or proof steps (+1 Mark)
• Step 3: Correct final value or conclusion with mandatory SI units (+1 Mark)