Question

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 3d and 4d metals is much greater than between the 4d and 5d metals. The atomic radii of 2^nd and 3^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.

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 (3d).

(ii) Transition metals show variable oxidation states.

Hint:

Part (i): the 4f electrons shield poorly, so lanthanoid contraction makes 4d and 5d radii equal. Part (ii): (n-1)d and ns energies are close, so both sets of electrons can bond.

Answer 1

Step 1: Understand what is asked:
We must give one reason for each statement. The passage already hints at lanthanoid contraction for part (i).

Step 2: Reason for part (i):
Between the 4d series (Zr) and the 5d series (Hf), fourteen electrons fill the 4f subshell.
The 4f orbitals have diffuse shapes, so they shield the outer electrons very poorly.
The nuclear charge keeps rising, and the poorly shielded outer electrons are pulled in. This steady shrinkage is called lanthanoid contraction.
It almost cancels the normal increase in size from the 4d to the 5d series.
So 4d and 5d metals such as Zr and Hf have nearly the same radii. Their ionic sizes and chemistry are also alike.
The 3d to 4d jump has no such 4f filling in between, so the 3d series differs much more.

Step 3: Reason for part (ii):
In transition elements the (n-1)d and ns subshells have very close energies.
So electrons from both subshells can take part in bonding.
Different numbers of electrons can be lost or shared, which gives different oxidation states.
For example, Mn shows +2 to +7. The states usually differ by one unit.

Final Answer:
(i) Lanthanoid contraction makes the 2nd and 3rd series almost equal in size, so they resemble each other.
(ii) The (n-1)d and ns electrons have nearly equal energy, so several of them can be used in bonding.