Question

A reduction in atomic size with increase in atomic number is a characteristic of elements of:
A.d-block
B.f-block
C.radioactive series
D.high atomic masses

Answer 1

Hint: The atomic radii increase down the group which means the atomic radii or size of the second transition series are larger than those of the first transition series.
But, the atomic radii of the second and third transition series are nearly the same because of lanthanoid contraction.

Complete step by step answer:
-Generally, the atomic radii of d block elements in a series tend to decrease with increase in atomic number but the decrease is small after mid-way. At the end of the period, there is a slight increase in the atomic radii. So, reduction in atomic size with increase in atomic number is not a characteristic of elements of d block. So, option A is not correct.
-f-block elements are called inner transition elements as the large electron enters the penultimate energy level and thus form a transition series within the transition series or d-block elements. They are divided into 2 series – lanthanoid and actinoid series.
-It has been found that in the lanthanide series, with increasing atomic number, there is a progressive decrease in the atomic as well as ionic radii of the trivalent ions from lanthanum to lutetium. This is called lanthanoid contraction.
-Along the lanthanide series, the nuclear charge tends to increase by one unit at each successive element. The new electron is added into the same 4f subshell. As a result, the attraction of the nucleus on the electrons increases and this leads to decrease in the atomic size.
-Moreover, as new electrons are added into the f subshell, there is imperfect shielding of one electron by another which is unable to counterbalance the effect of increased nuclear charge. So, the overall outcome is a contraction in size.
-The actinoids also show similar actinoid contraction. So, reduction in atomic size with increase in atomic number is a characteristic of elements of f block. So, option B is correct.
-The reduction in atomic size with increase in atomic number is not a characteristic of elements of radioactivity series or high atomic masses and so C and D are wrong.

Hence option B is correct.

Note:
-Because of the lanthanoid contraction, the change in ionic radii in lanthanide is very small and so their chemical properties are similar. This leads to difficulty in the separation of lanthanides in the pure state.
-Another consequence of lanthanide contraction is on the basic strength of hydroxides. Since the size of the lanthanoid ions decrease from ${\text{L}}{{\text{a}}^{{\text{3 + }}}}$ to ${\text{L}}{{\text{u}}^{{\text{3 + }}}}$ , the covalent character of the hydroxides increases and so the basic strength decreases from ${\text{La}}{\left( {{\text{OH}}} \right)_{\text{3}}}$ to ${\text{Lu}}{\left( {{\text{OH}}} \right)_{\text{3}}}$ .