Question

Because of lanthanoid contraction, which of the following pairs of elements have nearly the same atomic radii?
(A)- Zr (40) and Hf (72)
(B)- Zr (40) and Ta (73)
(C)- Ti (22) and Zr (40)
(D)- Zr (40) and Nb (41)

Answer 1

The series of chemical elements consisting of 15 metallic chemical elements with atomic number 57-71 are known as Lanthanide or Lanthanoid. Lanthanide elements begin from Lanthanum and end at Lutetium and with Scandium and Yttrium, they make up the rare-earth metals.

Complete answer:
-Lanthanides have the extra electron (also called differentiating electron) which enters in $(n-2)f$ orbital. Since $(n-2)f$ orbital lies comparatively deep within the inner to the penultimate shell, therefore the elements are also called Inner transition elements.
-The lanthanoid have the general electronic configuration as $4{{f}^{1-14}}5{{d}^{0-10}}6{{s}^{2}}$, with some exception due to full fill or half-filled electronic configuration, wherein the 4f electrons of lanthanides did not participate in bond formation. Lanthanoid atoms have similar configurations and similar physical and chemical properties, and the most common valences are 3 and 4.
-The atomic radii or the size of tri positive lanthanide ion lanthanide ions decreases steadily from Lanthanum due to Lutetium due to an increase in the nuclear charge and electrons entering in the inner $(n-2)f$ orbital.
-The greater than expected decrease in ionic radii of the elements in the lanthanide series from atomic number 51 to atomic number 71, resulting in smaller atomic radii than expected for subsequent elements starting with atomic number 72 is known as lanthanide contraction.
-Lanthanide contraction results due to poor shielding of the nuclear charge by 4f electrons due to which 6s electrons are drawn towards the nucleus thus leading to a decrease in atomic radius.
-Let us now see the atomic radius of the elements given us in options-

ELEMENT	RADIUS (ppm)
Zr (40)	145
Hf (72)	144
Ta (73)	134
Ti (22)	132
Nb (41)	134

So, it is clear from the table that Zr and Hf have nearly the same atomic numbers.

Therefore, the correct answer is option A.

Note:
The International Union of Pure and Applied Chemistry (IUPAC), which is the international body in charge of chemical nomenclature, prefers the term ‘Lanthanoid’ over ‘Lanthanide’, because ‘-ide’ ending is usually reserved for negatively charged ions.