Question

Similarity in between Nb and Ta is because they:
A. are transition metal.
B. belongs to the same group of periodic tables.
C. have the same mineral source.
D. have the same ionic and covalent radii.

Answer 1

Hint: The similarity in the properties of the metals depend upon the size of the metals. The size of the metal depends upon the effective nuclear charge on the outermost shell.

Complete answer:
Niobium is a 4d series metal and tantalum is a 5d series metal.
The similarity in properties of the niobium and tantalum does not depend upon the fact that both are transition metals, so option (A) is incorrect.
Both metals belong to the same group and down in the group, size increases so, this is not the reason for the same properties so, option (B) is incorrect.
Both are found in the form of silicates but this is not the reason for the same size so, option (C) is incorrect.
Both are placed after the lanthanides in the periodic table so, known as the post lanthanides.
In post lanthanides, lanthanide contraction works which decrease the size of the metals.
In lanthanides, the electrons also fill in the f -orbital and the shielding power of the f-orbital is very less. So, the attraction of the nucleus on the outermost shell increases thus the size of the metal decreases.
So, the reason for the similarity in between niobium and tantalum is the size which is the result of the lanthanide contraction.
Thus, the niobium and tantalum have the same size and due to the same size, tantalum and niobium have the same ionic and covalent radii.

Therefore, option (D) has the same ionic and covalent radii, which is correct.

Additional Information: The screening effect is the power of the electrons to shield to the outermost electrons from the attraction of the nucleus. The screening power of the electrons depends upon the orbitals in which the electrons are present.

Note: Due to lanthanides contraction the size of zirconium-hafnium, niobium-tantalum and molybdenum-tungsten become the same. These pairs are also known as twin pairs.