⇒Transition and Inner Transition Elements

Correct Option is (B)

$\mathrm{Mn}$ (Observed and expected electronic configurations): $[\mathrm{Ar}]3{\mathrm{d}}^{5}4{\mathrm{s}}^2$

Correct Option is (A)

Currently no explanation available

Correct Option is (C)

The statement that is NOT CORRECT regarding transition elements is :

Option C : "5d series consists of all elements from lanthanum to mercury of periodic table."

This statement is incorrect because the 5d series of the transition elements does not start with lanthanum. The first element of the 5d series is hafnium (Hf), not lanthanum (La). Lanthanum is actually the first element in the lanthanide series.

Correct Option is (C)

Chromium (Cr):

Expected electronic configuration: $[\mathrm{Ar}]3{\mathrm{d}}^{4}4{\mathrm{s}}^2$

Observed electronic configuration: $[\mathrm{Ar}]3{\mathrm{d}}^{5}4{\mathrm{s}}^1$

Hence, number of unpaired electrons in observed electronic configuration is 6.

Correct Option is (C)

Except promethium (Pm), all are non-radioactive in nature.