Question

The correct order of unpaired electrons present in d orbitals for elements Fe, Ti, Cr and Co is:
A. $Cr=Fe>Co>Ti$
B. $Fe>Co>Cr>Ti$
C. $Cr>Fe>Co>Ti$
D. $Cr=Fe>Ti>Co$

Answer 1

D block elements are the elements which can be found from the third group to the twelfth group of the modern periodic table. The valence electrons of these elements fall under the d orbital. D block elements are also referred to as transition elements or transition metals.

Complete Step by step solution: Elements having electrons (1 to 10) present in the d-orbital of the penultimate energy level and in the outermost ‘s’ orbital 1-2 electrons are present are known as d block elements. These elements have metallic qualities such as malleability and ductility, high values of electrical conductivity and thermal conductivity, and good tensile strength. There are four series in the d block corresponding to the filling up of 3d, 4d, 5d or 6d orbitals. In each series there are 10 elements present except 6d it is an incomplete series. General electronic configuration of d- block elements is shown by: $(n-1){{d}^{1-10}}n{{s}^{1-2}}$. Unpaired electrons are the electrons which remain as such in the outermost shell i.e. in which pairing not occurs.
Fe (26) showing electronic configuration as $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{6}}$which means it have 4 unpaired electrons.
Cr (24); $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{4}}$, showing 4 unpaired electrons
Co (27); $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{7}}$, showing 3 unpaired electrons
Ti (22); $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{2}}$, it shows 2 unpaired electrons.

Hence the order is: $Cr=Fe>Co>Ti$, Option A is right.

Note: Oxidation state is a hypothetical state, where the atom appears to release or gain electrons more than the usual valency state. It is still useful in explaining the properties of the atom/ion. Transition elements/ions may have electrons in both s and d-orbitals. Since the energy difference between s and d-orbital are small, both the electrons can involve in ionic and covalent bond formation and hence exhibit multiple(variable) valency states (oxidation states). Each transition element can hence exhibit a minimum oxidation state.