Question

The coordination number of a metal in a coordination compound is:
A.Same as primary valency
B.Sum of primary and secondary valencies
C.Same as secondary valency
D.None of the above

Answer 1

Coordination compounds: It is defined as the chemical substance in which a central metal atom is surrounded by non-metal atoms or group of atoms (which are known as ligands, which donate the electrons to the center metal), are known as coordination compounds.
Secondary valency: It is also known as coordination number. These are defined as the number of negative ions or neutral molecules attached to the central metal atom.

Complete step by step solution:
First of all we will read about transition elements.
Transition elements: Those elements which are in groups from three to eleven. They are called a transition because they are in between the s- block elements and p-block elements. For example: Scandium, iron, zinc, etc. They have fully or at least one electron in their d-orbits. For example: scandium has one d-electron, zinc has ten d-electrons. Due to the different number of electrons present in their d-shells they show different valencies. For example: Scandium having one electron can show three valencies as one, two and three. Because the atomic number of scandium is $21$ and its electronic configuration is $[Ar]3{d^1}4{s^2}$. By losing only one s-electron it will attain one valency and by losing both the s-electrons it will attain valency two. And by losing two s-electrons and one d-electron it will achieve three valencies which are stable states of scandium. Because after losing three electrons it will attain electronic configuration of noble gas argon.
Coordination compounds are formed by transition elements.
Coordination compounds: It is defined as the chemical substance in which a central metal atom is surrounded by non-metal atoms or groups of atoms (which are known as ligands, which donate the electrons to the center metal), are known as coordination compounds. For example: hexamminecobalt(${\text{III}}$) chloride.
There are two types of valencies of the central metal in the coordination compound: primary valency and secondary valency.
Primary valency: It is defined as the number of negative ions needed to satisfy the charge on the central metal ion. This valency is ionisable because this valency is written outside the coordination sphere.
Secondary valency: It is also known as coordination number. These are defined as the number of negative ions or neutral molecules attached to the central metal atom. They are non-ionisable because they are written in the coordination sphere. For example: In hexamminecobalt(${\text{III}}$) chloride, the primary valency is three and secondary valency (coordination number) is six.
The coordination number of a metal in a coordination compound is secondary valency.

Hence, option C is correct.

Note: Transition elements form coordination compounds because they have empty valence shell orbitals i.e. d-orbitals which can accept pairs of electrons from ligands (which donate electrons to the central metal atom also known as Lewis base).