Question

What is the coordination number?
A.The number of ligands attached to the central metal atom or ion.
B.The number of central atoms.
C.The total number of bonds in the compound.
D.The total number of covalent bonds in the compound.
E.The total number of hydrogens attached to the central metal atom or ion.

Answer 1

We must remember that ions or neutral molecules that bind to a central metal atom or ion are ligands. Lewis bases (electron pair donors) serve as ligands, and the central atom serves as a Lewis acid (electron pair acceptor). With an electron pair used to form covalent bonds with the central atom, ligands have at least one donor atom.
We have to know that in the molecular formula, the central atom is usually the atom with the lowest subscript, and the atom that can form the most bonds. If all of the atoms normally form the same number of bonds, the central atom is typically the least electronegative atom.

Complete step by step answer:
We need to remember that the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bound to it in chemistry, crystallography, and materials science. A ligand is called the ion/molecule/atom surrounding the central ion/molecule/atom. This number is calculated somewhat differently for molecules than for crystals.
Coordination numbers vary from two to nine for transition metal complexes . Because of their larger ionic radii and the availability of more orbitals for bonding, metals in the f-block (lanthanoids and actinoids) can tolerate higher coordination numbers. For f-block elements, coordination numbers of eight to twelve are commonly observed.
The number of ligands attached to the central metal atom or an ion is the coordination number.
For example, in the complex of $Ni{\left( {CO} \right)_4}$ , the number of ligands present in this complex is $4$ . So the coordination number of this complex is $4$ .
Option A is correct because coordination number is the number of ligands attached to the central metal atom or an ion.

Note:
We need to remember that for d-block transition metal complexes, the most common coordination number is six . CN, i.e. octahedral vs trigonal prismatic, does not differentiate geometry from such complexes.
For atoms in the interior of a crystal lattice, the coordination numbers are well defined: one counts the nearest neighbours in all directions. The bulk coordination number is called the number of neighbours of an interior atom. The number of neighbours is more limited for surfaces, so the surface coordination number is lower than the bulk coordination number.