Q.Define Coordination number.

The working vocabulary for reading a coordination formula. A coordination entity is the central atom/ion plus its attached ligands, always shown inside square brackets, e.g. [Fe(CN)₆]⁴⁻ in K₄[Fe(CN)₆]. The central atom/ion occupies the centre and accepts electron pairs from the ligands through coordinate bonds, making it a Lewis acid. Ligands are the atoms/groups bound to the central atom; the specific atom within a ligand that is actually bonded is the donor atom -- in CN⁻ the donor atom is carbon, not nitrogen, while in NH₃ it is nitrogen.

The coordination sphere is the complex ion (central atom + ligands) enclosed in brackets with its net charge; anything written outside the bracket is a counter ion. The coordination polyhedron is the 3-D shape traced by the donor atoms around the metal (octahedral for CN=6, tetrahedral for CN=4 with sp³, etc). The coordination number is the count of donor atoms (equivalently, sigma bonds) bonded to the metal -- NOT simply the ligand count, since a bidentate ligand like ethane-1,2-diamine (en) contributes two donor atoms from one ligand molecule. …