Explain with one example each: (i) coordination sphere (ii) coordination polyhedron (iii) oxidation number of the central atom.

1. 1 · Coordination sphere. The central atom and its ligands in square brackets. In K₄[Fe(CN)₆] it is [Fe(CN)₆]⁴⁻; the K⁺ ions are counter ions.
2. 2 · Coordination polyhedron. The spatial arrangement of donor atoms around the metal: octahedral in [Co(NH₃)₆]³⁺, square planar in [PtCl₄]²⁻.
3. 3 · Draw the shapes. Draw an octahedron with the metal at the centre and a ligand at each of the six corners; draw a square with Pt at the centre and Cl at the four corners.
4. 4 · Oxidation number. The charge left on the metal if all ligands are removed with their shared pairs. In [Cu(CN)₄]³⁻: x + 4(−1) = −3, so x = +1.
5. 5 · Close in one line. So the bracket shows the sphere, its shape is the polyhedron, and charge balance gives the oxidation number.

Scoring tip: Write the charge equation (x + ... = ...) for the oxidation number. The working earns the mark even if you slip on the sign.