What is meant by the term ‘coordination number’?

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Hint: In complexes, there are ions, molecules or atoms which are bonded to the central metal atom. In crystals, each ion is surrounded by a specific number of ions of opposite charge.

Complete step by step solution:
- Coordination number is also known as ligancy. This term is mainly used in crystallography and materials science. Coordination number is the number of atoms, molecules or ions bonded to a central metal atom. The ions, molecules or atom surrounding the central metal atom is called ligands. This is in the case of complexes.
- In complexes formed by d-block transition metals, the most common coordination number is 6 and the structure is octahedral. Metals of f-block can have higher oxidation numbers because of their greater ionic radii and availability of more orbitals for bonding. They have a coordination number varying between 8 to 12.
- In case of crystals, the coordination number is the number of ions that is present in the immediate surroundings of an ion of opposite charge in a crystal lattice. For example, NaCl has a face centered cubic lattice. Each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. So, the coordination number for NaCl is 6.
Thus, coordination number is the number of ions, atoms or molecules bonded to a central atom in coordination chemistry. In crystallography, it is the number of ions in immediate surroundings of an ion.

Note: The concept of coordination number in crystals and complexes is slightly different. In the former, it means the number of species bonded to a central metal atom and in the latter; it is the number of immediate negatively charged ions surrounding a positively charged ion and vice versa.
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