In chemistry, a coordination compound is the product of a Lewis acid-base reaction in which a neutral molecule (two or more atoms held together by covalent bonds) or ion (molecule with positive or negative charge) is bonded to a central metal atom or ion. This central metal is usually a transition metal (the 38 elements from group three through 12 of the periodic table) The coordination number refers to the number (or sum) of atoms in the nearest molecule, atom or ion bonded to the central metal atom of a compound.
Find the central metal atom in the chemical formula of the coordination compound. In general, the central metal atom is a transition metal. Looking at the periodic table of elements, find the d-block (the 38 elements in groups 3-12). Compare the atoms of the chemical formula and identify the central metal atom.
Identify the atom, molecule or ion nearest to the central metal atom. Looking at the chemical formula, find the atom, molecule or ion that is directly beside the symbol for the central metal atom. The central metal atom might be in the middle of the formula, in that case, note both atoms, molecules or ions that are nearest on both sides.
Find the number of atoms (or add the numbers of atoms) of the nearest neighboring atom(s), molecule(s) or ion(s). If the central metal atom is bonded directly to only one other element, you simply need to note the number of atoms of that element. If the central metal atom is directly bonded to a molecule (or ion), you will need to add the number of atoms in the whole molecule.
Find the total number of nearest neighboring atoms. If the metal has two directly bonded atoms, molecules or ions, add both numbers together.
For crystallography, you identify the coordination number with a slightly different procedure, but the same basic concept applies.