The central atom/ion is the one to which a fixed number of ions/groups are bound in a definite geometric arrangement. Examples are Ni²⁺ in [NiCl₂(H₂O)₄], Co³⁺ in [CoCl(NH₃)₅]²⁺, and Fe³⁺ in [Fe(CN)₆]³⁻, also referred to as Lewis acids.

A coordination entity consists of a central metal atom or ion bonded to a fixed number of ions or molecules. Examples include [CoCl₃(NH₃)₃], [Ni(CO)₄], [PtCl₂(NH₃)₂], [Fe(CN)₆]⁴⁻, and [Co(NH₃)₆]³⁺.

Double salts dissociate into simple ions when dissolved, whereas complex ions, like [Fe(CN)₆]⁴⁻ in K₄[Fe(CN)₆], do not dissociate into their constituent ions, maintaining their structural integrity in solution.

Examples of double salts include carnallite (KCl.MgCl₂.6H₂O), Mohr’s salt (FeSO₄.(NH₄)₂SO₄.6H₂O), and potash alum (KAl(SO₄)₂.12H₂O), formed by combining two or more stable compounds in specific stoichiometric ratios.

Double salts, like carnallite or potash alum, dissociate completely into simple ions when dissolved. In contrast, complex ions, such as [Fe(CN)₆]⁴⁻ in K₄[Fe(CN)₆], do not dissociate into individual ions.