An Atom or molecule having a net electrical charge. The Handbook of Chemistry and Physics, 91st edition (2010-2011) defines an ion as an atomic or molecular particle having a net electrical charge. An electrical charge may be expressed either as positive or negative. When large structures carry a net electrical charge, such as a protein, a macromolecule or a nanoparticulate cluster, the term "charged particle" distinguishes such a complex unit from common, small ionized atoms or small molecules.