ions are ubiquitous in nature and are responsible for diverse phenomena from the luminescence of the sun to the existence of the earth's ionosphere. atoms in their ionic state may have a different colour from neutral atoms, and thus light absorption by metal ions gives the colour of gemstones. in both inorganic and organic chemistry (including biochemistry), the interaction of water and ions is extremely important; an example is energy that drives the breakdown of adenosine triphosphate (atp). the following sections describe contexts in which ions feature prominently; these are arranged in decreasing physical length-scale, from the astronomical to the microscopic.