Description Atoms Atoms are represented by the standard abbreviation of the chemical elements, in square brackets, such as [Au] for gold. Brackets may be omitted in the common case of atoms which: are in the "organic subset" of B, C, N, O, P, S, F, Cl, Br, or I, and have no formal charge, and have the number of hydrogens attached implied by the SMILES valence model (typically their normal valence, but for N and P it is 3 or 5, and for S it is 2, 4 or 6), and are the normal isotopes, and are not chiral centers. All other elements must be enclosed in brackets, and have charges and hydrogens shown explicitly. For instance, the SMILES for water may be written as either O or [OH2]. Hydrogen may also be written as a separate atom; water may also be written as [H]O[H]. When brackets are used, the symbol H is added if the atom in brackets is bonded to one or more hydrogen, followed by the number of hydrogen atoms if greater than 1, then by the sign + for a positive charge or by - for a negative charge. For example, [NH4+] for ammonium (). If there is more than one charge, it is normally written as digit; however, it is also possible to repeat the sign as many times as the ion has charges: one may write either [Ti+4] or [Ti++++] for titanium(IV) Ti4+. Thus, the hydroxide anion () is represented by [OH-], the hydronium cation () is [OH3+] and the cobalt(III) cation (Co3+) is either [Co+3] or [Co+++]. Bonds A bond is represented using one of the symbols . - = # $ : / \. Bonds between aliphatic atoms are assumed to be single unless specified otherwise and are implied by adjacency in the SMILES string. Although single bonds may be written as -, this is usually omitted. For example, the SMILES for ethanol may be written as C-C-O, CC-O or C-CO, but is usually written CCO. Double, triple, and quadruple bonds are represented by the symbols =, #, and $ respectively as illustrated by the SMILES O=C=O (carbon dioxide ), C#N (hydrogen cyanide HCN) and [Ga+]$[As-] (gallium arsenide). An additional type of bond is a "non-bond", indicated with ., to indicate that two parts are not bonded together. For example, aqueous sodium chloride may be written as [Na+].[Cl-] to show the dissociation.