Heteroatom
In chemistry, a heteroatom (Template:Ety) is, strictly, any atom that is not carbon or hydrogen.<ref>Template:Cite book</ref>
Organic chemistry
In practice, the term is mainly used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure. Typical heteroatoms are nitrogen (N), oxygen (O), sulfur (S), phosphorus (P), chlorine (Cl), bromine (Br), and iodine (I),<ref>Template:Cite journal</ref><ref>Template:Cite encyclopedia</ref> as well as the metals lithium (Li) and magnesium (Mg).
Proteins
It can also be used with highly specific meanings in specialised contexts. In the description of protein structure, in particular in the Protein Data Bank file format, a heteroatom record (HETATM) describes an atom as belonging to a small molecule cofactor rather than being part of a biopolymer chain.<ref name="pdbformat">Template:Cite web</ref>
Zeolites
In the context of zeolites, the term heteroatom refers to partial isomorphous substitution of the typical framework atoms (silicon, aluminium, and phosphorus) by other elements such as beryllium, vanadium, and chromium.<ref>Template:Cite book</ref> The goal is usually to adjust properties of the material (e.g., Lewis acidity) to optimize the material for a certain application (e.g., catalysis).