| Name | Description |
|---|---|
| A | Any (any atom except hydrogen) |
| AH | Any atom, including hydrogen |
| Q | Hetero (any atom except hydrogen and carbon) |
| QH | Hetero atom or hydrogen (any atom except carbon) |
| M | Metal (contains alkali metals, alkaline earth metals, transition metals, actinides, lanthanides, poor(basic) metals, Ge, Sb and Po) |
| MH | Metal or hydrogen |
| X | Halogen (F,Cl,Br or I) |
| XH | Halogen or hydrogen |