Homotopic groups in a chemical compound are equivalent groups. Two groups A and B are homotopic if the molecule remains achiral when the groups are interchanged with some other atom (such as bromine) while the remaining parts of the molecule stay fixed. Homotopic atoms are always identical, in any environment. Homotopic NMR-active nuclei have the same chemical shift in an NMR spectrum.  For example, the four hydrogen atoms of methane (CH) are homotopic with one another, as are the two hydrogens or the two chlorines in dichloromethane (CHCl).