An atom of an element that substitutes for a hydrogen atom in a hydrocarbon chain, such as a halogen or oxygen, is called a:

1. Elements like halogens, oxygen, nitrogen and sulphur can replace hydrogen in a chain. 2. The atom that substitutes for hydrogen is referred to as a hetero atom. 3. These hetero atoms or groups containing them are called functional groups. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 9 "Carbon Compounds", p.126_