In organic chemistry,<unk> s (or moieties) are specific<unk> s of atoms within molecules, that  are responsible for the characteristic chemical reactions of those molecules.  The same<unk>  will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of.
Combining the names of<unk> s with the names of the parent alkanes generates a powerful systematic nomenclature for naming organic compounds.
The non-hydrogen atoms of<unk> s are always associated with each other and with the rest of the molecule by covalent bonds. When the<unk>  of atoms is associated with the rest of the molecule primarily by ionic forces, the<unk>  is referred to more properly as a polyatomic ion or complex ion. And all of these are called radicals, by a meaning of the term radical that predates the free radical.
The first carbon atom after the carbon that attaches to the<unk>  is called the alpha carbon.<unk> s are attached to the carbon backbone of organic molecules.  They determine the characteristics and chemical reactivity of molecules.<unk> s are far less stable than the carbon backbone and are likely to participate in chemical reactions.
The following is a list of common<unk> s. In the formulas, the symbols R and R' usually denote an attached hydrogen, or a hydrocarbon side chain of any length, but may sometimes refer to any<unk>  of atoms.
Functional group
Clinical site
Pregnancy category
Etiology
Performance status
Chronic pain classification
Idose
Serotype