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-NH2 or, in its ionized form, -NH3+.
Amino groups are the functional groups that bestow the "amino" in amino acid. They also readily ionize, gaining a proton (hydrogen ion), and thus serve as weak bases. Also when ionized they possess a full charge and even when not ionized are associated with partial charges (due to the relatively high electronegativity of nitrogen). As a consequence, amino groups unquestionably are hydrophilic functional groups.
The nitrogens making up amino groups can bind to more than one carbon atom simultaneously, giving rise to what are known as primary amines (two rather than three hydrogens bound in the ionized state), secondary amines (one hydrogen), and tertiary amines (no hydrogens). That is, C-NH3+ (iominized amino group), C-NH2R+ (primary amine), C-NH1R2+ (secondary amine), and C-NR3+ (tertiary amine), where the "R" represents various non-hydrogen atoms or groups. In addition, we can include the ammonium ion in this list, i.e., C-NH4+, and indeed ammonia, NH3.
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