Proteins are synthesized from monomers called amino acids, of which there are twenty different kinds.
The amino acids are small molecules, composed of only ten to twenty-seven atoms each, depending on kind, averaging about twenty, and averaging in mass about 2.35 * 10-22 gram, about one-quarter of one zeptogram (sextillionth of a gram), or 235 yoctograms (septillionths of a gram), about eight times as much as the three-atom water molecule, an oxygen atom single-bonded to each of two hydrogen atoms (an oxygen atom forms two single bonds or one double bond in a molecule, while a hydrogen atom forms one single), the total mass of which is about 2.99 * 10-23 gram, or thirty yoctograms.
Every amino acid molecule consists of a central carbon atom single-bonded to a hydrogen atom, an amino group, a carboxylic acid group and a prosthetic group (a carbon atom forms four single bonds in a molecule, or one double bond and two singles, or two double bonds, or one triple bond and one single).
An amino group is composed of a nitrogen atom (which forms three single bonds, or one double and one single, or one triple) single-bonded to each of two hydrogen atoms.
And a carboxylic acid group is composed of a carbonyl group or moiety, a carbon double-bonded to an oxygen atom, further single-bonded to a hydroxy or alcohol group, an oxygen atom single-bonded to a hydrogen atom.
Such bonds and groups are of course the ordinary molecular bonds and functional groups of carbon chemistry, called "organic chemistry" due to life's taking such advantage of the ability of carbon to form large and stable molecules that all carbon on the face of the earth is or has been part of a living organism.
And the amino acid amino and carboxylic acid groups are of course what gives it its name.
Every amino acid is identical to every other in the above, regardless of kind, in a nine-atom moiety called here its invariant moiety, composed of its central carbon atom and the hydrogen atom and amino and carboxylic acid groups bonded to it.
And an amino acid of one kind differs from one of another solely in the elemental composition (the kinds and number of atoms involved), structure (how those atoms are bonded together) and consequent properties of its prosthetic group.