= Amide bond
An \b[amide bond], in the <carboxamide> sense used here, is the <carbon>–<nitrogen> <covalent bond> in the unit $\mathrm{R-C(=O)-NR'R''}$. The <nitrogen> is directly attached to the <carbonyl group>'s carbon. Merely finding a <carbon>–<nitrogen> bond elsewhere in a <molecule> does not identify an <amide bond>. The broader IUPAC class name <amide> also covers other acid-derived compounds; this article concerns the carbonyl linkage.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[entries C00850, carboxamides, https://doi.org/10.1351/goldbook.C00850 ; A00266, amides]{ref}
For example, <acetamide> has connectivity $\mathrm{CH_3-C(=O)-NH_2}$. Its amide C–N bond is the bond from the carbonyl carbon to <nitrogen>, rather than the adjacent C–C bond. A <peptide bond> is a biologically important instance of this connectivity in a chain of <amino acids>.
<Chemical resonance> describes electron sharing through the <nitrogen> and <carbonyl group>. Two useful contributing structures are
$$
\mathrm{R-C(=O)-NH-R'}\quad\longleftrightarrow\quad
\mathrm{R-C(-O^{-})=N^{+}H-R'}.
$$
These are descriptions contributing to one electronic structure, rather than two molecules rapidly alternating. The <amide bond> consequently has partial <double bond> character, and the ordinary amide group favors approximate planarity. <Linus Pauling>'s lecture connects that structural picture to measured bond lengths and explains why hypothetical resonance forms remain useful.<Modern structural chemistry by Linus Pauling>[p.433, amide group structure and the interpretation of resonance]{ref}
This supplies a useful modeling constraint: a chain containing <peptide bonds> cannot be treated as if every backbone <single bond> rotates equally freely. Approximate amide planarity constrains the conformations available without asserting that every <amide> is perfectly planar.
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