Q.Define bond energy.

Four measurable quantities characterise every covalent bond.

Bond length is the distance between the nuclei of two bonded atoms, equal to the sum of their bonded radii ($r_A + r_B$), measurable by spectroscopy, X-ray diffraction, or electron diffraction. It GROWS with the size of the bonded atoms (C-C, 1.54 Å, is longer than C-N, 1.43 Å) and SHRINKS as bond multiplicity increases (C-C single 1.54 Å > C=C double 1.33 Å > C≡C triple 1.20 Å).

Bond order, in simple Lewis-theory terms, is the number of electron pairs shared between two bonded atoms: 1 for a single bond (H-H), 2 for a double bond (O=O), 3 for a triple bond (N≡N). Molecular orbital theory later generalises this to a formula, $BO = (N_b - N_a)/2$, that can also yield FRACTIONAL values (e.g. NO, bond order 2.5) — see Molecular Orbital Theory.

Bond angle is the fixed angle between two bonds meeting at a common atom, a consequence of covalent bonds being directional in space, measurable by spectroscopy. CH₄ (no lone pairs on C) has bond angle 109°28'; NH₃ (one lone pair on N) has 107°18'; H₂O (two lone pairs on O) has 104°35' — the progressive shrinkage as lone-pair count rises is explained by VSEPR theory's repulsion ordering. …