Q.What is called Bond Length? Name the techniques through which the length of a bond can be determined.

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. …