Bond length is the equilibrium state between the centers of the atoms (r).
1. With the increase in the serial number of the element, its covalent radius increases, and the lengths of the corresponding bonds increase: C-F (0.138 nm) < C-Cl (0.178 nm) < C-Br (0.194 nm) < C-I (0.214 nm)
2. The change in the hybrid orbitals of an atom leads to a change in its covalent radius and the length of its bonds with other atoms: CH3 - CH3 (Csp3 - Csp3) 0.154 nm CH2 = CH2 (Csp2 = Csp2) 0.134 nm HC ≡ HC (Csp ≡ Csp) 0.120 nm
With the increase of the s-orbitals into hybrid orbitals the bond length decreases