Lesson 3 of 12 · 7 min
Bond parameters: length, angle, enthalpy and order
NCERT § "Bond Parameters"
Two hydrogen atoms drifting towards each other settle exactly 74 pm apart. Not 60, not 100. What decides that distance, and how much energy do they give out when they get there?
The lesson in notes
In short
Bond length is the separation of the nuclei of two bonded atoms at equilibrium; covalent radius is half the distance between two identical bonded atoms, while the van der Waals radius, measured between non-bonded atoms in contact, is larger.
Typical carbon-carbon bond lengths are C–C 154 pm, C=C 134 pm and C≡C 120 pm; the H–H bond is 74 pm.
Bond angle is measured at the central atom, between the orbitals that carry its bonding pairs; spectroscopy gives its experimental value, and it helps fix the shape.
Bond enthalpy is the energy needed to break one mole of a given bond between two atoms in the gaseous state; the H–H value is 435.8 kJ mol⁻¹.
For polyatomic molecules with several identical bonds (like the O–H bonds of water), an average bond enthalpy is used because each successive bond needs a different energy to break.
Bond order in the Lewis picture is the number of bonds between two atoms: H₂ 1, O₂ 2, N₂ 3, CO 3.
Isoelectronic species have the same bond order; F₂ and O₂²⁻ both have bond order 1, while N₂, CO and NO⁺ all have bond order 3.
As bond order increases, bond enthalpy increases and bond length decreases.