Formula sheet: Chemical Bonding and Molecular Structure
Chemical Bonding and Molecular Structure12 min readLesson 1 of 7
1.1 Counting electrons first
Almost every question in this chapter starts with a count. Count the valence electrons of the atoms, add or subtract charge electrons, then arrange them as bond pairs and lone pairs. The octet rule explains why atoms combine, formal charge picks the best of several possible Lewis structures, and VSEPR converts an electron count into a shape.
1.2 Shapes from electron domains
The number of electron domains around the central atom is the total of sigma bond pairs and lone pairs. The five basic arrangements follow directly from that count.
| Domains | Electron arrangement | Molecular shape when all domains are bond pairs |
|---|---|---|
| 2 | linear | linear, 180 degrees |
| 3 | trigonal planar | trigonal planar, 120 degrees |
| 4 | tetrahedral | tetrahedral, 109.5 degrees |
| 5 | trigonal bipyramidal | trigonal bipyramidal, 90 and 120 degrees |
| 6 | octahedral | octahedral, 90 degrees |
Lone pairs occupy more space than bond pairs, so each lone pair pushes the bond angles down from the ideal value. Two lone pairs give a bent shape, one lone pair gives a trigonal pyramidal shape, and in a trigonal bipyramid the lone pairs always take equatorial positions because those positions have two neighbours at 90 degrees instead of three.
1.3 Hybridisation
The number of hybrid orbitals equals the number of domains. One s orbital plus one, two or three p orbitals gives sp, sp2 or sp3 hybrids. When five or six domains are present, d orbitals join in, giving sp3d and sp3d2 hybrids.
1.4 Bond order, bond length and bond energy
Bond order counts the number of electron pairs shared between two atoms. A higher bond order means a shorter, stronger bond. Molecular orbital theory gives the same idea through the orbital electron count.
1.5 Dipole moment
The dipole moment of a bond is the product of the charge separated and the distance between the charges. In a molecule the individual bond moments add as vectors, so a symmetrical molecule can have polar bonds and still show zero net dipole moment.
1.6 Lattice enthalpy
Lattice enthalpy is the energy released when one mole of an ionic solid forms from its gaseous ions. The Born-Haber cycle computes it from the sublimation, ionization, dissociation, electron gain and formation enthalpies.
1.7 Hydrogen bonding
A hydrogen bond forms between a hydrogen atom bonded to a highly electronegative atom and a lone pair on another electronegative atom. Its energy is typically 10 to 40 kJ mol^-1, much larger than a van der Waals interaction of a few kJ mol^-1 but much smaller than a covalent bond of a few hundred kJ mol^-1.
| Interaction | Typical energy |
|---|---|
| Covalent bond | 200 to 400 kJ mol^-1 |
| Hydrogen bond | 10 to 40 kJ mol^-1 |
| van der Waals force | 0.4 to 10 kJ mol^-1 |
Exam
Always draw the Lewis structure before you decide a shape. The shape comes from the number of domains, not from the number of atoms. A molecule such as H2O has three atoms, but four domains, so the shape is bent and not linear.
- Count valence electrons before drawing any structure.
- Use formal charge to choose the best resonance form.
- Count domains as sigma bonds plus lone pairs.
- Assign hybridisation from the domain count.
- Work out bond order from the molecular orbital electron count.
- Check whether a hydrogen bond is possible from the H and the lone pair.