Electronegativity decides who wins the tug-of-war.
Every bond is two nuclei pulling on the same electrons. Electronegativity (EN) measures pulling strength. If the difference (ΔEN) is small, they share evenly. If it's medium, one side hogs the electrons (polar). If it's large, one atom takes them outright (ionic).
The 0.4 and 1.7 cutoffs are rules of thumb, and some textbooks use 0.5 and 2.0. Bonding is a continuum, so use your course's numbers on the exam.
Drawing one, every time
- Count valence electrons for all atoms. Add one per negative charge and subtract one per positive charge.
- Pick the central atom: the least electronegative one, and never H. Connect everything to it with single bonds (2 e⁻ each).
- Fill the outer atoms' octets with lone pairs (H only gets 2).
- Put leftover electrons on the central atom.
- If the central atom is short of an octet, turn an outer lone pair into a double (or triple) bond.
- Check formal charges: FC = valence e⁻ − lone-pair e⁻ − ½(bonding e⁻). The best structure has formal charges closest to zero.
Electron domains spread out as far as possible
Count the electron domains on the central atom. Each bond counts as one, whether single, double, or triple, and each lone pair counts as one. That number sets the electron geometry. The molecular shape describes only where the atoms are, so lone pairs are invisible in the name but still push on the bonds.
Drag to rotate. Faint lobes are lone pairs.
| Domains | Electron geometry | 0 lone pairs | 1 lone pair | 2 lone pairs | 3 lone pairs |
|---|---|---|---|---|---|
| 2 | linear | linear 180° | |||
| 3 | trigonal planar | trigonal planar 120° | bent <120° | ||
| 4 | tetrahedral | tetrahedral 109.5° | trigonal pyramidal ~107° | bent ~104.5° | |
| 5 | trigonal bipyramidal | trig. bipyramidal 90°/120° | seesaw | T-shaped | linear |
| 6 | octahedral | octahedral 90° | square pyramidal | square planar |
Polarity is why fat and water don't mix, and why soap works.
Water is bent and polar. Its partial charges stick to other polar things and to ions, which is why salt and sugar dissolve. Milk fat is long nonpolar carbon chains with nothing for water to grab onto. Like dissolves like.
Detergents and emulsifiers (lecithin, mono- and diglycerides, the surfactants in your cleaners) have a polar head and a nonpolar tail. The tail buries itself in fat and the head faces the water, so fat can be carried off a surface or held in suspension. A fat-soil CIP problem is a polarity problem. Caustic adds a second mechanism: it saponifies fats into soap.
Intermolecular forces, weakest to strongest
| Force | Who has it | Example |
|---|---|---|
| London dispersion | Everything, and it's the only force for nonpolar molecules. Bigger molecules have more of it. | CH₄, fats, noble gases |
| Dipole–dipole | Polar molecules | SO₂, HCl |
| Hydrogen bonding | H bonded directly to N, O, or F | H₂O, NH₃, alcohols |
| Ion–dipole | Ions dissolved in a polar solvent | NaCl in water |
Hydrogen bonding is why water boils at 100 °C (methane, a similar-size molecule, boils at −162 °C) and why water's specific heat is so high.
Predict the shape
Bond type from ΔEN
Common mistakes
- Counting a double bond as two domains. It's one.
- Naming the shape by electron geometry. NH₃ has a tetrahedral electron geometry but a trigonal pyramidal shape.
- Calling any molecule with polar bonds polar. CO₂ and CCl₄ have polar bonds, but their symmetry cancels the dipoles.
- Forgetting lone pairs on the central atom when counting domains. Count all valence electrons first.
- Saying lone pairs don't matter. They're invisible in the shape name but squeeze the bond angles (109.5° → 107° → 104.5°).