This AP Chemistry lesson on vsepr theory notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on vsepr theory and check yourself with the quiz.
Quick answer: A Lewis structure shows how valence electrons are shared as bonds and lone pairs, and formal charges help choose the best one. VSEPR predicts shape by counting electron domains around the central atom, and hybridization (sp, sp², sp³) describes the orbitals that match that count.
Level: AP Chemistry · Curriculum: AP Chem Unit 2
Vsepr theory notes: the key ideas
How are chemical bonds classified?
A bond forms when atoms lower their energy by sharing or transferring valence electrons. Ionic bonds transfer electrons between a metal and a non-metal. Covalent bonds share electron pairs. Metallic bonds hold a lattice of cations in a sea of delocalised electrons.
Electronegativity differences guide the classification. A difference above about 1.7 usually means ionic. A difference between about 0.4 and 1.7 gives a polar covalent bond, with partial charges δ⁺ and δ⁻. Below 0.4 the bond is nonpolar covalent.
How do you draw a Lewis structure?
Count all valence electrons (adjust for charge). Join the atoms with single bonds, complete the octets of the outer atoms, put remaining electrons on the central atom, and form double or triple bonds if the central atom lacks an octet. Elements from period 3 down may hold more than eight electrons.
Formal charge = (valence electrons in the free atom) − (non-bonding electrons) − ½(bonding electrons). The charges add up to the charge of the species.
Carbonate, CO₃²⁻: valence electrons = 4 + 3(6) + 2 = 24. One structure has C=O and two C–O⁻. Formal charges: C = 4 − 0 − 4 = 0; double-bonded O = 6 − 4 − 2 = 0; single-bonded O = 6 − 6 − 1 = −1 each. The total is −2. The double bond can sit on any oxygen, so there are three resonance forms and an average bond order of 4/3.
When several structures are possible, the best has formal charges closest to zero, with any negative charge on the most electronegative atom. Resonance means the real structure is a hybrid, not a switch between the drawings.
How does VSEPR predict molecular shape?
Electron domains (bonds of any order and lone pairs) repel each other and spread as far apart as possible. The number of domains gives the electron geometry. Lone pairs are not included in the name of the molecular shape, but they squeeze the bond angles.
| Domains | Electron geometry | Examples (molecular shape) | Hybridization |
|---|---|---|---|
| 2 | linear, 180° | CO₂ (linear) | sp |
| 3 | trigonal planar, 120° | BF₃ (trigonal planar), SO₂ (bent) | sp² |
| 4 | tetrahedral, 109.5° | CH₄, NH₃ (pyramidal), H₂O (bent) | sp³ |
| 5 | trigonal bipyramidal | PCl₅, SF₄ (seesaw), ClF₃ (T-shaped) | sp³d |
| 6 | octahedral, 90° | SF₆, XeF₄ (square planar) | sp³d² |
Ammonia, NH₃: valence electrons = 5 + 3 = 8. Three N–H bonds use 6 electrons, and the other 2 form a lone pair. Four domains give a tetrahedral electron geometry, so the molecular shape is trigonal pyramidal, the angle is about 107° and the nitrogen is sp³.
What is hybridization?
Hybridization mixes atomic orbitals on one atom to make equivalent orbitals pointing toward the electron domains. The number of hybrid orbitals equals the number of domains. A single bond is a sigma (σ) bond, formed by end-on overlap. A double bond is one σ and one pi (π) bond. A triple bond is one σ and two π bonds.
Ethyne, H–C≡C–H, has two domains on each carbon, so each carbon is sp hybridized and the molecule is linear. The bonds are 3 σ (two C–H and one C–C) and 2 π.
Students count lone pairs when naming the molecular shape. The shape names the atoms only, so NH₃ is trigonal pyramidal, not tetrahedral, even though its electron geometry is tetrahedral.
Check your Lewis structure by counting: electrons drawn must equal electrons available. Then count domains around the central atom to get the shape.
Bond order also governs strength and length: triple bonds are shortest and strongest, single bonds longest and weakest. A polar molecule needs polar bonds and an asymmetric shape, so the bond dipoles do not cancel.
- Formal charge = valence − nonbonding − ½ bonding electrons
- Domains 2, 3, 4, 5, 6 give linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral
- Hybridization: sp, sp², sp³, sp³d, sp³d² match 2 to 6 domains
- Double bond = 1 σ + 1 π; triple bond = 1 σ + 2 π
- Higher bond order means a shorter and stronger bond
Lewis structures, shapes and hybridization of molecules: frequently asked questions
How do you find the formal charge of an atom?
Subtract the non-bonding electrons and half the bonding electrons from the valence electrons of the free atom. For a good Lewis structure the formal charges are as small as possible.
What is resonance in chemistry?
Resonance describes a species whose electron distribution is better represented by several Lewis structures than by one. The true structure is an average, and the bond lengths are equal.
How does VSEPR predict shape?
Count the electron domains around the central atom. Domains spread to minimise repulsion, which gives the electron geometry, and the lone pairs are then ignored to name the molecular shape.
How do you find the hybridization of an atom?
Count its electron domains: 2 is sp, 3 is sp², 4 is sp³, 5 is sp³d and 6 is sp³d². Lone pairs count as domains.
Why is CO₂ nonpolar when its bonds are polar?
The molecule is linear, so the two bond dipoles are equal and opposite and cancel. Polar bonds only give a polar molecule when the shape is asymmetric.
