Difficulty: Introductory to Intermediate. Prerequisites: understand valence electrons and electron configuration (see Measurements and Atomic Structure notes).
Big picture: Once you know how many valence electrons an atom has, the next question is what it does with them. This topic covers the two main ways atoms stick together (ionic and covalent bonding), how to draw those arrangements (Lewis structures), and how to predict the 3D shape of a molecule (VSEPR theory). Molecular shape determines physical properties, biological activity, and chemical reactivity, so this is not just abstract theory.
Atoms bond by transferring electrons (ionic) or sharing them (covalent). Lewis dot structures map out where valence electrons sit. VSEPR theory uses those structures to predict molecular geometry: electron groups repel each other and spread as far apart as possible, giving rise to shapes like linear, trigonal planar, tetrahedral, and their bent or pyramidal variants.
Ionic bond
A bond formed by the transfer of electrons from one atom to another, creating oppositely charged ions held together by electrostatic attraction. Think of it as: one atom gives, the other takes, and opposite charges glue them together.
Covalent bond
A bond formed when two non-metal atoms share electrons to complete their octets. In simple terms, neither atom wants to let go entirely, so they share.
Lewis dot structure
A diagram showing valence electrons as dots around an element's symbol. Dots are placed one per side first, then paired after four. Think of it as: a quick sketch of an atom's available electrons for bonding.
Octet rule
Atoms tend to gain, lose, or share electrons until they have eight in their valence shell (except hydrogen, which aims for two).
Resonance structures
Two or more valid Lewis structures for the same molecule, differing only in the placement of electrons (not atoms). The real molecule is a blend of all resonance forms. In simple terms, when a Lewis structure can be drawn in more than one way, the truth is the average of all versions.
VSEPR (Valence Shell Electron Pair Repulsion) theory
A model predicting molecular geometry based on the principle that electron groups around a central atom arrange themselves as far apart as possible to minimise repulsion. Think of it as: electron clouds are like balloons tied together at the centre, pushing away from each other.
Lone pair
A pair of valence electrons not involved in bonding, sitting on the central atom. Lone pairs still take up space and affect molecular shape.
Bond angle
The angle between two bonds originating from the same central atom, determined by the arrangement of electron groups.
Ionic bonds form between metals and non-metals. One atom transfers electrons to the other. The resulting cation (+) and anion (-) are held together by electrostatic forces. Ionic compounds typically form crystalline solids with high melting points.
Covalent bonds form between non-metals. Atoms share electron pairs to complete their octets. When the attractive forces between opposite charges (nucleus to shared electrons) exceed the repulsive forces between like charges (nucleus to nucleus, electrons to electrons), a covalent bond forms. Noble gases are already stable and do not form covalent bonds under normal conditions.
Place the element symbol in the centre.
Represent each valence electron as a dot around the symbol.
Place dots one per side first (up, down, left, right), then pair them after four dots are placed.
The number of dots equals the number of valence electrons.
For molecules, shared pairs become bonding pairs (lines), and unshared pairs remain as lone pairs (dots).
VSEPR stands for Valence Shell Electron Pair Repulsion. Electron groups (bonds and lone pairs) around a central atom arrange themselves to be as far apart as possible.
Step-by-step approach:
Draw the Lewis structure.
Count the total number of electron units on the central atom (single bonds, double bonds, triple bonds, and lone pairs each count as one unit).
Identify the parent (electron) geometry from the unit count.
Determine the molecular geometry by considering only the atoms (ignore lone pairs for the name).
Assign bond angles.
Geometry reference table:
Electron units | Parent geometry | Molecular geometry (if no lone pairs) | Bond angle |
|---|---|---|---|
2 | Linear | Linear | 180° |
3 | Trigonal planar | Trigonal planar | 120° |
3 (1 lone pair) | Trigonal planar | Bent | ~120° |
4 | Tetrahedral | Tetrahedral | 109.5° |
4 (1 lone pair) | Tetrahedral | Trigonal pyramidal | ~107° |
4 (2 lone pairs) | Tetrahedral | Bent | ~104.5° |
Lone pairs occupy more space than bonding pairs, so they compress bond angles slightly below the ideal values.
When more than one valid Lewis structure can be drawn for a molecule (differing only in electron placement, not atom positions), the molecule has resonance. The real structure is a hybrid, a blend of all contributing forms. No single Lewis structure captures the full picture.
For example, the carbonate ion (CO₃²⁻) can be drawn with the double bond on any of the three oxygen atoms. The true structure has three equivalent bonds, each somewhere between a single and a double bond.
Molecular shape determines how drugs bind to receptors, how enzymes recognise substrates, and why certain molecules have particular smells or tastes. VSEPR is the entry point for understanding why water is bent (and therefore polar), which explains most of water's unusual properties. The shape of the SARS-CoV-2 spike protein, and how it fits the ACE2 receptor, is a real-world example of molecular geometry in biology.
Students often confuse electron geometry with molecular geometry. Electron geometry counts all electron groups (including lone pairs). Molecular geometry describes only the positions of atoms. A molecule with tetrahedral electron geometry and one lone pair has trigonal pyramidal molecular geometry, not tetrahedral.
Double and triple bonds are frequently miscounted as multiple electron units. In VSEPR, a double bond or triple bond counts as one electron unit, not two or three.
Resonance structures are sometimes thought to show a molecule flipping between forms. The molecule does not switch. The real structure is a single, blended hybrid at all times.
Students sometimes forget that lone pairs compress bond angles. The ideal tetrahedral angle is 109.5°, but water's bond angle is about 104.5° because two lone pairs push the bonding pairs closer together.
⚠️ Drawing correct Lewis structures is a prerequisite for most bonding and geometry questions. If the structure is wrong, the geometry prediction will be wrong too.
⚠️ VSEPR questions frequently ask for both the electron geometry and the molecular geometry. Know the difference.
⚠️ Bond angle questions are common. Memorise the ideal angles (180°, 120°, 109.5°) and know that lone pairs reduce them slightly.
⚠️ Resonance is often tested by asking students to draw all valid structures for ions like carbonate (CO₃²⁻) or nitrate (NO₃⁻).
True or False: A double bond counts as two electron units in VSEPR.
Fill in the blank: An ionic bond involves the ______ of electrons, while a covalent bond involves the ______ of electrons.
True or False: The molecular geometry of a molecule with four electron units and one lone pair is tetrahedral.
Fill in the blank: The ideal bond angle for a trigonal planar molecule is ______.
True or False: Resonance structures represent a molecule switching between different forms.
Answers: 1. False (it counts as one). 2. Transfer, sharing. 3. False (it is trigonal pyramidal). 4. 120°. 5. False (the molecule is a constant hybrid of all forms).
Q: What is the molecular geometry of water (H₂O)?
A: Bent. Oxygen has four electron units (two bonding pairs and two lone pairs), giving a tetrahedral electron geometry, but the molecular geometry considers only the atoms, which form a bent shape with a bond angle of about 104.5°.
Q: How many resonance structures can be drawn for the nitrate ion (NO₃⁻)?
A: Three. The double bond can be placed between nitrogen and any one of the three oxygen atoms, giving three equivalent resonance structures.
Q: Explain why NH₃ has a bond angle less than 109.5°.
A: Ammonia has three bonding pairs and one lone pair around nitrogen. The lone pair repels the bonding pairs more strongly than bonding pairs repel each other, compressing the bond angle from the ideal tetrahedral 109.5° to approximately 107°.
Q: Carbon dioxide (CO₂) has two double bonds. What is its molecular geometry and bond angle?
A: Linear, 180°. Two electron units (each double bond counts as one) arrange themselves on opposite sides of the central carbon.
Lewis structures feed directly into oxidation-reduction chemistry (Document 3): you need to track electron movement to assign oxidation numbers. Molecular geometry also connects to polarity and intermolecular forces, which determine boiling points, solubility, and whether a substance conducts electricity in solution. VSEPR predictions become essential when studying molecular polarity later in the course.
Ionic bonding, covalent bonding, electron transfer, electron sharing, Lewis dot structure, Lewis structure, octet rule, VSEPR, valence shell electron pair repulsion, molecular geometry, electron geometry, linear, trigonal planar, tetrahedral, bent, trigonal pyramidal, bond angle, lone pair, resonance structure, resonance hybrid, carbonate ion, nitrate ion, CHEM 111, general chemistry, Purdue