How do you draw a molecule's electron-dot structure and predict its shape?
Lewis structures and molecular geometry: draw electron-dot (Lewis) structures for simple molecules and use VSEPR to predict molecular shapes.
A focused Virginia SOL Chemistry answer on structure under CH.3: drawing electron-dot (Lewis) structures for simple molecules, counting bonding and lone pairs, and using VSEPR to predict shapes such as linear, bent, trigonal planar and tetrahedral.
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What this topic is asking
Continuing standard CH.3, Virginia expects you to draw electron-dot (Lewis) structures for simple molecules and to use VSEPR theory to predict their shapes. The structure shows where bonding pairs and lone pairs sit; the shape follows from those electron pairs repelling each other. Shape then explains polarity and physical properties, so this skill threads through the rest of the bonding module.
Drawing a Lewis structure
Each line represents a shared (bonding) pair of two electrons; remaining pairs that are not shared are lone pairs. Hydrogen needs only two electrons (one bond), and the structure is complete when every atom has its full outer level and all the counted electrons are placed.
Bonding pairs and lone pairs
The electrons around the central atom come in two kinds. Bonding pairs are shared between two atoms and form the bonds; lone pairs belong to a single atom and are not shared. Both kinds repel each other, but lone pairs take up slightly more room, which is why they distort bond angles. Counting the bonding and lone pairs on the central atom is the key input to VSEPR.
VSEPR and molecular shape
The common shapes for the SOL:
| Electron pairs on central atom | Lone pairs | Shape | Example |
|---|---|---|---|
| 2 | 0 | linear | |
| 3 | 0 | trigonal planar | |
| 4 | 0 | tetrahedral | |
| 4 | 1 | trigonal pyramidal | |
| 4 | 2 | bent |
A molecule with two bonds and no lone pairs on the center is linear; the same two bonds with two lone pairs (water) is bent, because the lone pairs push the bonds together.
Try this
Q1. How many valence electrons are in the Lewis structure of methane, ? [1 point]
- Cue. electrons, drawn as four single C to H bonds.
Q2. Predict the shape of a molecule whose central atom has four bonding pairs and no lone pairs. [1 point]
- Cue. Tetrahedral, because four bonding pairs spread out to the corners of a tetrahedron.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksHow many valence electrons must be shown in the Lewis structure of ? (A) 4 (B) 12 (C) 16 (D) 22Show worked answer →
The answer is (C) 16.
Add the valence electrons of every atom. Carbon has valence electrons; each oxygen has . So the total is valence electrons. The Lewis structure of shows these arranged as two carbon-to-oxygen double bonds with two lone pairs on each oxygen, satisfying the octet on each atom.
The trap is counting only bonding electrons or forgetting one oxygen; total the valence electrons of all three atoms.
SOL (tech-enhanced, fill in the blank)2 marksA water molecule, , has two bonding pairs and two lone pairs on the central oxygen. (a) State the molecular shape. (b) Explain why the shape is not linear.Show worked answer →
A 2-point VSEPR item.
(a) Shape (1 point): bent (angular).
(b) Explanation (1 point): the two lone pairs on the oxygen repel the bonding pairs and push them closer together, bending the molecule away from a straight line.
Markers reward naming the shape as bent and linking it to lone-pair repulsion. VSEPR predicts shape by arranging all electron pairs (bonding and lone) as far apart as possible; lone pairs occupy more space and distort the angle.
Related dot points
- Types of chemical bonds: explain ionic, covalent and metallic bonding in terms of valence electrons and electronegativity, and predict bond type from the periodic table.
A focused Virginia SOL Chemistry answer on bonding under CH.3: why atoms bond to reach a stable octet, how ionic, covalent and metallic bonds form, and how to predict the bond type from electronegativity difference and position on the periodic table.
- Polarity and intermolecular forces: determine molecular polarity from shape and bond polarity, and compare dispersion, dipole-dipole and hydrogen-bonding forces and their effect on properties.
A focused Virginia SOL Chemistry answer on polarity under CH.3: how bond polarity and molecular shape combine to make a molecule polar or nonpolar, the three intermolecular forces (dispersion, dipole-dipole, hydrogen bonding), and how they set boiling and melting points and solubility.
- Naming compounds and writing formulas: name and write formulas for ionic compounds (including polyatomic ions), binary molecular compounds and simple acids.
A focused Virginia SOL Chemistry answer on nomenclature under CH.3: writing formulas for ionic compounds by balancing charges (the crossover method), using polyatomic ions and roman numerals, and naming binary molecular compounds with prefixes and simple acids.
- Electron configuration and energy levels: describe how electrons occupy energy levels, write electron configurations, identify valence electrons, and relate ground and excited states to spectra.
A focused Virginia SOL Chemistry answer on electron arrangement under CH.2: energy levels and sublevels, writing electron configurations, counting valence electrons, and the difference between ground state and excited state and how it produces line spectra.
- States of matter and kinetic molecular theory: describe solids, liquids and gases in terms of particle arrangement and motion, and state the assumptions of kinetic molecular theory.
A focused Virginia SOL Chemistry answer on the states of matter under CH.4: how particles are arranged and move in solids, liquids and gases, the link between temperature and average kinetic energy, and the assumptions of kinetic molecular theory.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)