How do you draw a Lewis electron-dot structure, and why is a molecule like water polar but carbon dioxide nonpolar?
Lewis structures and molecular polarity: draw Lewis electron-dot diagrams for simple atoms, ions and molecules, and decide whether a molecule is polar or nonpolar from its bonds and shape.
A focused Regents Chemistry answer on Lewis electron-dot diagrams and molecular polarity: how to draw dot structures for atoms, ions and small molecules, and how bond polarity together with molecular symmetry decides whether the whole molecule is polar.
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What this topic is asking
The Core Curriculum asks you to draw Lewis electron-dot diagrams for atoms, ions and simple molecules, and to decide whether a molecule is polar or nonpolar. Drawing dot diagrams is a frequent Part B-2 task worth one or two points, and deciding molecular polarity from bonds and shape is a classic Part A question. Both skills rest on valence electrons and the bond-polarity page.
Drawing Lewis electron-dot diagrams
To draw a small molecule: count the total valence electrons, place the least electronegative atom (never hydrogen) in the center, connect the outer atoms with single bonds, then distribute the remaining electrons as lone pairs to complete octets, forming double or triple bonds if needed. For water, oxygen ( valence electrons) bonds to two hydrogens with two shared pairs and keeps two lone pairs; for , carbon forms a double bond to each oxygen.
What makes a molecule polar
Two ingredients decide molecular polarity. First, the bonds: a bond is polar if the two atoms differ in electronegativity (covered on the previous page). Second, the shape: even with polar bonds, a symmetrical arrangement can make the individual bond dipoles cancel. So you must consider both.
Symmetry decides whether dipoles cancel
This is why is nonpolar despite two polar bonds, while water is polar: the bent shape of water leaves a net dipole pointing from the hydrogens toward the oxygen. The Regents does not require formal VSEPR geometry names, but you should recognize the bent shape of water and the pyramidal shape of ammonia as the reason their dipoles do not cancel.
Try this
Q1. State the number of lone pairs on the oxygen atom in a water molecule. [1 point]
- Cue. Two lone pairs (oxygen has valence electrons; two are shared, four are lone-pair electrons).
Q2. Explain why is nonpolar even though it has polar bonds. [1 point]
- Cue. It is linear and symmetrical, so the two equal bond dipoles point in opposite directions and cancel.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part B-2 style)2 marksIn the space in your answer booklet, draw a Lewis electron-dot diagram for a molecule of water, . (Worth 2 points: bonding and nonbonding electrons.)Show worked answer →
A 2-point constructed-response item asking for an electron-dot diagram.
A correct diagram shows the oxygen atom in the center bonded to two hydrogen atoms by single covalent bonds (a shared pair shown as two dots or a line between O and each H), plus two nonbonding (lone) pairs of electrons on the oxygen. Oxygen has valence electrons: two are shared in the O-H bonds and the remaining four are shown as two lone pairs. Each hydrogen shows its one shared electron and no lone pairs.
Markers reward showing the correct bonding pairs between O and each H (1 point) and the two lone pairs on oxygen so that oxygen has eight electrons around it (1 point).
Regents (Part A style)1 marksWhich molecule is nonpolar even though it contains polar bonds? (1) (2) (3) (4) Show worked answer →
A 1-point Part A item on molecular polarity. The answer is (3) .
Carbon dioxide has two polar bonds, but the molecule is linear and symmetrical, so the two bond dipoles point in exactly opposite directions and cancel, leaving the molecule nonpolar overall. In contrast, water and ammonia are bent or pyramidal, so their bond dipoles do not cancel and the molecules are polar; has a single polar bond and is polar.
Markers reward recognizing that symmetry can cancel bond dipoles, making a molecule with polar bonds nonpolar overall.
Related dot points
- Electronegativity and bond polarity: use electronegativity differences from Table S to classify bonds as ionic, polar covalent or nonpolar covalent.
A focused Regents Chemistry answer on electronegativity difference and bond polarity: how subtracting Table S electronegativities classifies a bond as nonpolar covalent, polar covalent or ionic, and how that difference shapes the unequal sharing of electrons.
- Types of chemical bonds: distinguish ionic, covalent and metallic bonding in terms of electron transfer or sharing, and relate bond type to the elements involved.
A focused Regents Chemistry answer on ionic, covalent and metallic bonding: how electrons are transferred or shared, why bonds form to reach stability, the role of energy, and how to predict bond type from the elements involved.
- Intermolecular forces: describe hydrogen bonding, dipole-dipole forces and weak dispersion forces, and use them to explain trends in boiling point and the properties of water.
A focused Regents Chemistry answer on intermolecular forces: hydrogen bonding, dipole-dipole attractions and weak dispersion (van der Waals) forces, how they differ from chemical bonds, and how they explain boiling points and water's high boiling point and surface tension.
- Properties of ionic, molecular and metallic substances: relate melting point, electrical conductivity, hardness and solubility to the type of bonding and structure.
A focused Regents Chemistry answer on how bonding type explains properties: why ionic solids have high melting points and conduct only when molten or dissolved, why molecular substances are soft and low-melting, and why metals conduct and are malleable.
- Electron configuration and energy levels: write Regents electron configurations, distinguish ground state from excited state, and explain how electrons absorb and emit specific amounts of energy as photons.
A focused Regents Chemistry answer on electron configuration the New York way (shell notation such as 2-8-1), the ground state versus excited state distinction, valence electrons, and how absorbed and emitted energy produces bright-line spectra.
Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)