Grade 10-11 (age 15-17)

high school Molecular Structure

Molecular Structure (VSEPR)

A molecule's shape is set by the electron pairs around the central atom pushing one another as far apart as they can; this is valence shell electron pair repulsion (VSEPR) theory. Depending on how many atoms are bonded to the central atom and how many lone pairs it carries, the shape comes out linear, trigonal planar, tetrahedral, trigonal pyramidal or bent, and more lone pairs squeeze the bond angle smaller. Even when every bond is polar, a symmetric shape lets the bond polarities cancel and the molecule is nonpolar, so judging polarity requires knowing the shape. Changing the structure in the diagram changes the example molecule's atoms, bond angle and polarity together.

Why Do Molecules Have Definite Shapes?

💡 Think of Balloons Tied Together

①Tie two balloons together and they point in opposite directions, in a straight line (180°)

②Tie three and they spread 120° apart in one plane

③Tie four and a plane no longer has room, so they spread in three dimensions into a tetrahedron (109.5°)

④Electron pairs push on each other like balloons and settle into the arrangement that keeps them farthest apart: that is VSEPR theory

Six Shapes from VSEPR

Molecular shapeLinear

🔍 Comparing the Six Structures

①Linear (CO₂): 2 bonded atoms, 0 lone pairs. A double bond counts as one direction, so C has only two electron domains → nonpolar

②Trigonal planar (BF₃): 3 bonded atoms, 0 lone pairs → nonpolar

③Bent (SO₂): 2 bonded atoms, 1 lone pair. Three electron domains sit in a trigonal plane and one of them is a lone pair, so the shape bends → polar

④Tetrahedral (CH₄): 4 bonded atoms, 0 lone pairs → nonpolar

⑤Trigonal pyramidal (NH₃): 3 bonded atoms, 1 lone pair. Four electron domains sit at the corners of a tetrahedron and the lone pair takes the top → polar

⑥Bent (H₂O): 2 bonded atoms, 2 lone pairs → polar

Lone Pairs and Bond Angle

The Core of VSEPR

Repulsion: lone–lone > lone–bond > bond–bond

Lone pairs take up more space and crowd the bonding pairs

Bond Angle Comparison

CH₄(109.5°) > NH₃(107°) > H₂O(104.5°)

Among four electron domains, going from 0 to 1 to 2 lone pairs narrows the bond angle step by step

💡 Why Lone Pairs Narrow the Bond Angle

①A bonding pair is held between two nuclei and stretches out thinly

②A lone pair is bound to the central nucleus only, so it spreads more widely close to the central atom

③The wider lone pair pushes harder on the bonding pairs, closing the angle between bonds

④That is why, within the same tetrahedral arrangement, the bond angle drops from CH₄ 109.5° to NH₃ 107° to H₂O 104.5°. Stepping through these three structures in the diagram adds one lone pair at a time

Judging Polarity

Judging Molecular Polarity

Shape and Polarity

Shape	Examples	Symmetric	Polarity
Linear	CO₂	Yes	Nonpolar
Trigonal planar	BF₃	Yes	Nonpolar
Bent	H₂O, SO₂	No	Polar
Tetrahedral	CH₄, CCl₄	Yes	Nonpolar
Trigonal pyramidal	NH₃	No	Polar

💡 Three Steps to Spot a Polar Molecule

①Check whether each bond is polar: atoms with different electronegativities form a polar bond

②Check whether the shape is symmetric, using VSEPR to find the shape

③If it is symmetric, the bond dipoles cancel and the molecule is nonpolar; the two C=O bonds in CO₂ point in opposite directions and cancel

④If it is not symmetric, the dipoles do not add to zero and the molecule is polar; H₂O is bent, so its two O–H dipoles do not cancel

⑤So molecules built from the same kind of polar bonds can differ in polarity depending on their shape

Work It Out

Example 1

Find the shape and bond angle of methane (CH₄).

Count the electron domains around the central C: it is bonded to 4 H and has no lone pairs.

4 electron domains around C (4 bonds, 0 lone pairs)

Four domains are farthest apart in a tetrahedron, and since all four are bonds, the shape is tetrahedral too.

tetrahedral, 109.5°

▸ tetrahedral, 109.5°

With no lone pairs, nothing narrows the angle and it stays at 109.5°. In the tetrahedron in the diagram, the wedge and the dashes are the two bonds reaching out in three dimensions.

Example 2

Compare the shapes of water (H₂O) and carbon dioxide (CO₂).

H₂O has 2 bonded atoms and 2 lone pairs on the central O, four electron domains in all.

H₂O: two of the four tetrahedral positions are lone pairs → bent, 104.5°

CO₂ has two double bonds and no lone pairs on the central C, so it has two electron domains.

CO₂: 2 electron domains, 0 lone pairs → linear, 180°

▸ H₂O bent, CO₂ linear

Both central atoms are bonded to two atoms, but the presence or absence of lone pairs splits them into bent and linear. That is why H₂O is polar and CO₂ is nonpolar.

Summary

Finding a Molecule's Shape

① count electron domains around the central atom → ② check the number of lone pairs → ③ decide the shape

electron domains = bonded atoms (a multiple bond counts as 1) + lone pairs

2022 KICE mock Chemistry I type, adapted

Which of the following molecules has a linear shape?

①H₂O

②NH₃

③CH₄

④CO₂

⑤SO₂

▸ ④ CO₂

For each molecule, count the electron domains around the central atom and the lone pairs among them.

H₂O, NH₃ and SO₂ have lone pairs and come out bent or pyramidal, and CH₄ is tetrahedral. Only CO₂ has just two electron domains and no lone pairs.

O=C=O: 2 electron domains, 0 lone pairs → linear, 180°

🎯 Exam Points

①VSEPR: electron domains around the central atom arrange themselves as far apart as possible

②A multiple bond counts as one electron domain

③Lone pairs take more space and reduce bond angles

④CH₄ tetrahedral (nonpolar), NH₃ trigonal pyramidal (polar), H₂O bent (polar), CO₂ linear (nonpolar)

⑤Molecular polarity is judged from bond polarity and the symmetry of the shape together

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Covalent Bond

Electronegativity & Polarity

Last updated 2026-10-17

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