# Molecular Geometry and Polarity

Chemistry I · Atoms, Bonds and Reaction Rates · https://tryals.app/learn/chemistry-i/molecular-geometry-and-polarity

## Shape from Repulsion

**VSEPR** theory rests on one idea: electron domains around a central atom push each other as far apart as possible. A domain is any single bond, any multiple bond (counted once), or any lone pair.

| Domains | Arrangement | Ideal angle |
|---|---|---|
| 2 | Linear | $180^\circ$ |
| 3 | Trigonal planar | $120^\circ$ |
| 4 | Tetrahedral | $109.5^\circ$ |
| 5 | Trigonal bipyramidal | $120^\circ$ and $90^\circ$ |
| 6 | Octahedral | $90^\circ$ |

The **molecular shape** is what you see when only the atoms are drawn, so lone pairs change the name without changing the domain count. Four domains give a tetrahedral arrangement, but methane (0 lone pairs) is tetrahedral, ammonia (1) is trigonal pyramidal, and water (2) is bent.

Lone pairs also squeeze the angles. A lone pair is held by one nucleus only, so it spreads wider than a bonding pair and pushes harder. The tetrahedral $109.5^\circ$ therefore falls to $107^\circ$ in ammonia and $104.5^\circ$ in water. Repulsion strength ranks: lone-lone > lone-bond > bond-bond.

**Molecular polarity** needs two things: polar bonds, and a geometry that fails to cancel them. Bond dipoles are vectors, so a symmetric arrangement of identical bonds sums to zero. $\mathrm{CO_2}$ has two strongly polar C=O bonds pointing in exactly opposite directions and is nonpolar; $\mathrm{H_2O}$ has two similar bonds at $104.5^\circ$ that add to a large net dipole. $\mathrm{CCl_4}$ is nonpolar for the same reason as $\mathrm{CO_2}$, while $\mathrm{CHCl_3}$ is polar because one substituent differs and the cancellation fails.

> **Common pitfall:** deciding polarity from the bonds alone. Polar bonds are necessary but not sufficient, the geometry decides whether they cancel. Carbon dioxide is the standard counterexample: very polar bonds, zero net dipole.

## Practice questions

8 of this lesson's 12 practice questions, with answers. The full set is in the app.

### 1. Water and methane both have four electron domains around the central atom, yet their bond angles are $104.5^\circ$ and $109.5^\circ$. Why?

A. Intramolecular hydrogen bonding between hydrogen atoms draws them closer together
B. Water’s two lone pairs repel more strongly than bonding pairs and compress the angle
C. Oxygen is far more electronegative than carbon, pulling bonded pairs closer to widen the angle
D. Water has only two bonding electron domains, so its ideal arrangement is non-tetrahedral

**Answer:** B. Water’s two lone pairs repel more strongly than bonding pairs and compress the angle

**Why:** A lone pair is bound to a single nucleus and occupies more angular space than a shared pair. Water’s two lone pairs squeeze the H-O-H angle below the ideal tetrahedral value; methane, with none, keeps it.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/water-and-methane-both-have-four-electron-domains-around-the-central

### 2. Carbon dioxide contains polar bonds but has no overall dipole moment.

**Answer:** True

**Why:** True, each C=O bond is strongly polar, but the molecule is linear, so the two bond dipoles are equal and opposite and sum to zero. Geometry, not bond polarity, decides the net result.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/carbon-dioxide-contains-polar-bonds-but-has-no-overall-dipole-moment

### 3. Which molecules are polar overall?

A. $\mathrm{H_2O}$
B. $\mathrm{CHCl_3}$
C. $\mathrm{NH_3}$
D. $\mathrm{CCl_4}$

**Answer:** A. $\mathrm{H_2O}$; B. $\mathrm{CHCl_3}$; C. $\mathrm{NH_3}$

**Why:** Water and ammonia are polar because lone pairs make them asymmetric; CHCl3 is polar because one of four substituents differs. CCl4 has four identical bonds in a symmetric tetrahedron, which cancel exactly.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/which-molecules-are-polar-overall

### 4. Which arrangement of electron domains gives ideal angles of both $90^\circ$ and $120^\circ$?

A. Trigonal pyramidal
B. Square bipyramidal
C. Trigonal bipyramidal
D. Pentagonal bipyramidal

**Answer:** C. Trigonal bipyramidal

**Why:** The trigonal bipyramid is the only common arrangement with two distinct site types: three equatorial positions $120^\circ$ apart, and two axial positions $90^\circ$ from the equatorial plane.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/which-arrangement-of-electron-domains-gives-ideal-angles-of-both-90

### 5. How many electron domains surround the central atom in a water molecule, $\mathrm{H_2O}$?

**Answer:** 4

**Why:** Water has two bonding domains and two lone pairs, giving **4** domains in total. That is why its underlying arrangement is tetrahedral even though the visible shape is bent.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/how-many-electron-domains-surround-the-central-atom-in-a-water

### 6. Molecular polarity requires both polar bonds and asymmetry to prevent vector cancellation. How does this rule distinguish the polarity of carbon tetrachloride from chloroform?

A. Identical peripheral atoms cancel dipoles only in symmetric frames
B. Tetrahedral symmetry always forces an equal cancellation of vectors
C. A single differing bond prevents cancellation of identical dipoles
D. Peripheral chlorine atoms draw density away to leave no net dipole

**Answer:** C. A single differing bond prevents cancellation of identical dipoles

**Why:** Symmetry cancels identical vectors only when all surrounding positions match. Introducing a distinct substituent disrupts vector balance, whereas treating tetrahedral shapes as intrinsically nonpolar ignores how substituent identity dictates net dipole summation.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/molecular-polarity-requires-both-polar-bonds-and-asymmetry-to-prevent

### 7. A double bond counts as two electron domains when predicting molecular geometry.

**Answer:** False

**Why:** False, a double bond is a single domain because its electrons all lie in one region between the same two nuclei. This is why CO2, with two double bonds, is linear rather than tetrahedral.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/a-double-bond-counts-as-two-electron-domains-when-predicting

### 8. Match each repulsion pairing to its relative strength.

**Answer:**

- Lone pair with lone pair → Strongest repulsion
- Lone pair with bonding pair → Intermediate repulsion
- Bonding pair with bonding pair → Weakest repulsion
- Two domains at 180 degrees → Repulsion already minimised

**Why:** Lone pairs are held by a single nucleus, spread wider and push hardest, so lone-lone > lone-bond > bond-bond. Two domains at $180^\circ$ are already as far apart as geometry allows.

Page: https://tryals.app/practice/chemistry-i/molecular-geometry-and-polarity/match-each-repulsion-pairing-to-its-relative-strength