Molecular Geometry & Polarity — Practice · Grade 11 Science

Chapter: Bonding, Formulas & Molecular Structure

What this practice covers

By the end of Molecular Geometry & Polarity you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: VSEPR theory, molecular geometry, electronegativity, polar bond, nonpolar bond, dipole, intermolecular force, hydrogen bond.

Problem 1

VSEPR theory predicts molecular shape based on the idea that the electron groups around a central atom…
  1. always stay at fixed 90° angles
  2. have no effect on the shape
  3. attract each other
  4. repel and spread as far apart as possible
Answer and explanation

repel and spread as far apart as possible

Electron groups are all negative, so they repel and spread out as far as possible, which sets the shape.

Problem 2

A molecule with a central atom bonded to two others and no lone pairs (like CO₂) has which shape?
  1. linear
  2. tetrahedral
  3. trigonal pyramidal
  4. bent
Answer and explanation

linear

Two electron groups push to opposite sides, giving a linear shape with a 180° bond angle.

Problem 3

A methane molecule (CH₄) is tetrahedral. What is its approximate bond angle, in degrees?
Answer and explanation

109.5 °

Four electron groups spread to the corners of a tetrahedron, giving bond angles of about 109.5°.

Problem 4

Why is a water molecule polar while carbon dioxide is nonpolar, even though both have polar bonds?
  1. Water is bent, so its bond dipoles don't cancel, while CO₂ is linear and symmetric, so its dipoles cancel
  2. Water has no polar bonds
  3. Carbon dioxide has no definite shape
  4. Water is actually an ionic compound
Answer and explanation

Water is bent, so its bond dipoles don't cancel, while CO₂ is linear and symmetric, so its dipoles cancel

Molecular polarity depends on both bond polarity and shape; water's bent shape leaves a net dipole, while CO₂'s symmetric linear shape cancels its dipoles.

Problem 5

What is a hydrogen bond, and why does it give water an unusually high boiling point?
Answer and explanation

A hydrogen bond is a strong attraction between a hydrogen atom bonded to N, O, or F in one molecule and a lone pair on the N, O, or F of a neighboring molecule. Water molecules form many hydrogen bonds with one another, so it takes a lot of extra energy — a higher temperature — to pull them apart into a gas, giving water an unusually high boiling point.

Problem 6

Ammonia (NH₃) has a central nitrogen with three bonding pairs and one lone pair. Using VSEPR, what is its molecular shape?
  1. tetrahedral
  2. trigonal pyramidal
  3. linear
  4. trigonal planar
Answer and explanation

trigonal pyramidal

The four electron groups arrange tetrahedrally, but with only three atoms attached the visible shape is trigonal pyramidal; the lone pair pushes the N–H bonds down to about 107°.

Problem 7

Why is water polar in the bent molecular model? Show the reasoning, not just the conclusion.
Hints

Compare CO₂ with bent H₂O, where dipoles do not cancel.

Answer and explanation

Its bond dipoles have components that add rather than completely cancel.

Problem 8

CH₄ has symmetric tetrahedral geometry. What does that imply for its net dipole in the simple model?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The symmetric distribution gives no permanent net dipole.

Problem 9

Molecular polarity depends on how bond contributions combineMolecular polarity depends on how bond contributions combineCO₂: opposing contributions cancelNet dipole = 0H₂O: bent; nonzero sumOCOOHH
Original qualitative vector comparison. Connectors show bond directions and are not complete Lewis structures; water angles are schematic here.
Interpret the supplied visual. Why do two polar C–O bonds not automatically make CO₂ a polar molecule?
Answer and explanation

In the symmetric linear molecule, equal opposing contributions cancel. Geometry is needed along with bond polarity.

A dipole moment has direction as well as magnitude. Symmetry can make equal bond contributions cancel, as in an isolated ideal linear CO₂ molecule. In bent H₂O, the corresponding contributions do not cancel, producing a net molecular dipole. A bond can therefore be polar while its entire molecule has no permanent net dipole. The selected arrows point toward the negative ends as a classroom convention; a quantitative dipole calculation needs the actual charge distribution rather than these unscaled arrows alone.

In the symmetric linear molecule, equal opposing contributions cancel. Geometry is needed along with bond polarity.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

A molecule has four electron domains around its center, with two bonding pairs and two lone pairs. Predict electron-domain geometry and molecular shape; explain why bond angles need not be 109.5°.
Answer and explanation

The electron-domain arrangement is approximately tetrahedral and the molecular shape is bent. Lone-pair interactions and the actual chemical species alter angles; 109.5° is an ideal tetrahedral reference rather than an exact universal bond angle.

The electron-domain arrangement is approximately tetrahedral and the molecular shape is bent. Lone-pair interactions and the actual chemical species alter angles; 109.5° is an ideal tetrahedral reference rather than an exact universal bond angle.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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