6 Shapes of Molecules and Intermolecular Forces

Question 1

Describe the

“electron-pair repulsion theory”.

Answer 1

Electron-pair repulsion theory is a model used in chemistry for exploaining and predicting the shapes of molecules and polyatomic ions.

• Electron pairs around a central atom determine the shape
• Electron pairs repel each other so arrange as far apart as possible.
• The minimium repulsion is the most stable so it gives a definite shape.
• Different numbers of electron pairs result in different shapes.
• Known as VSEPR (Valence shell electron pair repulsion) but this is not needed at A level.

Question 2

Explain the shape of a molecule of methane.

Answer 2

• CH₄ is symmetrical with four C-H covalent bonds.
• There are four bonded pairs of electrons around the central atom.
• The four electrons pairs repel each other.
• The furthest apart they can get results in a tetrahedral shape.
• Tetrahedral shape means there will be bond angles of 109.5°

Question 3

Which occupies more space: a lone pair of electrons or a covalently bonded pair of electrons?

Answer 3

A lone pair takes up more space.

It is also slightly closer to the central atom.

Question 4

Lone pairs repel bonded pairs slightly _____________, which ______________ the bond angle.

Answer 4

Lone pair repel bonded pairs slightly closer together, which decreases the bond angle.

Question 5

How much does the bond angle decrease in the presence of a lone pair?

Answer 5

2.5° for each lone pair.

Question 6

Give the shape and bond angle for CO₂

Answer 6

Linear and 180°

Question 7

The greater the number of electron pairs around the central atom, the ______________ the bond angle.

Answer 7

Smaller

Question 8

Give the formula, bond angles, and molecular shape that arises when boron combines with fluorine.

Answer 8

BF₃, trigonal planar, 120°