2.2.2 Bonding And Structure

Question 1

Electron pair repulsion theory

Answer 1

• electron pairs repel one another so they are arranged as far apart as possible
• the arrangement of electron pairs minimises repulsion and holds the bonded atoms in a definite shape
• the electron pairs surrounding a central atom determine the shape of the molecule or ion

Question 2

Bond angles

Answer 2

Lone pairs repel more strongly than bond pairs, so the angle between the bonded pair is reduced by ~2.5° per lone pair
- lone pairs are closer to the central atom and occupy more space

Increasing repulsion:

1. bonded-bonded
2. lone-bonded
3. lone-lone

Question 3

Shapes

Answer 3

4 bond pairs 0 lone pairs - tetrahedral (109.5°)
3 bond pairs 0 lone pairs - trigonal planar (120°)
3 bond pairs 1 lone pair - pyramidal (107°)
2 bond pairs 2 lone pairs - non-linear (104.5°)
2 bond pairs 0 lone pairs - linear (180°)
6 bond pairs 0 lone pairs - octahedral (120°)

Question 4

Electronegativity

Answer 4

Ability of an atom to attract the bonding electrons in a covalent bond
Changes when:
- nuclear charges are different
- atoms are different sixes
- shared pair of electrons closer to one nucleus than another

Measured using Pauling electronegativity values:

• increases upwards towards H
• increases across periodic table towards F (highest value 4.0)

Question 5

Non polar bond

Answer 5

Bonded electron pair shared equally between bonded atoms
Formed when:
- bonded atoms are the same
- have same/similar electronegativity 
From pure covalent bonds

Question 7

Polar molecules

Answer 7

Molecules with 2/more polar bonds can have dipoles that can:
- reinforce each other to produce a larger dipole
- cancel out if dipoles act in opposite directions
Depending on the shape of the molecule

Polar solvents can dissolve polar/ionic solutes

Question 8

Induced dipole-dipole interactions (London forces)

Answer 8

Weak intermolecular forms existing between all molecules

• movement of electrons produce a changing dipole in a molecule
• an instantaneous dipole will exist at a moment (temporary), position constantly shifting
• induces a dipole on a neighbouring molecule
• induced dipole induced further dipoles on neighbouring molecules, which then attract one another

The more electrons in each molecule, the higher the b.p.

• larger induced dipoles
• stronger London forces between molecules that require more energy to overcome

Question 9

Permanent dipole-dipole interactions

Answer 9

Interactions between permanent dipoles in different polar molecules

Permanent dipole-dipole interactions are stronger than London Forces - higher b.p.

• extra energy is needed to break the additional permanent dipole-dipole interactions
• higher bond enthalpy

Question 10

Simple molecular substance

Answer 10

Substance made of simple molecules in a solid simple molecular lattice

• molecules held in place by weak intermolecular forces
• atoms within each molecule are bonded strongly by covalent bonds

Properties:

• low m.p. and b.p. (Weak intermolecular forces)
• non-polar simple molecular compound + non polar solvent = dissolve
• non-polar simple molecular compound + polar solvent = does not dissolve
• insulators (cannot conduct electricity)

Question 11

Polar bond

Answer 11

Bonded electron pair is shared unevenly between bonded atoms
Partial positive and negative charges separation called a dipole
- Permanent dipoles form within covalently bonded atoms with different electronegativity values

Question 12

Hydrogen bond

Answer 12

Type of permanent dipole-dipole interaction between molecules containing
- an electronegative atom with a lone pair of electrons e.g. O, N, F
- a hydrogen atom attached to an electronegative atom e.g. H—O, H—N, H—F
H bond between lone pair of electrons on an electronegative atom in one molecule and a H atom in a different molecule
Strongest type of intermolecular attraction

Question 13

Ice is less dense than water

Answer 13

4 H bonds hold water molecules apart in an open tetrahedral lattice structure with holes

• water molecules in ice are further apart than water
• solid ice is less dense than liquid water and floats

Allows ice to form an insulating layer on lakes and ponds to prevent the water from freezing solid so aquatic animals can survive

Question 14

High b.p. and m.p.

Answer 14

Water has H bonds extra to the London forces
- significant amount of energy required to overcome the attractive forces, so much higher m.p. and b.p. than just London forces
When water boils, H bonds completely break
When ice melts, rigid arrangement of H bonds in open lattice breaks

Without H bonds b.p. would be ~ -75°C (gas at RTP)
Liquid water required for aquatic life, movement in blood, solvent properties, etc.