Chapter 6 - Shapes of Molecules and Intermolecular Forces

Question 1

What does molecular shape depend on?

Answer 1

Electron pairs around the central atom.

Question 2

In what way do electron pairs interact?

Answer 2

They repel each other.

Question 3

Which type of pair repels the most?

Answer 3

Lone pairs

Question 4

What is the order of pair repulsion, from strongest to weakest?

Answer 4

Lone pair/lone pair
Lone pair/bonding pair
Bonding pair/bonding pair

Question 5

What is the name of the way to predict a molecule’s shape?

Answer 5

Electron pair repulsion theory

Question 6

What shape name is given to a molecule with no lone pairs?

Answer 6

Tetrahedral

Question 7

What is the bond angle in a tetrahedral molecule?

Answer 7

109.5

Question 8

What shape name is given to a molecule with one lone pair?

Answer 8

Trigonal pyramidal

Question 9

What is the bond angle in a trigonal pyramidal molecule?

Answer 9

107

Question 10

What shape name is given to a molecule with two lone pairs?

Answer 10

Non-linear

Question 11

What is the bond angle in a non-linear molecule?

Answer 11

104.5

Question 12

What shape name is given to a molecule with 2 electron pairs around the central atom?

Answer 12

Linear

Question 13

What is the bond angle in a linear molecule?

Answer 13

180

Question 14

What shape name is given to a molecule with 3 electron pairs around the central atom?

Answer 14

Trigonal planar

Question 15

What is the bond angle in a trigonal planar molecule?

Answer 15

120

Question 16

What shape name is given to a molecule with 5 electron pairs around the central atom?

Answer 16

Trigonal bipyramidal

Question 17

What is the bond angle in a trigonal bipyramidal molecule?

Answer 17

There are actually two, 120 in one plane and 90 in the other plane.

Question 18

What shape name is given to a molecule with 6 electron pairs around the central atom?

Answer 18

Octahedral

Question 19

What is the bond angle in an octahedral molecule?

Answer 19

90

Question 20

What is electronegativity?

Answer 20

An atom’s ability to attract the electron pair in a covalent bond.

Question 21

How is electronegativity measured?

Answer 21

On the Pauling scale.

Question 22

What does a higher Pauling value mean?

Answer 22

A higher electronegativity and thus a greater attraction for an electron pair in a covalent bond.

Question 23

What makes a bond polar?

Answer 23

Two atoms with electronegativities differing by at least 0.5, the bonding electrons are pulled towards the more electronegative atom, making it polar.

Question 24

What does a polar bond form?

Answer 24

A permanent dipole

Question 25

How is a dipole formed?

Answer 25

A difference in charge between the two atoms, caused by a shift in electron density in the bond.

Question 26

Are diatomic gases polar or non-polar, why?

Answer 26

Non-polar

Because the atoms have equal electronegativities and so the electrons are equally attracted to both nuclei.

Question 27

What happens if the polar bonds are arranged symmetrically in a molecule?

Answer 27

The dipoles cancel each other out, so the molecule has no overall dipole and is non-polar.

Question 28

What happens if the polar bonds are arranged unsymmetrically in a molecule?

Answer 28

Uneven distribution of charge and the molecule will have an overall dipole.

Question 29

What can electronegativity be used to predict about bonding?

Answer 29

Whether it will be covalent or ionic bonding.

Question 30

What is the general strength of intermolecular forces?

Answer 30

Very weak

Question 31

What is the order of the intermolecular forces from strongest to weakest?

Answer 31

Hydrogen bonding
Permanent dipole-dipole interactions
Induced dipole-dipole

Question 32

What are induced dipole-dipole forces?

Answer 32

Fluctuation in the electron density around a molecule creates an instantaneous dipole in a molecule. The instantaneous dipole induces a dipole in a neighbouring molecule.

Question 33

What is the effect of stronger induced dipole-dipole forces?

Answer 33

Higher boiling points

Question 34

Why are higher boiling points brought about?

Answer 34

Larger molecules have larger electron clouds so stronger induced dipole-dipole forces.
Boiling point is overcoming the intermolecular forces so stronger ones means a higher boiling point.

Question 35

How are permanent dipole-dipole interactions labelled?

Answer 35

With charges of delta+ and delta- on the atoms.

Question 36

When can hydrogen bonding occur?

Answer 36

When hydrogen is covalently bonded to fluorine, nitrogen or oxygen.

Question 37

Why are hydrogen bonds brought about?

Answer 37

Hydrogen has a high charge densiy and F, N and O are very electronegative. The bond is so polarised that a weak bond forms between the hydrogen of one molecule and a lone pair on a neighbouring molecule’s F, N or O.

Question 38

State and explain two anomalous properties of water.

Answer 38

• Ice is less dense than liquid water because hydrogen bonds hold molecules apart in open lattice structure
• Higher melting and boiling points than expected because a lot of energy is needed to overcome hydrogen bonds

Question 39

Explain why simple molecular compounds have low melting and boiling points.

Answer 39

Weak intermolecular forces are broken by the energy present at low temperatures

Question 40

Explain why simple molecular compounds do not usually dissolve readily in water.

Answer 40

There is little interaction between the molecules in the lattice and the polar solvent molecules

Question 41

How do intermolecular forces explain simple covalent compounds not conducting electricity?

Answer 41

Overall covalent molecules are uncharged, permanent dipoles are not strong enough.

Question 42

What happens to nuclear charge and atomic radius across the periodic table?

Answer 42

• The nuclear charge increases

- The atomic radius decreases