Chapter 6 (2.2.2)

Question 1

What is the electron pair repulsion theory?

Answer 1

• explains the shapes of molecules + polyatomic ions
• electron pairs surrounding a central atom determine the shape of the molecule or ions by repelling one another so they are arranged as far apart as possible
• arrangement minimises repulsion + thus holds the atoms in a definite shape

Question 2

Wedges for 3D shapes

Answer 2

solid line: bond in plane of paper
solid wedge: comes out of plane of paper
dotted wedge: into the plane of paper

Question 3

Bond and lone pair repulsions

Answer 3

• lone pairs are slightly closer to the central atom + occupy more space than a bonded pair
• thus a lone pair repels more strongly than a bond pair
• bonded/bonded < bonded/lone < lone/lone (relative strength of repulsions)

Question 4

What does adding lone pairs do?

Answer 4

since lone pairs repel more strongly than bonded pairs, they repel the bonded pairs slightly closer together, decreasing the bond angle by 2.5° for each lone pair.

Question 5

What are the molecular shapes + bond angles for methane, ammonia, water?

Answer 5

Methane: tetrahedral, 109.5°
Ammonia: Pyramidal, 107°
Water: Non-linear (bent) 104.5°

Question 6

What happens to the molecular shape with multiple bonds?

Answer 6

• each multiple bond is treated as a bonding region

- e.g. 2 double bonds = 2 bonding regions

Question 7

carbon dioxide, CO2

Answer 7

linear, 180°

Question 8

boron trifluoride, BF3

Answer 8

trigonal planar, 120°

Question 9

phosphorus pentachloride, PCl5

Answer 9

trigonal bipyramidal, some 120° and some 90°

Question 10

sulfur hexafluoride, SF6

Answer 10

octahedral, 90°

Question 11

ammonium, NH4+

Answer 11

tetrahedral, 109.5°

Question 12

carbonate, CO32-

Answer 12

trigonal planar, 120°

Question 13

nitrate, NO3-

Answer 13

trigonal planar, 120°

Question 14

sulfate, SO42-

Answer 14

tetrahedral, 109.5°

Question 15

What is the basis for electronegativity?

Answer 15

• in bonds like H2 or O2, the atoms are the same element and the bonded electron pair is shared evenly
• this changes when the atoms are different elements: since nuclear charges are different, atoms may be different sizes, the shared pair may be closer to one nucleus than the other
• shared pair may experience more attraction from one atom

Question 16

What is electronegativity?

Answer 16

The ability of an atom to attract the bonding electrons in a covalent bond

Question 17

Measure of electronegativity

Answer 17

The Pauling scale

Question 18

Describe the Pauling scale values

Answer 18

Across the periodic table:
- nuclear charge increases
- atomic radius increases
Thus are more electronegative 
Noble gases not included

Question 19

Patterns on electronegativity

Answer 19

• non metals: N, O, F, Cl = most electronegative

- G1 metals: Li, Na, K = least electronegative

Question 20

bond types based on electronegativity difference

Answer 20

covalent = 0
polar covalent = 0-1.8
ionic: >1.8

Question 21

When does a bond become ionic?

Answer 21

If the EN difference is large, one bonded atom will have a much greater attraction for the shared pair than the other bonded atom. the more electronegative atom will have gained control of the electrons + the bond becomes ionic.

Question 22

Describe non-polar bonds

Answer 22

• electron pair shared equally between the bonded atoms
• occurs when atoms have the same or similar electronegativity
• when bonded atoms come from the same element it is a pure covalent bond
• non-polar solvents do not mix with water

Question 23

Describe polar bonds

Answer 23

• electron pair shared unequally
• occurs when bonded atoms are different + have different EN values - polar covalent bond
• a dipole in a polar covalent bond does not change + is a permanent dipole

Question 24

What is a dipole

Answer 24

The separation of opposite charges

Question 25

What makes a molecule polar?

Answer 25

• if there are 2 or more polar bonds, their direction is important
• depending on the shape of the molecule, the dipoles may reinforce one another to produce a larger dipole - making the molecule polar
• or cancel out if the dipoles act in opposite directions - making the molecule non-polar

Question 26

What are London forces?

Answer 26

• induced dipole - dipole interactions
• weak intermolecular forces that exist between all molecules, whether they are polar or non-polar
• act between induced dipoles in different molecules

Question 27

The origin of induced dipoles

Answer 27

• quick movement of electrons means that at any instant, the electrons in an atom are likely to be more to one side than the other. thus an instantaneous dipole will exist, but will be constantly shifting
• this dipole induces a dipole on a neighbouring atom, the 2 are then attracted
• the induced dipole induced further dipoles on neighbouring molecules, which then attract one another

Question 28

Overall effect of London forces

Answer 28

• induced dipoles are temporary
• electrons are constantly moving so dipoles are being created + destroyed all the time
• even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other

Question 29

Why does boiling point increase down a group?

Answer 29

down a group there are more electrons meaning:
- larger instantaneous + induce dipoles
- greater induced dipole-dipole interactions
- stronger attractive forces between molecules
Thus more energy needed so higher boiling point

Question 30

When do permanent dipole-dipole interactions occur?

Answer 30

• they act between permanent dipoles in different polar molecules
• in addition to London forces

Question 31

Boiling points in polar molecules

Answer 31

polar molecules have additional permanent dipole-dipole interactions between molecules so extra energy needed to break them, thus higher boiling points

Question 32

What is a hydrogen bond?

Answer 32

Special type of permanent dipole-dipole interactions found between molecules containing:
- an electronegative atom with a lone pair of electrons, N, O, F
- a hydrogen atom attached to an electronegative atom
Strongest type of intermolecular attractions

Question 33

What does a hydrogen bond act between?

Answer 33

A lone pair of electrons on an electronegative atom in one molecule + a hydrogen atom in a different molcule