Module 2: Shapes Of Molecules And Intermolecular Forces (chapter 6)

Question 1

What is electron pair repulsion theory?

Answer 1

The shape of the molecule is determined by the number of electron pairs surrounding the central atom.
Electrons repel each other due to being negatively charged.
Electrons arrange themselves as far apart in space as possible.

Question 2

What is the difference between lone pairs and bonding electrons?

Answer 2

Lone pairs repel more that bonded pairs, meaning bond angles are smaller with lone pairs. This is because lone pairs are closer to the central atom and occupy more space than a bonded pair.
Lone/lone - bonded/lone - bonded/bonded

Question 3

Describe a linear shaped molecule

Answer 3

2 bonding pairs and 0 lone pairs.
180º bond angle
E.g. CO2

Question 4

Describe a trigonal planar shaped molecule

Answer 4

3 bonded pairs and 0 lone pairs.
120º bond angles.
E.g. BF3

Question 5

Describe tetrahedral shaped molecules

Answer 5

4 bonding pairs and 0 lone pairs.
109.5º bond angles.
E.g. CH4

Question 6

Describe trigonal bipyramidal shaped molecules

Answer 6

5 bonding pairs and 0 lone pairs.
90º and 120ºbond angles.
E.g. PCl5

Question 7

Describe octahedral shaped molecules

Answer 7

6 bonding pairs and 0 lone pairs.
90º bond angles.
E.g SF6

Question 8

Describe trigonal pyramidal shaped molecules.

Answer 8

3 bonding pairs and 1 lone pair.
107º bond angles.
E.g. NH3

Question 9

Describe non-linear shaped molecules

Answer 9

2 bonding pairs and 2 lone pairs.
104.5ºbond angles.
E.g. H2O

Question 10

How much is the bond angle reduced by for each lone pair?

Answer 10

2.5 degrees

Question 11

Define electronegativity

Answer 11

A measure of the tendency of an atom to attract a bonding pair of electrons in a covalent bond

Question 12

What affects electronegativity?

Answer 12

Nuclear charge, atomic radius and shielding.

Question 13

What happens with increasing electronegativity?

Answer 13

There is greater attraction of the electrons towards the atom with greater electronegativity.

Question 14

How is electronegativity compared?

Answer 14

Pauling scale

Question 15

What is the most electronegative element? What is the trend in the periodic table?

Answer 15

Fluorine

Up and across to the right

Question 16

What happens between bonds of identical atoms?

Answer 16

The atoms share the same electronegativity, so the electrons are shared equally. E.g. hydrogen and chlorine.

Question 17

What term is given to molecules of elements that come form the same element and the electron pair is shared equally?

Answer 17

Pure covalent bond

Question 18

What term is said about the electronegativity difference between carbon and hydrogen?

Answer 18

Negligible difference

Question 19

How are polar bonds formed? Give an example

Answer 19

2 atoms have differences in electronegativity. The electrons are attracted to the atom with greater electronegativity. The bond becomes polarised, creating small partial charges. The charge difference is called a permanent dipole. E.g. HCl - hydrogen is electron deficient in this bond, due to being less electronegative.

Question 20

Define dipole

Answer 20

The separation of opposite charges

Question 21

How can types of bonding be determined by electronegativity?

Answer 21

No difference = non-polar covalent
Small difference = polar covalent 
Large difference (+1.8) = ionic

Question 22

What is the trend in electronegativity across a period? Why?

Answer 22

Increases due to increased nuclear charge coming into effect, whilst shielding and atomic radius remains relatively similar. E.g. Na to Cl are all n=3 (similar atomic radius) and have 1s 2s and 2p electrons.

Question 23

What is the trend in electronegativity down a group?

Answer 23

Decreases due to increased atomic radius ad shielding, with nuclear charge remaining the same.
F - At the bonding pair is shielded from the fluorines nucleus of 1s2.