6.1 Shapes of molecules and ions

Question 1

What is the charge of an electron and what does this cause to occur?

Answer 1

Electrons are negatively charged and therefore they repel one another

Question 2

What is the electron-pair theory?

Answer 2

It is a model used in chemistry to explain and predict the shape of molecules and polyatomic ions.

• the electron pairs surrounding a central atom determine the shape of the molecule or ion.
• The electron pairs repel one another so that they are arranged as far apart as possible
• the arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape
• Different numbers of electrons pairs result in different shapes

Question 3

describe the shape of a molecule of methane, CH4

Answer 3

It is symmetrical with 4 C-H covalent bonds
Four bonded pairs of electrons surround the central carbon atom
The four electron pairs repel one another as far apart as possible in the 3D space.
The result is a tetrahedral shape with four equal C-H bond angles of 109.5o

Question 4

What does a solid line represent?

Answer 4

A bond in the plane of the paper

Question 5

HWat does a solid wedge reppresent?

Answer 5

The bond coming out of the paper

Question 6

What does a dotted wedge represent?

Answer 6

The bond goes into the paper

Question 7

What is the difference between the position of the lone pair and the bonded pair od electrons?

Answer 7

The lone pair of electrons are slightly closer to the central atom. As a result it repels more strongly.

Question 8

Which of the three repels the most and which repels the least:

Bonded-pair/Lone-pair
Lone-pair/Lone-pair
Bonded-pair/Bonded-pair

Answer 8

Repels the most:
Lone-pair/Lone-pair

Repels the least:
Bonded-pair/Bonded-pair

Question 9

Describe the structure of a tetrahedral arrangement

Answer 9

The four electron pairs around the central atoms repel one another as far apart as possible
Becuase lone pairs repel more than bonded pairs of electrons, lone pairs repel bonded pairs slightly closer together decreasing the bond angle

Question 10

What is a bond angle?

Answer 10

It is the angle between the bonded pairs of electrons

Question 11

What degree does the bond angle decrease pair extra lone pair added?

Answer 11

2.5o

Question 12

What are the bond pairs, lone pairs, bond angle and electron pairs/regions for a tetrahedral arrangement?

Answer 12

Bonded pairs:
4

Lone pairs:
0

Bond angle:
109.5o

Electron pairs/regions:
4

Question 13

What are the bond pairs, lone pairs and bond angle for a pyramidal arrangement?

Answer 13

Bonded pairs:
3

Lone pairs:
1

Bond angle:
107

Question 14

What are the bond pairs, lone pairs and bond angle for a non-linear arrangement?

Answer 14

Bonded pairs:
2

Lone pairs:
2

Bond angle:
104.5o

Question 15

What si the bond angle od a linear arrangement?

Answer 15

180o

Question 16

What is the shape of a carbon dioxide molecule and why?

Answer 16

It is linear because the 2 bonded regions repel one another as far apart as possible. This give CO2 a linear shape with 3 atoms aligned in a straight line

Question 17

What is the principle of electron-pair repulsion theory?

Answer 17

Electron pairs around the central atom repel each other as far apart as possible
The greater the number of electron pairs, the smaller the bond angle
lone pairs of electrons repel more strongly than bonded pairs of electrons

Question 18

Give an example and explain the shape of a molecule with three electron pairs (include the bond angle)

Answer 18

Boron trifluoride (BF3) has only 3 bonded pairs around the central Boron atom. Electron-pair repulsion gives a trigonal planar shape with equal bond angles of 120o

Question 19

Give an example and explain the shape of a molecule with six electron pairs (include the bond angle)

Answer 19

Sulfide hexafluoride (SF6) has 6 bonded pairs of electrons around the central sulfur atom. Electron-pair repulsion gives an octahedral shape with equal bond angles of 90o

Question 20

What are the electron pairs and bond angle of a linear shape?

Answer 20

Electron pairs/regions:
2
Bond angle:
180o

Question 21

What are the electron pairs and bond angle of a trigonal planar shape?

Answer 21

Electron pairs/regions:
3
Bond angle:
120o

Question 22

What are the electron pairs and bond angle of a tetrahedral shape?

Answer 22

Electron pairs/regions:
4
Bond angle:
109.5o

Question 23

What are the electron pairs and bond angle of an octahedral shape?

Answer 23

Electron pairs/regions:
6
Bond angle:
90o

Question 24

How is an octahedral shape obtained?

Answer 24

By joining together six corners occupied in a fluorine atom

Question 25

Describe the structure, shape and bond angle of an ammonium ion.

Answer 25

the ammonium ion (NH4+), ahs four bonded pairs surrounding the central nitrogen atom.
An NH4+ ion has the same number od bonded pairs of electrons around the central atoms as a methane molecule
It ahs the same tetrahedral shape and bond angle of 109.5o as a methane molecule

Question 26

Describe the structure, shape and bond angle of a CO3 2- ion.

Answer 26

It has three regions of electron density surrounding the centre carbon atom.
It has a trigonal planar shape with an angle of 120o

Question 27

Describe the structure, shape and bond angle of a NO3 - ion.

Answer 27

It has three regions of electron density surrounding the central nitrogen atom.
It has a trigonal planar shape with an angle of 120o

Question 28

Describe the structure, shape and bond angle of a SO4 2- ion.

Answer 28

It has four centres of electron density around the central sulphur atoms
It has a tetrahedral shape with a bond of 109.5o