2.2.9 Shapes of Molecules and Ions

Question 1

What is “electron pair repulsion theory”?

Answer 1

Each electron pair repels other electron pairs
The shape adopted by a simple molecule or ion will be:
1. The shape which keeps repulsive forces to a minimum.
2. The shape which allows electrons to be as far apart as possible.

Question 2

What is a “bonding region,” why has it gained this terming?

Answer 2

• The region of space where a bond (double or triple) is fixed in the space between the central atom and the other bonded atom.

Question 3

Give two examples of linear molecules, bond angles and numbers of bonded electron pairs around the central atom.

Answer 3

Hydrogen: one bonded electron pair

Carbon dioxide: 180 degree bonding angles, two bonded electron pairs

Question 4

What does a wedged line symbolise?

Answer 4

The wedge indicates a 3D model of the atom, showing an electron pair aimed towards the viewer.

Question 5

What does a dashed projection line with six marks mean?

Answer 5

A dash projection indicates a 3D model of the atom, showing an electron pair aimed away from the viewer.

Question 6

Name the: 
A) Shape 
B) Number of bonded electron pairs 
C) Bonding angles
D) Number of planes bonded on of:

CH₄

Answer 6

A) Tetrahedral
B) Four
C) 109.5 degrees
D) Three planes (two normal, one H on wedged and one on dashy plane)

Question 7

Name the: 
A) Shape 
B) Number of bonded electron pairs 
C) Bonding angles
D) Number of planes bonded on of:

PCl₅

Answer 7

A) Trigonal pyramid
B) Five
C) 90 degrees and 120 degrees
D) Three on one plane, at 90 degrees and two on the other planes, at 120 degrees

Question 8

Name the: 
A) Shape 
B) Number of bonded electron pairs 
C) Bonding angles
D) Number of planes bonded on of:

PCl₅

Answer 8

A) Trigonal bipyramid
B) Five
C) 90 degrees and 120 degrees
D) Three on one plane, at 90 degrees and two on the other planes, at 120 degrees

Question 9

Name the: 
A) Shape 
B) Number of bonded electron pairs 
C) Bonding angles
D) Number of planes bonded on of:

SF₆

Answer 9

A) Octahedral
B) Six
C) 90 degree bonding angles
D) Two on each plane, at 90 degrees

Question 10

Why are lone pairs of electrons repel more than bonded pairs? What effect does this have on the molecule?

Answer 10

1. A lone pair of electrons is more electron-dense than a bonded pair. Therefore, a lone pair repels more than a bonded pair.
2. Each lone pair reduces the bond angle by approx. 2.5 degrees (because each lone pair has an extra repulsive effect)
3. If 1/4 areas of electrons is a lone pair, the tetrahedral shape becomes pyramidal. If this increases to 2/4, the shape becomes non-linear.

Question 11

How do lone pairs impact bonding angles between molecules? Give examples.

Answer 11

1. Tetrahedral Methane had bond angles of 109.5 degrees
2. Pyramidal Ammonia has bond angles of 107 degrees
3. Non-linear water had a bond angle of 104.5 degrees

Question 12

If an atom has four electron pairs, what can you tell about its structure?

Answer 12

It is tetrahedral

Question 13

If an atom has three electron pairs, what can you tell about its structure?

Answer 13

It is trigonal planar

Question 14

If an atom has five electron pairs, what can you tell about its structure?

Answer 14

It is trigonal bipyramidal

Question 15

If an atom has six electron pairs, what can you tell about its structure?

Answer 15

It is octahedral

Question 16

How do you show charge in a atom like this?

Answer 16

Square brackets