2.2 : Electrons, bonding and structure - shapes of molecules and ions

Question 1

What is the electron pair repulsion theory?

Answer 1

The shape of a molecule/ion is determined by the number of electron pairs in the outer shell surrounding the central atom.

Since all electrons have a negative charge, each electron pair repels other electron pairs. The shape adopted will be the shape that allows the pairs of electrons to be as far apart as possible.

Question 2

What is the name of the shape and the bond angles of molecules with…?
- 2 bonding pairs
- 3 bonding pairs
- 4 bonding pairs
- 3 bonding pairs and 1 lone pair
- 2 bonding pairs and 2 lone pairs
- 5 bonding pairs
- 6 bonding pairs

Give an example for each

Answer 2

• linear, bond angle of 180, eg CO2
• trigonal planar, bond angles of 120, eg BF3
• tetrahedral, bond angles of 109.5 - eg CH4
• pyramidal, bond angles of 107 - eg NH3
• bent/non-linear, bond angle of 104.5 - eg H2O
• trigonal bipyramid, bond angles of 120 and 90, eg PCl5
• octahedral, bond angles of 90, eg SF6

Question 3

The extra repulsion from the lone pair reduces the bond angle by roughly how much?

Answer 3

2.5 degrees

Question 4

The bond angles found in ammonium are 109.5°
whereas those in ammonia are 107°. Account
for this difference.

Answer 4

• In ammonia, the electron pairs are a mixture of bonded pairs and lone pairs. - Lone pairs are more repulsive than the bonded pair and so ammonia adopts a pyramidal structure
  with smaller bond angles whereas ammonium is tetrahedral.

Question 5

What is electronegativity?

Answer 5

The ability of a bonded atom to attract the pair of electrons in a covalent bond.

Question 6

What is the most electronegative element?

Answer 6

Fluorine

Question 7

What is a permanent dipole?

Answer 7

A permanent dipole is a small charge difference across a bond that happens because of a difference in the electronegativities of the bonded atoms, a polar bond has a permanent dipole.

Question 8

Describe what is meant by a non-polar bond

Answer 8

• the two bonding atoms are identical : the nucleus of each bonded atom is equally attracted to the bonding electron pair and so the electrons in the bond are evenly distributed between the bonding atoms

Question 9

Describe what is meant by a polar bond

Answer 9

• the two bonding atoms are different so one of the atoms is likely to attract the bonding electrons more
• the bonding atom with a greater attraction for the electron pair is more electronegative
• this causes the electrons in the bond to be unevenly distributed between the bonding atoms

Question 10

What scale is used to determine electronegativity?

Answer 10

The Pauling electronegativity values

Question 11

A C=O bond is a polar bond. Why is carbon dioxide not a polar molecule?

Answer 11

The molecule is symmetrical so the dipoles cancel out.

Question 12

CCl4 is a non-polar molecule but it contains polar C-Cl bonds. How is this possible?

Answer 12

A CCl4 molecule is symmetrical so the dipoles act in different directions and cancel each other out.

Question 13

How does the size of the difference in electronegativity affect the bond formed?

Answer 13

If the difference in electronegativity is only small, the atoms share the electrons unequally and form a polar covalent bond.

The greater the difference in electronegativity, the greater the permanent dipole.

If the difference is very large, the most electronegative element may take the electron completely and form an ionic bond.