1.2.2 - Bonding and Structure

Question 1

Both graphite and diamond have a giant covalent structure made of carbon atoms. Explain why graphite can conduct electricity but diamond cannot?

Answer 1

Graphite has hexagonal layers with delocalised electrons between these layers. This means it can conduct electricity as the delocalised electrons are free to carry the current.

Diamond has a tetrahedral structure, meaning the electrons are not delocalised and cannot carry the flow of electricity.

Question 2

As you go down the elements in the noble gas group in the Periodic table, the boiling point increases. Explain why this happens using your knowledge of intermolecular forces.

Answer 2

• As you go down the group, the number of electrons increase.
• This means the van der Waals’ forces increase because a larger dipole is induced, creating a greater attraction between each molecule.
• This means more energy is required to break these van der Waals’ forces apart, thus a higher boiling point.

Question 3

The shape of a simple molecule is determined by the ______ between _______ pairs surrounding the _______ _______.

Answer 3

The shape of a simple molecule is determined by the **repulsion **between electron pairs surrounding the central atom.

Question 3

Describe the structure of NaCl.

Answer 3

Structure of NaCl - is a giant ionic lattice structure where there is the attraction between positive metal ions and negative non-metal ions.

Question 4

Define what an intermolecular force is and list the three common types of intermolecular forces.

Answer 4

Intermolecular force - an attractive force between neighbouring molecules.

Three common types include:

• Hydrogen bonds
• Permanent dipole-dipole
• Van der Waals’ forces

Question 5

Explain why a HCl molecule is a polar molecule.

Answer 5

• The Cl atom = more electronegative.
• Cl atom = has a greater attraction for the electron pairs.
• Cl atom = bonding electrons are closer so it has a slight negative charge.
• This creates a permanent dipole where the H atom has a slight positive charge.
• This results in a polar bond.

Question 6

Define the term ‘covalent bonding’.

Answer 6

Covalent bonding - A bond formed by the sharing pairs of electrons.

Question 6

Construct a dot-and-cross diagram for NH₄⁺.

What bonds are present in this molecule?

Answer 6

There are three covalent bonds and one dative covalent bond.

Question 7

Explain how hydrogen bonding accounts for the fact that ice is less dense than water.

Answer 7

• Ice has an open lattice with hydrogen bonds holding the water molecules apart. This means the molecules cannot align themselves like they would in a normal solid and are less dense.
• When it melts, the rigid hydrogen bonds collapse, allowing the water molecules to move closer together.

Question 8

Describe what causes a hydrogen bond to arise.

Answer 8

Hydrogen bonding - is a strong dipole-dipole attraction between:

• an electron deficient hydrogen atom (O-H^σ+, N-H^σ+) on one molecule
• and a lone pair of electrons of a highly electronegative atom from another molecule.

Question 8

Explain how hydrogen bonding accounts for water’s relatively high boiling point.

Answer 8

• The hydrogen bonds are strong.
• They are also extra forces of attraction in addition to the van der Waals’ forces.
• This means more energy is required to break these forces apart, thus a higher boiling point.

Question 9

State the formulae and the charges for the following ions:

• Nitrate
• Carbonate
• Sulfate
• Ammonium.

Answer 9

• Nitrate - NO₃^-
• Carbonate - CO₃^2-
• Sulfate - SO₄^2-
• Ammonium - NH₄⁺

Question 10

1. Name of the shape of this molecule:

It has 4 pairs of electrons surrounding the central atom. 3 of the pairs are bonded electrons whilst the 1 remaining pair is a lone pair.

2. Give the bond angle of this shape.

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Answer 10

1. Pyramidal.
2. The bond angle - 107^o.

Question 11

Define the term ‘metallic bonding’.

Answer 11

Metallic bonding - the electrostatic attraction of positive metal ions and delocalised electrons.

Question 13

Rearrange the order of the types of elecron pairs in terms of how much they repel each other. Start with the smallest repulsion first:

• Bonded pair with another bonded pair.
• Lone pair with another lone pair.
• Bonded pair with a lone pair.

Answer 13

• Bonded pair with another bonded pair will repel each other the least.
• Bonded pair with a lone pair.
• 2 lone pairs of electrons will repel each other the most.

Question 14

CH₄ has a tetrahedral shape.

1. What does this indicate about the number of electron pairs surrounding the central atom and the bond angle?
2. Suggest another molecule that has this shape.

Answer 14

• Tetrahedral - This means it has 4 bonded electron pairs surrounding the central atom and no lone pairs. It also means each bond angle is at 109.5^o
• Another molecule with the same shape - NH₄

Question 15

Construct a dot-and-cross diagram of a BF₃ molecule.

State what shape this molecule is, thus giving the bond angle and give reasons for your answer.

Answer 15

• BF₃ - is a trigonal planar with 120^o. This is because it has:
• 3 bonded pairs of electrons surrounding the central atom.
• No lone pairs.

Question 16

Define the term ‘electronegativity’.

Answer 16

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

Question 18

Construct a dot-and-cross diagram for the bonding in N_2.

What bond is this?

Answer 18

This bond is a triple bond of covalent bonds.

Question 19

Define the term ‘ionic bonding’.

Answer 19

Ionic bonding - the electrostatic attraction between oppositely-charged ions.

Question 20

Describe the structure of a diamond.

Answer 20

Structure of a diamond - is a giant covalent structure (a tetrahedral structure) held together by strong covalent bonds throughout the lattice.

Question 21

Describe the structure of graphite.

Answer 21

Structure of graphite - is a giant covalent structure consisting of strong hexagonal layers with weak van der Waals’ forces between each layer. There are also delocalised electrons between the layers.

Question 23

What determines the fact that a SF₆ molecule is an octahedral?

Answer 23

• Octahedrals have 6 bonded pairs of electrons surrounding the central atom and no lone pairs.
• This creates bond angles at 90^o each.

Question 24

1. What is the shape of a H₂O molecule? Give reasons for your answer.
2. Why does this shape differ from a CO₂ molecule?

Answer 24

1. The shape of a H₂O molecule = non-linear. This is because it has 2 bonded pairs of electrons surrounding the central atom and 2 lone pairs, creating a bond angle of 104.5^o.
2. This differs from the shape of a CO₂ molecule because the shape of a CO₂ molecule is linear. This means it has 2 bonding regions but no lone pairs, giving a bond angle of 180^o.

Question 25

Describe how van der Waals’ forces arise.

Answer 25

• Van der Waals’ forces are caused by the movement of electrons within the shells.
• An uneven distribution of electrons causes an instantaneous dipole across a molecule.
• This induces a dipole in neighbouring molecules.
• The small induced dipoles attract causing van der Waals’ forces.