Covalent Bonding

Question 1

How are covalent molecules/compounds formed?

Answer 1

Non-metal atoms share pairs of electrons to complete both of their outer shells, making them stable

They keep the electrons so no ions are formed

Question 2

What holds the atoms in a covalent molecule together?

Answer 2

Strong intramolecular electrostatic forces between the shared pairs of electrons and the nuclei involved

Question 3

How might you draw a covalent dot and cross diagram?

Answer 3

• Draw all the atoms in the molecule, showing the outer shells of the electrons with each of the shared pair of electrons overlapping
• These molecules can be diatomic, or have as many as five atoms and can have double shared pairs in some of the bonds
• Each atom must have a full outer shell, which includes the shared pair of electrons

Question 4

Why do simple molecular structures (water, carbon dioxide etc.) have low melting and boiling points?

Answer 4

• While there are strong covalent bonds between the atoms in the substance, the individual covalent molecules that comprises the substance are only held together by weak intermolecular forces
• This is why most small covalent molecules are liquids or gas at room temperature

Question 5

What happens to the melting and boiling point of a simple covalent molecule as its size (relative molecular mass) increases and why?

Answer 5

The melting and boiling point increase as there are more electrons so more intermolecular forces which needs an increasing amount of energy to overcome

Question 6

Do covalent compounds conduct electricity and why/why not?

Answer 6

• They are usually poor conductors (and are generally insulators)
• This is because they usually have no free ions or delocalised electrons (no charged particles) which can move and carry charge because each outer shell is full

Question 7

Why do giant covalent structures have high boiling and melting points?

Answer 7

• Each atom in the structure is held together by strong covalent bonds with strong electrostatic forces of attractions which take alot of energy to break
• There are no intermolecular forces

Question 8

What is diamond and what is its structure like?

Answer 8

• Diamond is an allotrope of carbon, and is made up of only carbon
• It can have an unlimited number of carbon atoms
• Each carbon atom is bonded with four other carbon atoms
• There are no intermolecular forces

Silica (silicon dioxide) is very similar to diamond and has similar properties, but contains silicon and oxygen atoms instead of carbon

Question 9

What are the properties of diamond and why does it have these properties?

Answer 9

• It has an extremely high melting point due to strong covalent bonds and no intermolecular forces
• It is very dense and has no intermolecular forces so that makes it extremely hard
• It cannot conduct electricity because it has no free/delocalised electrons

Question 10

What is graphite and what is its structure like?

Answer 10

• Graphite is an allotrope of carbon, and is made up of only carbon where there is not a defined number of carbon atoms
• It is comprised of layers of carbon hexagons
• Each carbon atom is bonded with three other carbon atoms, which leaves one free delocalised electron
• Each layer is held together by weak intermolecular forces

Each single layer of graphite is graphene, which is 1 atom thick and extremely strong and hard

Question 11

What are the properties of graphite and why does it have these properties?

Answer 11

• It has a very high melting and boiling point due to strong covalent bonds
• It can conduct electricity due to its valence electron as each carbon atom is only bonded to three others
• It is less dense and soft and slippery due to the weak intermolecular forces between each layer of carbon

Question 12

What are fullerenes and what are there structures and properties?

With specific reference to C₆₀ fullerence

Answer 12

• They are allotropes of carbon
• C₆₀ fullerene (or buckminsterfullerene) has 60 carbon atoms all covalently bonded together to three other carbon atoms and is shaped like a football
• They are hollow in shape and have low melting and boiling points due to the weak intermolecular forces between each fullerene molecule (as they are simple covalent structures)
• They can conduct electricity as there is a free valence electron due to each carbon atom only being bonded to three others