Covalent Substances

Question 1

simple molecular substances

Answer 1

• the atoms within a molecule are held together by very strong covalent bonds
• however, the intermolecular forces of attraction between the molecules are very weak
• this means the melting and boiling points are very low, because the molecules are easily separated

Question 2

why are intermolecular forces stronger between molecules with a high relative molecular mass than between smaller molecules?

Answer 2

• this is because there are more points along the larger molecules for intermolecular forces to act between them, so more energy is needed to break the forces
• due to the increasing strength of the forces, the melting and boiling points of simple molecular substances increase as the relative molecular mass increases
• most molecular substances are gases or liquids at room temperature

Question 3

giant covalent structuress

Answer 3

• giant covalent structures are similar to giant ionic structures except they don’t have any charged ions
• all of the atoms are bonded to each other by strong covalent bonds
• there are lots of these bonds which means it takes a lot of energy to break them, so giant covalent structures are solids with very high melting and boiling points
• they do not conduct electricity, not even when molten (except graphite)
• they are usually insoluble in water
• examples include diamond and graphite, which are made only from carbon atoms

Question 4

can simple molecular substances conduct electricity?

Answer 4

no - because there molecules are neutral and charged particles are required to conduct electricity

Question 5

diamond

Answer 5

• giant covalent structure
• diamond is made up of a network of carbon atoms that each form 4 covalent bonds
• the strong covalent bonds take lots of energy to break, so diamond has a high melting point
• the strong covalent bonds hold the atoms in a very rigid lattice structure, so it is really hard
• it doesn’t conduct electricity because it has no free electrons or ions

Question 6

graphite

Answer 6

• giant covalent structure
• in graphite, each carbon atom only forms 3 covalent bonds, creating layers of carbon atoms
• the layers are only held together weakly by intermolecular forces, so they are free to slide over each other - this makes graphite soft and slippery
• graphite has a high melting point - the covalent bonds in the layers need lots of energy to break
• only 3 out of each carbon’s 4 outer electrons are used in bonds, so each carbon atom has one electron that is delocalised and can move - so graphite is a non-metal that conducts electricity

Question 7

C60 fullerene

Answer 7

• simple molecular substance
• C60 fullerene molecules are hollow spheres made up of 60 carbon atoms
• unlike diamond and graphite, C60 isn’t a giant covalent structure - it is just made up of large covalent molecules
• the C60 molecules are only held together by intermolecular forces and so they can slide over each other - this means the material is soft
• like graphite, each carbon in C60 fullerene has 1 delocalised electron - however, the electrons can’t move between the molecules, so C60 fullerene is a poor conductor of electricity