Simple molecular and giant covalent structuresIB MYP Chemistry: Revision notes
Section 1
Simple molecular structures
In a simple molecular substance, the atoms in each molecule are joined by strong covalent bonds, but the molecules themselves are held to each other by weak intermolecular forces. Examples are water, carbon dioxide, methane, iodine and chlorine.
When a simple molecular substance melts or boils, only the weak intermolecular forces are overcome. The covalent bonds inside the molecules do not break. Little energy is needed, so these substances have low melting and boiling points and are often gases or liquids at room temperature.
They do not conduct electricity because they have no ions and no free electrons.
Never say the covalent bonds break when a simple molecular substance melts or boils. The weak forces between the molecules are overcome.
Section 2
Giant covalent structures
In a giant covalent structure (also called a macromolecule) a huge number of atoms are joined by covalent bonds in a continuous network. There are no separate small molecules.
Because many strong covalent bonds must be broken to melt it, a giant covalent substance has a very high melting point. Diamond, graphite and silicon dioxide are the three you need to know.
Section 3
Diamond
Diamond is a form of carbon. Each carbon atom forms four covalent bonds with other carbon atoms in a rigid three-dimensional network.
- Very hard, because the strong covalent bonds in all directions are hard to break. It is used on cutting tools and drill bits.
- Very high melting point, because a lot of energy is needed to break the bonds.
- Does not conduct electricity, because all four outer electrons are used in bonds, so there are no free electrons.
Section 4
Graphite
Graphite is another form of carbon. Each carbon atom forms three covalent bonds, giving flat layers of atoms. The fourth outer electron of each atom is delocalised (free to move).
- Conducts electricity, because the delocalised electrons can move through the layers and carry charge.
- Soft and slippery, because there are only weak forces between the layers, so they slide over each other. It is used as a pencil lead and a lubricant.
- Very high melting point, because the covalent bonds within the layers are strong.
Graphite and diamond are both pure carbon. They differ only in how the atoms are bonded, so they have very different properties.
Section 5
Silicon dioxide
Silicon dioxide, SiO₂, is the main compound in sand and quartz. Each silicon atom is bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms in a giant covalent structure. The ratio of silicon to oxygen is 1 : 2, giving the formula SiO₂.
- Very high melting point (about 1700 °C) and very hard, because many strong covalent bonds must be broken.
- Does not conduct electricity, because it has no ions and no free electrons.
Must know
- Simple molecules: low melting point because the weak forces between molecules are overcome. They do not conduct.
- Giant covalent: high melting point because many strong covalent bonds are broken.
- Diamond: four bonds per carbon; very hard; no conduction.
- Graphite: three bonds per carbon; layers; delocalised electrons conduct; soft.
- Silicon dioxide: giant structure; very hard; no conduction.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Simple molecular and giant covalent structures
- A technician in Dubai gently warms a few crystals of iodine, I₂, in a fume cupboard. The grey-black solid easily turns into a purple vapour at a temperature far below 200 °C. Solid iodine does not conduct electricity.Explain why iodine changes into a vapour at such a low temperature.2 marks
- A jeweller in Antwerp compares a diamond with a piece of graphite from a pencil. Both are made of carbon atoms only, but diamond is the hardest natural material and graphite is soft and slippery.Explain why diamond is so hard.2 marks
- A student tests three unknown solids, P, Q and R. P melts at 114 °C and does not conduct electricity when solid or molten. Q is very hard, melts at about 1700 °C and does not conduct electricity in any state. R is a soft black solid that does not melt even above 3000 °C and conducts electricity when solid. The student predicts that any solid with a high melting point will conduct electricity.Identify the type of structure of each of P, Q and R, choosing from simple molecular, a giant covalent structure like silicon dioxide, and a giant covalent structure like graphite.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).