Covalent bondingIB MYP Sciences: Revision notes
Section 2
Dot-and-cross diagrams for molecules
Draw the outer shells overlapping, with dots for one atom's electrons and crosses for the other's. Shared electrons go in the overlap.
- H₂: one shared pair
- Cl₂: one shared pair, and each Cl has three non-bonding pairs
- H₂O: oxygen shares two pairs, one with each hydrogen, and has two non-bonding pairs
- CO₂: two double bonds, O=C=O, so four electrons are shared in each
- CH₄: four single bonds, one to each hydrogen
- NH₃: three single bonds and one non-bonding pair on nitrogen
Check that every atom has a full outer shell: hydrogen has 2 and the others have 8.
Count the electrons. In a correct diagram, every atom except hydrogen has eight outer electrons, shared and unshared together.
Section 3
Simple molecular substances
Substances made of small molecules, such as water, carbon dioxide and methane, have low melting and boiling points. The strong covalent bonds are inside the molecules, but the forces between molecules are weak, so little energy is needed to separate them.
They do not conduct electricity, because they have no ions and no free electrons.
When a simple molecular substance melts or boils, the covalent bonds do not break. Only the weak forces between molecules are overcome.
Section 4
Giant covalent structures
In a giant covalent structure, a huge number of atoms are joined by covalent bonds in a continuous network.
- Diamond: each carbon atom forms four bonds. Very hard, very high melting point, does not conduct. Used on cutting tools and drill bits.
- Graphite: each carbon atom forms three bonds in flat layers. One outer electron per atom is free (delocalised), so it conducts. The layers slide, so it is soft and slippery. Used in pencils, lubricants and electrodes.
- Silica (SiO₂): each silicon atom is bonded to four oxygen atoms. Hard, very high melting point, does not conduct. Used to make glass and as sand in building.
Section 5
Why the structure explains the properties
Giant covalent structures have very high melting points because a lot of energy is needed to break so many strong bonds. Simple molecules have low melting points because only weak forces between molecules must be overcome.
Compare the two forms of carbon: diamond has no free electrons, so it does not conduct. Graphite has delocalised electrons, so it does.
Must Know
- Covalent bonds are shared pairs of electrons between non-metal atoms.
- Simple molecules: low melting and boiling points, do not conduct.
- Giant covalent structures: diamond, graphite and silica have high melting points.
- Graphite conducts because of delocalised electrons. Diamond and silica do not conduct.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Covalent bonding
- Chlorine gas, Cl₂, is added to the water in public swimming pools in Singapore to kill bacteria. A chlorine atom has seven electrons in its outer shell, and a molecule of chlorine contains two chlorine atoms joined by a covalent bond.Describe, in terms of outer electrons, how the covalent bond forms between two chlorine atoms.2 marks
- A fertiliser factory in Gujarat makes ammonia, NH₃, from nitrogen and hydrogen. A nitrogen atom has five electrons in its outer shell and a hydrogen atom has one. Ammonia is a gas at room temperature.Explain why ammonia has a low boiling point and does not conduct electricity.2 marks
- A student in Nairobi compares two forms of carbon, diamond and graphite. She tries to scratch a glass slide with a sample of each. The diamond scratches the glass. The graphite does not scratch the glass and instead leaves a grey mark on it. She also finds that graphite conducts electricity but diamond does not, and both have very high melting points.Explain why graphite conducts electricity but diamond does not.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).