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IB DemystifiedMYP Sciences
Covalent bonding and structures
Diamond and pencil lead are both pure carbon, yet one is the hardest natural substance and the other smears onto paper. Carbon dioxide (CO₂) is a gas, yet silicon dioxide (SiO₂, sand), with a very similar formula, melts at 1710 °C. Structure explains the difference.
Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
Figure 1. Shared pairs of electrons sit in the overlaps between atoms.
explain a covalent bond as a shared pair of electrons, including double and triple bonds
draw and interpret dot-and-cross diagrams of simple molecules
explain the low melting points and non-conductivity of simple molecular substances
explain the properties of diamond, graphite and silicon dioxide from their structures
classify substances as simple molecular, giant covalent, ionic or metallic using data
evaluate uses of covalent substances and their impacts
Before you start
You will use these skills. If any feel shaky, review them first.
electron shells and outer electrons from the periodic table
ionic bonding and giant ionic lattices (see Ionic bonding)
the particle model of melting and boiling (see States of matter)
Key vocabulary
Covalent bond
A shared pair of electrons between two atoms, attracted to both nuclei.
Double / triple bond
Two / three shared pairs between the same two atoms.
Lone pair
A pair of outer electrons that is not shared in a bond.
Intermolecular forces
Weak forces of attraction between separate molecules.
Simple molecular substance
A substance made of small, separate molecules, such as water or carbon dioxide.
Giant covalent structure
A continuous network of atoms joined by covalent bonds, with no separate molecules, such as diamond.
Delocalised electrons
Electrons not fixed to one atom or bond, free to move through a structure.
Understanding covalent bonding
What is it?
When non-metal atoms bond, neither can easily lose electrons, so they share pairs of outer electrons. Each shared pair is a covalent bond. Most atoms form enough bonds to reach eight outer electrons (hydrogen reaches two).
Why does it hold?
The shared electrons are attracted to the positive nuclei of both atoms at once, which holds the atoms strongly together. The number of bonds an atom usually forms is the number of electrons it needs to fill its outer shell: hydrogen 1, oxygen 2, nitrogen 3, carbon 4.
How do we know?
Covalent substances do not conduct when melted, so they contain no ions. Measurements of molecules show fixed bond lengths and angles, and X-ray studies revealed the tetrahedral network of diamond and the layers of graphite.
Why does it matter?
Covalent bonding holds together almost every molecule in living things: DNA, proteins, sugars and fats. It also explains materials from plastics to silicon chips and graphene.
What does it connect to?
Covalent bonding links to organic chemistry, polymers and fuels in chemistry; to biological molecules and enzymes in biology; and to semiconductors and electrical conduction in physics.
Simple and giant structures
Simple molecular substances (such as H₂O, CO₂, CH₄, O₂) contain strong covalent bonds within each molecule, but only weak forces between molecules. Melting and boiling overcome these weak forces, not the bonds, so melting and boiling points are low. Larger molecules generally have stronger intermolecular forces and higher boiling points. They do not conduct: molecules have no charge and no free electrons.
Giant covalent structures have no separate molecules. Melting them means breaking many strong covalent bonds, so their melting points are very high.
Figure 2. Diamond and graphite are both pure carbon, with different structures.
Diamond: each carbon bonded to four others in a rigid 3D network. Extremely hard; does not conduct.
Graphite: each carbon bonded to three others in flat layers, with weak forces between the layers. Soft and slippery; one delocalised electron per atom lets it conduct.
Silicon dioxide (sand, quartz): each silicon bonded to four oxygens in a giant network. Hard; very high melting point; does not conduct.
Covalent substances in the real world
Industrial diamonds on drill bits cut through rock for tunnels and wells. Graphite electrodes carry huge currents in steel furnaces and aluminium smelters. Quartz crystals keep time in watches, and ultra-pure silicon, another giant covalent structure, is the base of every computer chip.
Worked examples
Example 1: predicting a molecule
Predict the formula of the compound of nitrogen and hydrogen, and describe its bonding.
Nitrogen has 5 outer electrons and needs 3 more; hydrogen needs 1 more.
Nitrogen forms three single bonds, each to one hydrogen: NH₃.
Nitrogen keeps one lone pair, giving 8 outer electrons (6 shared + 2 in the lone pair).
Example 2: classifying from data
A substance melts at 115 °C and does not conduct when solid or melted. What is its structure?
The low melting point means only weak forces are overcome: not a giant structure.
It does not conduct when melted, so it is not ionic or metallic.
It is a simple molecular substance (this is sulfur, S₈).
In the eAssessment
Structure questions test whether you can link particles, forces and properties. Expect:
Interpret dot-and-cross diagrams: count bonds, shared electrons and lone pairs, including double and triple bonds.
Classify unknown substances using melting point, boiling point and conductivity data.
Explain properties by naming the structure, the particles and the forces overcome.
Analyse trends such as boiling points in a family of molecules.
Common ways to lose marks: saying covalent bonds break when a simple molecular substance melts or boils; calling diamond or sand "molecules"; saying graphite conducts because of ions; and assuming similar formulae (CO₂ and SiO₂) mean similar structures.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: evaporation and cooling
Partially guided investigation · about 50 minutes · pairs
Research question
How does the size of an alcohol molecule (ethanol, propan-1-ol, butan-1-ol) affect the fall in temperature of a thermometer bulb wrapped in paper soaked in the alcohol?
Scientific background
Evaporating molecules take energy away from the liquid left behind, so evaporation cools a surface. How fast a liquid evaporates depends on the strength of the forces between its molecules.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Thermometers or temperature probes, strips of filter paper, elastic bands, dropping pipettes, the three alcohols in small labelled bottles, clamp stand, stopwatch.
Method
Wrap an identical strip of filter paper round each thermometer bulb and hold it with an elastic band.
Record the starting temperature.
Soak the paper with alcohol, clamp the thermometer, and record the temperature every 30 seconds for 4 minutes.
Repeat for each alcohol, then repeat the whole set; work out the largest temperature fall for each.
Safety. Alcohols are highly flammable: no flames anywhere in the room. Work in a well-ventilated space, wear eye protection, avoid breathing the vapour, and keep bottles closed when not in use.
Then evaluate: were your results reliable, and what would you change?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
