Covalent bonding — AQA GCSE Combined Science
Test yourself on Covalent bonding with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Covalent bonding explained
Covalent bonding happens between non-metal atoms that need more electrons to complete their outer shells.
Read the full explanation
Instead of transferring electrons, the atoms share one or more pairs. A single shared pair is one covalent bond, shown as a line in a displayed formula. For example, two hydrogen atoms each contribute one electron to a shared pair, giving H₂, where each H has the electron arrangement of helium. In H₂O, oxygen shares one pair with each hydrogen, giving two single bonds and two lone pairs on oxygen. Each shared pair is attracted to both nuclei, so the bond holds the atoms together strongly. This strong attraction is the covalent bond; it is not a physical force between molecules.
Covalently bonded substances may consist of small molecules.
Many covalently bonded substances are made of small, separate molecules rather than giant structures. A small molecule contains a fixed, small number of atoms joined by strong covalent bonds, for example H₂O, CO₂, O₂, N₂, CH₄ and HCl. The atoms within each molecule are held together strongly, but the molecules themselves are separate and attracted to one another only by weak intermolecular forces. This explains why small-molecule substances often have low melting and boiling points: little energy is needed to overcome the weak forces between molecules, not the strong covalent bonds within them. For example, iodine and carbon dioxide are simple molecular substances that are gases or easily vaporised solids at room temperature.
Students should be able to recognise common substances that consist of small molecules from their chemical formula.
A small-molecule covalent substance consists of a discrete, finite number of non-metal atoms joined by covalent bonds, such as H₂O, CO₂, NH₃, CH₄, O₂, N₂, HCl, Cl₂ and C₆H₁₂O₆. To recognise one from its formula, check that all elements are non-metals and that the formula represents a discrete particle. A formula with a specific number of atoms, whether a few (like H₂O) or several (like C₆H₁₂O₆), indicates a small molecule. This contrasts with giant covalent structures (like SiO₂), which use empirical formulae for continuous lattices, or polymers, which show repeating units with a subscript n. For example, CO₂ contains one carbon and two oxygen atoms, so it is a small molecule; NaCl contains a metal, so it is ionic. The skill involves inspecting the formula to identify discrete molecules.
Some covalently bonded substances have very large molecules, such as polymers.
Not all covalent substances are small molecules. In a polymer, many identical small units called monomers are joined by strong covalent bonds into a very large molecule. The formula is often written with a repeating unit and the subscript n, for example poly(ethene) as –(C₂H₄)ₙ–. While the atoms within each chain are held by strong covalent bonds, the intermolecular forces between separate polymer molecules are relatively strong compared to those between small molecules, due to the large size of the polymer chains. This cumulative strength of intermolecular forces explains why polymers are usually solids at room temperature with higher melting points than small-molecule substances like CH₄. To recognise a large covalent molecule, look for a repeating unit and the subscript n.
Some covalently bonded substances have giant covalent structures, such as diamond and silicon dioxide.
Most covalently bonded substances are simple molecules with weak forces between them, but some form giant covalent structures in which every atom is joined to its neighbours by strong covalent bonds throughout the whole solid. Diamond is a giant covalent structure of carbon atoms, each bonded to four others in a tetrahedral network, giving it extreme hardness and a very high melting point. Silicon dioxide (SiO₂) is also giant covalent: each silicon atom bonds to four oxygen atoms and each oxygen to two silicon atoms, forming a continuous three-dimensional network. Because melting requires breaking many strong covalent bonds, these substances have high melting points and do not conduct electricity, as there are no free electrons or ions.
The covalent bonds in molecules and giant structures can be represented in the following forms:
Covalent bonds can be shown in several ways, and you must recognise and evaluate each representation. A dot-and-cross diagram shows outer-shell electrons, with shared pairs in the overlap between atoms (e.g. H₂O has two bonding pairs and two lone pairs). A displayed formula shows every atom and bond as a line in 2D (e.g. H–O–H). A structural formula shows atom arrangement without drawing all bonds (e.g. CH₃CH₂OH). 3D models, including ball-and-stick models, represent the spatial arrangement of atoms and bonds. While 2D formulae are useful for simple molecules, they cannot show the 3D shape. Giant covalent structures are best represented by 3D ball-and-stick models showing a small section of the continuous network, as structural formulae cannot represent their infinite lattices.
Polymers can be represented in the form:
Polymers are very large covalent molecules built from many small repeating units called monomers. In a displayed formula, the polymer is shown with brackets around the repeating unit and the letter n written outside the brackets. The bonds at the two ends of the repeating unit pass through the brackets, showing that the unit joins to identical units on both sides. For example, poly(ethene) is represented with the repeating unit —CH₂—CH₂— inside brackets and n outside. The value of n is not fixed for a sample; it shows the number of monomer units joined together. This representation is a shorthand that avoids drawing thousands of atoms and lets chemists see the repeating pattern clearly.
where n is a large number.
In the shorthand formula of a polymer, n is written outside the brackets and stands for a large number of repeating units joined together. It is not a small fixed value such as 2 or 3; a polymer sample contains chains of many different lengths, so n is variable and large. For example, in poly(ethene), n might be several thousand, meaning thousands of —CH₂—CH₂— units are covalently bonded in one chain. The value of n affects the size and properties of the polymer, such as its strength and melting point, because longer chains have stronger intermolecular forces between them. Understanding n helps explain why polymers are macromolecules rather than simple molecules.
draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane
This statement requires you to draw accurate dot and cross diagrams for eight simple molecules. Start by working out the outer-shell electrons for each atom: hydrogen has one, chlorine has seven, oxygen has six, nitrogen has five, and carbon has four. Then pair electrons so that each atom reaches a full outer shell, using dots for one atom and crosses for the other. Hydrogen forms a single bond with one shared pair, chlorine forms a single bond with three lone pairs on each atom, oxygen forms a double bond with two lone pairs on each atom, and nitrogen forms a triple bond with one lone pair on each atom. Hydrogen chloride has a single bond and three lone pairs on chlorine. Water has two single bonds and two lone pairs on oxygen. Ammonia has three single bonds and one lone pair on nitrogen. Methane has four single bonds and no lone pairs on carbon.
represent the covalent bonds in small molecules, in the repeating units of polymers and in part of giant covalent structures, using a line to represent a single bond
A covalent bond is a shared pair of electrons between two non-metal atoms. Chemists often draw each shared pair as a single straight line between the atom symbols, so H₂ becomes H—H, and each line counts as one shared pair. In small molecules such as H₂O, CH₄ and CO₂, draw the central atom, attach each bonded atom with one line per shared pair, and add any unbonded outer-shell electrons as dots if required. In polymers, show the repeating unit by drawing the two backbone atoms with a line between them and extending bonds through brackets with n outside. In giant covalent structures such as diamond, graphite and silicon dioxide, draw a representative fragment: each carbon in diamond uses four single lines to four neighbours, while each silicon in silicon dioxide bonds to four oxygens and each oxygen to two silicons.
describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures
Every model of a molecule or giant structure is a simplification, and each type has particular limitations. Dot and cross diagrams show which electrons are shared and can show lone pairs, but they do not show the shape of the molecule, the relative sizes of atoms or the three-dimensional arrangement. Ball and stick models show the shape and the angles between bonds clearly, but the balls are not the real size of the atoms and the sticks suggest a solid connection where there is actually empty space. Two-dimensional diagrams are quick to draw and show bonding, but they flatten a three-dimensional structure and can hide which atoms are in front. Three-dimensional models show arrangement in space, but they can still misrepresent bond lengths, electron clouds and the scale of the structure.
deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule.
A molecular model or diagram uses spheres or letters for atoms and lines or sticks for covalent bonds. To deduce the molecular formula, identify each distinct element present, then count how many atoms of each element appear in one molecule. Write the element symbols with the counts as subscripts, using the conventional order and omitting the subscript 1. For example, a model with one carbon atom bonded to four hydrogen atoms gives CH₄; a model with two hydrogen atoms and two oxygen atoms gives H₂O₂. Check that every atom in the diagram has been counted exactly once, including atoms at the ends of chains and any atom shown without a label. The formula must describe one molecule, not the whole mixture or the number of bonds.
Your focus
- Describe covalent bonding as the sharing of pairs of electrons between atoms.
- Represent simple covalent bonds using dot-and-cross diagrams and displayed formulae.
- Explain why a covalent bond is strong in terms of attraction between the shared electrons and the nuclei.
Show all 36 objectives
- Recognise that some covalently bonded substances exist as small, discrete molecules.
- Distinguish between strong covalent bonds within a molecule and weak forces between molecules.
- Relate the small-molecule structure of a substance to its low melting point and boiling point.
- Identify the elements present in a given chemical formula and classify each as a metal or non-metal.
- Recognise common small-molecule substances such as H₂O, CO₂, NH₃, CH₄, O₂, N₂, HCl, Cl₂ and C₆H₁₂O₆ from their formulae.
- Distinguish small molecules from ionic compounds, polymers, and giant covalent structures using formula and composition.
- Describe polymers as covalently bonded substances containing very large molecules built from repeating monomer units.
- Recognise a polymer from a formula that shows a repeating unit or the subscript n.
- Compare the structure and properties of polymers with those of small-molecule covalent substances, focusing on intermolecular forces.
- Describe the giant covalent structures of diamond and silicon dioxide.
- Explain how the network of strong covalent bonds leads to high melting points.
- Explain why giant covalent substances such as diamond and silicon dioxide do not conduct electricity.
- Draw dot-and-cross diagrams for simple covalent molecules.
- Write displayed and structural formulae for simple covalent molecules.
- Represent the bonding in a giant covalent structure using a 3D ball-and-stick model of a section of the network.
- Draw the shorthand representation of a polymer from a given monomer.
- Identify the repeating unit and the meaning of n in a polymer structure.
- Explain how the shorthand representation shows many monomer units joined by covalent bonds.
- State that n represents a large number of repeating units.
- Explain how the value of n relates to the length and properties of a polymer chain.
- Interpret polymer formulae containing n and describe what the brackets and n show.
- Draw accurate dot and cross diagrams for hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane.
- Distinguish between bonding pairs and lone pairs in each of the eight molecules.
- Use dot and cross diagrams to show how each atom achieves a full outer shell.
- Draw small molecules such as H₂O, CH₄ and CO₂ with one line for each shared pair of electrons.
- Draw the repeating unit of a polymer with bonds crossing the brackets and n outside.
- Draw a labelled fragment of diamond, graphite or silicon dioxide showing the correct number of lines around each atom.
- State one limitation of dot and cross diagrams for showing molecular shape or atom size.
- Explain why ball and stick models do not show the true size of atoms or the empty space between them.
- Compare two-dimensional and three-dimensional diagrams in terms of how well they show the arrangement of atoms in a giant structure.
- Identify the elements present in a molecular model or diagram.
- Count the atoms of each element in one molecule accurately.
- Write the molecular formula using correct symbols and subscripts.
Covalent bonding exam tips
Marking Points
- Covalent bonds form between non-metal atoms, for example H and O or C and H.
- A covalent bond is a shared pair of electrons, with each atom contributing at least one electron to the pair.
- Atoms share electrons to achieve a full outer shell, often eight electrons (or two for hydrogen and helium).
- A single covalent bond is one shared pair; double and triple bonds involve two and three shared pairs respectively.
- The shared pair is attracted to both nuclei, which makes the bond strong.
- Displayed formulae use lines to represent shared pairs, for example H—H, and dot-and-cross diagrams show the origin of each electron.
- A small molecule is a discrete group of a limited number of atoms joined by covalent bonds, such as H₂O or CO₂.
- Small molecules contain strong covalent bonds between the atoms inside the molecule.
- Separate small molecules are held near each other by weak intermolecular forces.
- The weak forces between molecules explain low melting and boiling points and ready vaporisation.
- When a simple molecular substance melts or boils, the covalent bonds within molecules are not broken; the molecules separate from one another.
- Examples include H₂, O₂, N₂, Cl₂, H₂O, CO₂, CH₄ and HCl.
- Identify all elements present in the formula and confirm they are non-metals before deciding that the substance is covalent.
- Recognise that a formula showing a specific, finite number of atoms (e.g. H₂O, CO₂, NH₃, CH₄ or C₆H₁₂O₆) indicates a discrete small molecule.
- Recognise common small molecules from their formulae, including H₂, O₂, N₂, Cl₂, HCl, H₂O, CO₂, NH₃ and CH₄.
- Distinguish small molecules from giant covalent structures (e.g. SiO₂) which form continuous lattices, and from polymers which have repeating units.
- Explain that the formula of a small molecule shows the actual number of atoms covalently bonded together in one discrete particle.
- Use the presence of a metal combined with a non-metal as evidence that a substance is ionic, not a small covalent molecule.
- State that polymers are covalently bonded substances made from many small monomer units joined together.
- Recognise a polymer from a formula showing a repeating unit and the subscript n, such as –(C₂H₄)ₙ–.
- Explain that the atoms within a polymer molecule are joined by strong covalent bonds.
- State that the intermolecular forces between large polymer molecules are relatively strong compared to those between small molecules.
- Compare polymers with small molecules such as CH₄, noting that polymers have much larger molecules and higher melting points due to stronger intermolecular forces.
- Use the idea of large molecules and relatively strong intermolecular forces to explain why polymers are solids at room temperature.
- Giant covalent structures contain many atoms joined by strong covalent bonds extending continuously through the substance, not discrete small molecules.
- In diamond, each carbon atom forms four covalent bonds to four other carbon atoms in a rigid tetrahedral network.
- In silicon dioxide, each silicon atom bonds to four oxygen atoms and each oxygen atom bonds to two silicon atoms, giving the formula SiO₂.
- Melting a giant covalent substance requires breaking many strong covalent bonds, so melting points are very high.
- Giant covalent substances such as diamond and silicon dioxide do not conduct electricity because they contain no free electrons or ions.
- Dot-and-cross diagrams show outer-shell electrons, using different symbols for electrons from different atoms to highlight shared pairs.
- Displayed formulae show every atom and every covalent bond as a line in 2D, for example H–O–H for water.
- Structural formulae show the arrangement of atoms without drawing every bond, for example CH₃CH₂OH for ethanol.
- 3D models and ball-and-stick models show the spatial arrangement of atoms and bonds, which is essential for visualising molecular shapes.
- Giant covalent structures are represented by drawing a 3D section of the network, as 2D structural formulae cannot show the continuous lattice.
- Evaluate the limitations of models, such as 2D diagrams not showing 3D shape, and dot-and-cross diagrams not showing relative atom sizes.
- A polymer is a large covalent molecule formed when many small monomer molecules join together by covalent bonds.
- The repeating unit is the smallest section of the polymer chain that repeats many times.
- In the shorthand representation, the repeating unit is placed inside brackets.
- The letter n is written outside the brackets to show that the unit repeats a large number of times.
- The bonds at each end of the repeating unit cross the brackets, showing continuation of the chain in both directions.
- For addition polymers such as poly(ethene), the repeating unit has the same atoms as the monomer but with the double bond opened to form single bonds.
- n represents the number of repeating units in the polymer chain.
- n is a large number, not a small fixed number such as 2 or 3.
- Different polymer molecules in a sample can have different values of n, so n is variable.
- The size of n affects the length of the polymer chain and therefore properties such as strength and melting point.
- The repeating unit inside the brackets is joined many times, which is why the polymer is a macromolecule.
- Draw hydrogen as H–H with one shared pair and no lone pairs.
- Draw chlorine as Cl–Cl with one shared pair and three lone pairs on each chlorine atom.
- Draw oxygen as O=O with two shared pairs and two lone pairs on each oxygen atom.
- Draw nitrogen as N≡N with three shared pairs and one lone pair on each nitrogen atom.
- Draw hydrogen chloride as H–Cl with one shared pair and three lone pairs on chlorine.
- Draw water as two O–H bonds with two lone pairs on oxygen.
- Draw ammonia as three N–H bonds with one lone pair on nitrogen.
- Draw methane as four C–H bonds with no lone pairs on carbon.
- A single line between two atom symbols represents one shared pair of electrons, so a double bond needs two parallel lines and a triple bond needs three.
- Small molecules are drawn with the correct number of bonds per atom: carbon forms four single bonds, oxygen two, hydrogen one and nitrogen three.
- In a polymer repeating unit, the two bonds that join to the next unit are drawn passing through the brackets, with n written outside to show repetition.
- In a giant covalent fragment, every atom shown must have its full bonding, for example four lines around each carbon in diamond and two lines around each oxygen in silicon dioxide.
- Unbonded outer-shell electrons may be shown as dots, but the line itself already represents the shared pair, so no extra dots should be placed on the line.
- Dot and cross diagrams show shared and lone pairs of electrons but do not show molecular shape or the relative sizes of atoms.
- Ball and stick models show bond angles and three-dimensional shape but the balls are not to scale and the sticks wrongly suggest a physical rod between atoms.
- Two-dimensional diagrams are easy to draw and show connectivity but lose the three-dimensional arrangement of atoms.
- Three-dimensional models show spatial arrangement but may still misrepresent bond lengths, electron density and the true scale of a giant structure.
- A good answer names the model, states one specific limitation and explains why that limitation matters for understanding the structure.
- Identify every different element shown in the model or diagram before counting any atoms.
- Count the atoms of each element in a single molecule, including terminal atoms and atoms joined by multiple bonds.
- Write the correct element symbols in the accepted order, usually carbon first, then hydrogen, then other elements alphabetically.
- Use subscript numbers for counts greater than one and omit the subscript 1, for example H₂O not H₂O₁.
- Check the total number of atoms counted against the total number of spheres or labelled atoms in the diagram.
Examiner Tips
- 💡When asked to explain bonding, name the atoms involved, state that electrons are shared, and say how this gives a full outer shell.
- 💡Use dot-and-cross diagrams to show which atom supplies each electron in a shared pair, and check that outer shells are complete.
- 💡If a question asks why a bond is strong, refer to the attraction between the shared pair and both nuclei, not to the molecule as a whole.
- 💡When comparing melting points, identify whether the substance is simple molecular or giant covalent before explaining the energy needed.
- 💡Use the phrase weak intermolecular forces for the attractions between small molecules, and strong covalent bonds for the attractions within them.
- 💡Give a named example, such as H₂O or CO₂, to support an explanation about small molecules.
- 💡Underline the element symbols in a formula and label each as metal or non-metal before deciding on the type of substance.
- 💡Learn a short list of common small molecules and their formulae so recognition is quick and accurate.
- 💡If asked to explain, state that the formula shows a specific number of non-metal atoms covalently bonded in a discrete molecule.
- 💡Look for a repeating unit or the subscript n in a displayed formula to identify a polymer.
- 💡When comparing polymers with small molecules, refer to the size of the molecule and the relative strength of the intermolecular forces.
- 💡Use the terms monomer and polymer correctly, and link polymerisation to the joining of many small units.
- 💡When asked why diamond has a high melting point, link the answer to breaking many strong covalent bonds rather than to intermolecular forces.
- 💡Use a labelled diagram of the diamond tetrahedral arrangement or the SiO₂ network to support your written explanation.
- 💡Check whether the question asks about structure, bonding or properties, and make sure each part of your answer addresses the correct one.
- 💡Label shared pairs clearly in dot-and-cross diagrams and keep the symbols for each atom's electrons distinct.
- 💡When drawing displayed formulae, include every bond as a line and check each atom's bonding total.
- 💡Be prepared to evaluate the advantages and limitations of different models, such as 2D vs 3D representations.
- 💡When asked to draw a polymer, draw the repeating unit with its two free bonds crossing the brackets, then write n outside.
- 💡Check that the atoms in the repeating unit match the monomer for an addition polymer, with the C=C changed to C—C.
- 💡If asked what n represents, state that it is a large number of repeating units, not a fixed small number.
- 💡When defining n, use the phrase 'a large number of repeating units' rather than just 'many'.
- 💡Link n to polymer properties such as strength or melting point to gain explanation marks.
- 💡Avoid giving a single numerical value for n unless the question provides one; n is generally variable.
- 💡Use a key or label to show which atom provides the dots and which provides the crosses.
- 💡Draw the outer shell only; do not draw inner shells unless the question asks for them.
- 💡Check that each atom in the completed diagram has a full outer shell of electrons (two for hydrogen, and eight for carbon, nitrogen, oxygen and chlorine).
- 💡For water and ammonia, place lone pairs on the central atom and ensure bond angles are shown clearly enough to count the bonds.
- 💡Count the outer-shell electrons each atom needs before drawing, then check that every atom has the right number of lines around it.
- 💡For polymers, draw the repeating unit only once inside the brackets and place the n clearly outside, not inside.
- 💡For giant covalent structures, label the diagram as a fragment or part of the structure so the examiner sees you are not trying to draw the whole lattice.
- 💡Use the structure 'model X is useful because ..., but it does not show ...' to make the limitation clear.
- 💡Give a different limitation for each model rather than repeating the same point.
- 💡Link each limitation to a consequence, such as not being able to predict the shape or the reactivity of a molecule.
- 💡Annotate the diagram lightly with a tally for each element before writing the formula.
- 💡If the diagram uses letters, treat each letter as one atom of that element unless a key states otherwise.
- 💡Re-count after writing the formula to confirm the subscripts match the diagram.
Common Mistakes
- Thinking atoms transfer electrons in covalent bonding; correct this by stressing that covalent bonding involves sharing, while ionic bonding involves transfer.
- Drawing a bond as a line but forgetting the shared pair of electrons; correct this by always linking the line to one pair of electrons.
- Confusing the strength of the covalent bond with the strength of forces between molecules; correct this by stating that covalent bonds are strong attractions within a molecule, whereas intermolecular forces are much weaker.
- Saying that covalent bonds break when a simple molecular substance boils; correct this by stating that the weak forces between molecules are overcome, while covalent bonds remain intact.
- Treating a small molecule as a giant lattice; correct this by describing it as a discrete particle with a fixed small number of atoms.
- Assuming all covalent substances are small molecules; correct this by noting that some covalent substances, such as diamond and silicon dioxide, are giant covalent structures.
- Treating any formula containing only non-metals as a small molecule, including giant covalent substances such as SiO₂; correction: check whether the structure is discrete or giant, noting that SiO₂ is a giant covalent lattice.
- Assuming that a formula with several atoms, such as C₆H₁₂O₆, must be a polymer or giant structure; correction: C₆H₁₂O₆ represents a discrete small molecule, whereas a polymer contains many repeating units denoted by 'n'.
- Classifying NaCl as a small molecule because it is written with two symbols; correction: sodium is a metal, so NaCl is an ionic compound, not a covalent molecule.
- Thinking that all covalent substances are small molecules; correction: polymers are covalent but contain very large molecules made of many repeating units.
- Stating that covalent bonds are broken when a polymer melts; correction: it is the intermolecular forces between the polymer chains that are overcome, not the strong covalent bonds within the chains.
- Assuming intermolecular forces in polymers are weak like in small molecules; correction: because polymer molecules are so large, the intermolecular forces between them are relatively strong, making them solid at room temperature.
- Saying diamond is a molecule of carbon: correct this by describing it as a giant covalent lattice of many carbon atoms joined by covalent bonds.
- Stating that giant covalent substances melt easily because covalent bonds are weak: correct this by explaining that individual covalent bonds are strong and many must be broken to melt the solid.
- Describing silicon dioxide as simple molecules of SiO₂: correct this by explaining it is a giant covalent network in which the ratio of silicon to oxygen atoms is 1:2.
- Drawing dot-and-cross diagrams with the wrong number of outer electrons; correction: count outer electrons from the group number, such as four for carbon and six for oxygen.
- Using a structural formula to represent a giant covalent structure; correction: giant structures form continuous lattices, so they are represented using 3D ball-and-stick models of a section, not discrete structural formulae.
- Assuming a 2D displayed formula shows the actual shape of a molecule; correction: displayed formulae only show connectivity; 3D models or ball-and-stick diagrams are needed to show the true spatial arrangement.
- Drawing the repeating unit without brackets or without n outside: correct this by enclosing the repeating unit in brackets and writing n outside.
- Putting n inside the brackets: correct this by placing n outside the brackets, because n counts how many repeating units are joined.
- Showing a double bond inside the repeating unit of an addition polymer: correct this by drawing single bonds only, because the double bond opens when monomers join.
- Stating that n is a fixed number such as 2: correct this by saying n is a large, variable number of repeating units.
- Confusing n with the number of atoms in one repeating unit: correct this by explaining that n counts repeating units, not atoms.
- Thinking n affects only the drawing and not the real molecule: correct this by linking larger n to longer chains and different physical properties.
- Forgetting lone pairs on chlorine, oxygen or nitrogen; correct this by counting all outer-shell electrons and showing those not used in bonding.
- Drawing oxygen with a single bond; correct this by showing a double bond so each oxygen has eight outer-shell electrons.
- Drawing nitrogen with a double bond; correct this by showing a triple bond so each nitrogen has eight outer-shell electrons.
- Drawing methane with lone pairs on carbon; correct this by showing four single bonds and no lone pairs on carbon.
- Drawing a double bond as one line: the error is that one line means one shared pair, so the correction is to draw two parallel lines for O=O or C=O.
- Leaving a hydrogen atom with two bonds in a small molecule: the error is that hydrogen can only form one single bond, so the correction is to check each hydrogen has exactly one line.
- Drawing polymer brackets without bonds crossing them: the error is that the repeating unit would not connect to its neighbours, so the correction is to extend the two backbone bonds through the brackets and write n outside.
- Saying dot and cross diagrams are wrong: the error is confusing a limitation with an error, so the correction is to say they are useful but do not show shape or relative size.
- Claiming ball and stick models show the real size of atoms: the error is that the balls are only representative, so the correction is to state that they are not to scale.
- Describing a limitation without linking it to the model: the error is a vague statement such as 'it is not accurate', so the correction is to name the model and say exactly what it fails to show.
- Counting bonds instead of atoms: a molecule with four C–H bonds has four hydrogen atoms, not four hydrogen molecules; correct by counting atom labels or spheres.
- Forgetting terminal atoms in a chain: for example missing the final hydrogen in a displayed formula; correct by tracing every bond to its end atom.
- Writing a subscript of 1, such as CO₁; correct by omitting the 1 and writing CO.