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K324 / 2027
Bonding and Structure

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Bonding and Structure

Why salt, candle wax and copper behave differently: the particles, attractions and moving charges behind their properties.

O-Level 2026 SEC G3 2027. Extensions are labelled Pure only.

One idea connects the chapter

Particles. Attractions. Properties.

Salt melts at a high temperature. Wax softens easily. Copper conducts while solid. The difference starts with what each is made of and what can move.

  1. Identify the particlesAtoms, molecules or ions?
  2. Find the attractionsWhat holds them together?
  3. Explain the propertyWhat needs energy? What moves?
01

First, identify the particles

An element can contain molecules. A compound can contain ions.

An atom is a single particle of an element. A molecule is a discrete group of atoms joined by covalent bonds. An ion is a charged particle. These describe particles; element, compound and mixture describe what a substance or sample contains.

Atom

One helium atomA single circle labelled He represents one neutral helium atom. The circle is a particle symbol, not a diagram of its electrons.He
One atom of an element. Helium exists as separate atoms.

Molecule

One water moleculeOne oxygen atom joined to two hydrogen atoms by covalent bonds. The three atoms together form one neutral water molecule. The bent drawing is not to scale.OHH
In water, one oxygen and two hydrogen atoms form a discrete H2O molecule.

Ion

A positive sodium ion and a negative chloride ionSeparate particle symbols labelled Na plus and Cl minus. An ion has a net charge because the numbers of protons and electrons are unequal.Na+Cl-
Charged particles. Na+ has lost an electron; Cl- has gained one.

Particle symbols, not shell diagrams. Sizes and spacing are not to scale.

Read the labels and connections: a molecule contains joined atoms; an ion has an overall charge.
Classifying a sample
SampleWhat makes it different?Example
ElementOnly one type of atom; cannot be chemically broken into simpler substances.O2: pairs of oxygen atoms.
CompoundDifferent elements chemically combined in a fixed proportion; separation requires chemical change.H2O: two hydrogen atoms per oxygen atom.
MixtureSubstances together without being chemically combined; proportions can vary and physical separation is possible.Air: several gases mixed together.

Check your understanding

A diagram shows only identical pairs of oxygen atoms. Is it an element, a compound or a mixture?

  1. An element, made of O2 molecules
  2. A mixture, since there are many separate pairs
  3. A compound, since atoms are bonded in pairs
Show the answer

An element, made of O2 molecules

Every atom is oxygen, so it is an element. Each pair is a molecule, but two atoms joined together do not make two elements.

02

Ionic bonding: transfer, then attraction

Electron transfer creates ions. Attraction between opposite charges holds them together.

A metal atom usually loses outer-shell electrons to form a positive ion. A non-metal atom usually gains electrons to form a negative ion. The resulting ions commonly have the electron arrangement of a noble gas: a full outer shell. The nucleus does not change.

Sodium chloride

Na (2,8,1) loses one electron. Cl (2,8,7) gains that electron.

Sodium chloride: ions after electron transferA sodium ion with charge plus one and a chloride ion with charge minus one. Each has a full outer shell of eight electrons. The chloride has seven dots from chlorine and one cross transferred from sodium. Sodium's remaining outer shell has eight crosses, which were previously in an inner shell.Na+Cl-
One Na+ for each Cl-. The total charge is zero.

Magnesium chloride

Mg (2,8,2) loses two electrons. Two Cl atoms each gain one.

Magnesium chloride: ions after electron transferOne magnesium ion with charge plus two and two chloride ions, each with charge minus one. All three ions have full outer octets. Each chloride shows seven dots from chlorine and one cross transferred from magnesium. Magnesium's eight remaining outer-shell electrons were previously in an inner shell.Mg2+Cl-Cl-
One Mg2+ for every two Cl- ions: +2 - 1 - 1 = 0.
  • Electron from chlorine
  • Electron from the metal

Each bracket shows the ion's full outer shell. The eight electrons on Na+ and Mg2+ were previously in an inner shell. Dots and crosses track origin; all electrons are the same kind of particle.

These groups show the ion ratio, not separate NaCl or MgCl2 molecules. The ions form an extended lattice.

NaCl: one sodium electron transfers to chlorine. MgCl2: magnesium transfers one electron to each of two chlorine atoms. Show brackets and charges on the resulting ions.
Follow the electrons and the charge
Starting atomElectron changeResulting ion
Na: 2,8,1Loses one electronNa+: 2,8
Mg: 2,8,2Loses two electronsMg2+: 2,8
Cl: 2,8,7Gains one electronCl-: 2,8,8

An ionic bond is the strong electrostatic attraction between oppositely charged ions. Na+ and Cl- balance in a 1:1 ratio: NaCl. One Mg2+ needs two Cl- ions: MgCl2. The compound has no overall charge.

Check your understanding

Which ion has the same electron arrangement as a neon atom, 2.8?

  1. S2-
  2. Al3+
  3. K+
  4. Cl-
Show the answer

Al3+

Aluminium (2.8.3) loses its three outer electrons, leaving 2.8, like neon.

Worked exampleDot-and-cross diagram for sodium oxide

Draw a dot-and-cross diagram for sodium oxide, Na2O. Show outer electrons only. Then explain why the formula has two sodium ions.

Try it first. Then show the worked stepsHide the worked steps
  1. Each sodium atom (2,8,1) gives its one outer electron to the oxygen atom (2,6). Oxygen needs two electrons to fill its outer shell, so it takes one from each of two sodium atoms.
  2. Each Na+ ion is now 2,8; its emptied third shell is not drawn. The O2- ion has 8 outer electrons: 6 of its own (dots) and 2 from sodium (crosses). Use square brackets and write the charge on every ion. Two Na+ charges balance one O2- charge, so the formula is Na2O.
Answer

Each Na gives one electron, and O needs two, so two Na+ balance one O2-.

Check your understanding

Why does sodium oxide have the formula Na2O?

  1. Each Na gives one electron, and O needs two, so two Na+ balance one O2-.
  2. Na gives two electrons, and O needs one, so two Na balance the extra charge.
  3. Two Na atoms share a pair of electrons with O, forming two covalent bonds.
Show the answer

Each Na gives one electron, and O needs two, so two Na+ balance one O2-.

Sodium (2.8.1) loses one electron and oxygen (2.6) gains two. Two Na+ charges balance one O2- charge, so the formula is Na2O.

Worked exampleDot-and-cross diagram for the hydride ion

Magnesium hydride, MgH2, is an ionic compound that contains the hydride ion, H-. Draw a dot-and-cross diagram for H-, and explain why this ion is stable.

Try it first. Then show the worked stepsHide the worked steps
  1. A hydrogen atom has one electron. It gains one electron from magnesium to form H-.
  2. The first shell holds only two electrons, so H- has a full shell, the same arrangement as helium. That is why hydrogen can form a negative ion with a metal as reactive as magnesium. Each Mg atom gives one electron to each of two H atoms, so the formula is MgH2.
Answer

Its first shell holds two electrons, so H- has a full shell like helium.

Check your understanding

Magnesium hydride, MgH2, contains the hydride ion, H-. Why is H- stable?

  1. Its first shell holds two electrons, so H- has a full shell like helium.
  2. It has lost its single electron, so it has an empty shell like a bare proton.
  3. It has eight outer electrons, so it has the arrangement of a noble gas.
Show the answer

Its first shell holds two electrons, so H- has a full shell like helium.

Hydrogen has one electron and gains one from magnesium. The first shell holds only two, so H- has the stable arrangement of helium. Each Mg gives one electron to each of two H atoms: MgH2.

03

Why salt melts high but conducts only when ions move

The attractions explain melting. The mobility of charged particles explains conductivity.

Solid sodium chloride has a giant ionic lattice: a repeating arrangement of positive and negative ions extending throughout the crystal. Each ion attracts surrounding ions of the opposite charge. There are no separate NaCl molecules; its formula gives the simplest ion ratio.

Solid: ions fixed in place

Sodium chloride solid: can the ions carry charge?Eight alternating sodium and chloride ions occupy fixed positions in a regular two-dimensional slice. Positive and negative electrodes show an applied voltage, but no ion movement is drawn. The real sodium chloride lattice extends in three dimensions; each ion has six nearest neighbours of opposite charge.+-Na+Cl-Na+Cl-Cl-Na+Cl-Na+
The ions cannot move through the solid, so they cannot carry charge through it.

Molten: mobile ions

Sodium chloride molten: can the ions carry charge?Four sodium ions and four chloride ions have a disordered liquid arrangement. Arrows on two sodium ions point towards the negative electrode on the right. Arrows on two chloride ions point towards the positive electrode on the left. Sodium chloride remains ionic on melting; it does not become separate neutral atoms or molecules.+-Na+Cl-Na+Cl-Cl-Na+Cl-Na+
The lattice is disrupted. Mobile ions carry charge through the liquid.

Dissolved in water: mobile ions

Sodium chloride dissolved in water: can the ions carry charge?Four sodium ions and four chloride ions are separated in a pale blue region representing water. Sodium ions move towards the negative electrode on the right and chloride ions towards the positive electrode on the left when a voltage is applied. Water molecules and hydration shells are omitted. Electrode products are not shown.+-Cl-Na+Cl-Na+Na+Cl-Na+Cl-Water
Water separates ions from the lattice. The dissolved ions can carry charge.

Conductivity needs charged particles that can move. In molten and aqueous sodium chloride, those particles are ions.

The signed bars are electrodes under an applied voltage. Arrows show ion movement, not bond formation. These are simplified slices, not to scale; water molecules and electrode products are omitted.

Solid: ions vibrate about fixed positions. Molten: ions can move. Aqueous: dissolved ions can move through water. The lattice view is a simplified slice of a three-dimensional crystal.
Link the property to its cause
PropertyExplanation
High melting and boiling pointsStrong electrostatic attractions act throughout the lattice; much energy is needed to overcome them.
Does not conduct when solidThe ions are charged but cannot move through the solid to carry charge.
Conducts when molten or dissolvedMobile ions carry charge through the liquid or solution.

Watch out for this

A student says: "Salt disappears in water because it melts, and every ionic compound dissolves like this." What is wrong with this?

Show the answer

Melting uses heat to turn the compound into a liquid. Dissolving separates its ions into a solvent. Many ionic compounds dissolve in water, but some are insoluble; ionic bonding alone does not guarantee solubility.

Check your understanding

"Solid salt has no charged particles, so it does not conduct." What is wrong with this explanation?

  1. Solid salt has charged particles, but they are electrons, not ions.
  2. Nothing; the ions form when salt is melted or dissolved, not before.
  3. Solid salt has ions, but they are fixed in the lattice and cannot move.
Show the answer

Solid salt has ions, but they are fixed in the lattice and cannot move.

Conduction needs charged particles that can move. Solid NaCl contains Na+ and Cl- ions, but they cannot move until it is melted or dissolved.

04

Covalent bonding: count shared pairs

Each shared pair is one bond. Unshared electrons still count towards the outer shell.

Non-metal atoms form a covalent bond by sharing a pair of electrons. Usually each atom contributes one electron. The shared pair counts towards both outer shells, commonly giving a noble-gas arrangement: two electrons for hydrogen, eight for the other atoms shown here.

Each dot or cross is one outer-shell electron. A pair between atoms is shared; a pair beside one atom is a lone pair.

Hydrogen (H2)

Hydrogen dot-and-cross diagramTwo hydrogen atoms share one pair of electrons: one dot from the left hydrogen and one cross from the right. Each hydrogen has two electrons in its shared first shell.HH
  • Left H
  • Right H
One shared pair. Each hydrogen has a full first shell of two electrons.

Oxygen (O2)

Oxygen dot-and-cross diagramTwo oxygen atoms share two electron pairs, a double bond. The left oxygen contributes six dots and the right six crosses. Each oxygen has two lone pairs as well as the two shared pairs, giving eight electrons around each atom.OO
  • Left O
  • Right O
Two shared pairs form a double bond. Each oxygen also has two lone pairs.

Water (H2O)

Water dot-and-cross diagramOxygen contributes six dots: one in each of two bonds and four in two lone pairs. Each hydrogen contributes one cross. Oxygen has an octet and each hydrogen a duet. The flat layout shows electron accounting; a water molecule is bent.HOH
  • Oxygen
  • Hydrogens
Two shared pairs and two lone pairs around oxygen. Water is bent; this layout only counts electrons.

Methane (CH4)

Methane dot-and-cross diagramCarbon contributes four dots, one to each of four shared pairs. Each of four hydrogens contributes one cross. Carbon has an octet and each hydrogen a duet. Methane is tetrahedral; the flat cross layout does not show its shape.CHHHH
  • Carbon
  • Hydrogens
Four shared pairs, with no lone pair on carbon. Methane is tetrahedral, not flat.

Carbon dioxide (CO2)

Carbon dioxide dot-and-cross diagramCarbon shares two pairs with each oxygen, making two double bonds. Carbon contributes four dots in total. Each oxygen contributes six crosses: two in the shared pairs and four in two lone pairs. All three atoms have octets.OCO
  • Carbon
  • Oxygens
Two double bonds. Each oxygen has two lone pairs; carbon has none.

Ammonia (NH3)

Ammonia dot-and-cross diagramNitrogen contributes five dots: three in shared pairs and two in one lone pair. Each hydrogen contributes one cross. Nitrogen has an octet and every hydrogen a duet. Ammonia is pyramidal, not flat.NHHH
  • Nitrogen
  • Hydrogens
Deduce three shared pairs and one lone pair from nitrogen's five outer electrons. Ammonia is pyramidal.

Only outer-shell electrons are shown. Shared electrons count towards both bonded atoms' outer shells. Dots and crosses identify the source atom, not different types of electron.

H2, O2, H2O, CH4 and CO2: count every shared pair and every lone pair. These flat electron diagrams do not show three-dimensional molecular shapes.
Five patterns worth recognizing
MoleculeShared pairsLone pairs
H2One pair between the H atoms: a single bond.None.
O2Two pairs between the O atoms: a double bond.Two on each O.
H2OOne pair in each of two O-H bonds.Two on O.
CH4One pair in each of four C-H bonds.None on C.
CO2Two pairs in each C=O bond: O=C=O.Two on each O; none on C.

Worked exampleDeduce an unfamiliar diagram: ammonia

Nitrogen has five outer electrons. How are electrons arranged in NH3?

Try it first. Then show the worked stepsHide the worked steps
  1. Nitrogen needs three more electrons around it; each hydrogen needs one.
  2. Form three shared pairs, one between N and each H. These give three N-H bonds.
  3. Six electrons surround N in bonds. Its remaining two electrons form one lone pair.
  4. Check: eight electrons around N, two around each H, and eight outer electrons in total.
Answer

Three single N-H bonds and one lone pair on nitrogen. Sharing fills shells without transferring electrons to form ions.

Worked exampleDot-and-cross diagram for nitrogen trifluoride

Nitrogen trifluoride, NF3, is a covalent molecule. Nitrogen has 5 outer electrons and fluorine has 7. Draw its dot-and-cross diagram, outer electrons only.

Try it first. Then show the worked stepsHide the worked steps
  1. Each fluorine atom needs one more electron, so it shares one pair with nitrogen. Nitrogen needs three more electrons, so it forms three shared pairs, one with each fluorine.
  2. Check the count. Nitrogen has 3 shared pairs plus 1 lone pair: 8 electrons. Each fluorine has 1 shared pair plus 3 lone pairs: 8 electrons. This is the same pattern as ammonia, with F in place of H.
Answer

3 shared pairs and 1 lone pair, so nitrogen has 8 outer electrons

Check your understanding

In the dot-and-cross diagram of NF3, how many shared pairs and lone pairs does nitrogen have?

  1. 5 shared pairs and no lone pair, so nitrogen has 10 outer electrons
  2. 3 shared pairs and no lone pair, so nitrogen has 6 outer electrons
  3. 3 shared pairs and 1 lone pair, so nitrogen has 8 outer electrons
Show the answer

3 shared pairs and 1 lone pair, so nitrogen has 8 outer electrons

Nitrogen has 5 outer electrons and needs 3 more, so it shares one pair with each fluorine. The two unshared electrons form a lone pair, as in ammonia.

Worked exampleDot-and-cross diagram for the hydroxide ion

The hydroxide ion, OH-, has a covalent bond between O and H and an overall charge of 1-. Draw its dot-and-cross diagram, outer electrons only.

Try it first. Then show the worked stepsHide the worked steps
  1. Oxygen (6 outer electrons) shares one pair with hydrogen. The extra electron that gives the 1- charge sits on the oxygen.
  2. Oxygen now has 8 outer electrons: one shared pair and three lone pairs (5 of its own unshared electrons plus the gained one). Hydrogen has 2. Put the whole ion in square brackets with the charge outside, because the charge belongs to the whole ion.
Answer

On oxygen, which then has one shared pair and three lone pairs

Check your understanding

In a dot-and-cross diagram of the hydroxide ion, OH-, where does the extra electron that gives the 1- charge go?

  1. On hydrogen, which then holds three electrons in its first shell
  2. On oxygen, which then has one shared pair and three lone pairs
  3. Between O and H, which makes a double bond in the ion
Show the answer

On oxygen, which then has one shared pair and three lone pairs

Oxygen shares one pair with H. With its other 5 electrons plus the gained one, it has 8 outer electrons. Bracket the whole ion with the charge outside.

05

Strong bonds inside; weaker attractions between

Melting a molecular substance separates molecules without breaking them apart.

Methane and iodine have simple molecular structures: many separate molecules. Strong covalent bonds hold atoms together within each molecule. Weaker intermolecular forces attract one molecule to another. These are two different levels of attraction.

Solid iodine

Ordered iodine molecules in a solidSix iodine molecules, each formed from two iodine atoms joined by a solid covalent-bond line. Their positions are ordered. Dashed lines between separate molecules indicate intermolecular attractions. The drawing is schematic and does not show motion.IIIIIIIIIIII
The molecules vibrate around fixed positions.

Liquid iodine

Intact iodine molecules in a liquidSix iodine molecules, each formed from two iodine atoms joined by a solid covalent-bond line. Their arrangement is less regular after melting. Every molecule still has its two atoms joined: the covalent bonds have not broken. The drawing is schematic and does not show motion.IIIIIIIIIIII
The molecules can move past one another. Each I2 molecule stays intact.
  • Covalent bond within a molecule
  • Attraction between molecules

Melting overcomes enough intermolecular attractions to let the molecules move past one another. It does not break the covalent bonds inside them.

Simplified snapshots, not the measured crystal structure or a simulation. Only a few intermolecular attractions are marked; they are still present in the liquid.

On melting, molecules move apart and past one another. The covalent bonds within each molecule remain intact.

Simple molecular substances generally melt and boil at low temperatures compared with ionic lattices: relatively little energy overcomes their intermolecular forces. Methane is a gas at room temperature, while iodine is a solid. Simple molecular does not mean always gaseous; intermolecular attraction varies between substances.

Their pure solids and liquids generally do not conduct electricity: neutral molecules have no mobile ions or delocalised electrons to carry charge. A solution needs separate consideration: a molecular substance may form ions by reacting with water.

Watch out for this

A student says: "Iodine melts easily because its covalent bonds are weak." What is wrong with this?

Show the answer

A low melting point indicates weak attractions between molecules. It does not show that the covalent bonds inside the molecules are weak. Melting and breaking a molecule chemically are different changes.

Check your understanding

Iodine turns into a vapour on gentle warming. Why is so little energy needed?

  1. The covalent bond inside each I2 molecule is weak and breaks
  2. The ions in solid iodine are only weakly attracted to each other
  3. Only weak forces between the I2 molecules have to be overcome
  4. Iodine has a giant covalent structure with only a few bonds
Show the answer

Only weak forces between the I2 molecules have to be overcome

Iodine is simple molecular. Warming separates the molecules; the covalent I-I bonds stay intact, so the vapour is still I2.

06Pure only

A long molecule is different from a giant network

Compare poly(ethene), diamond, graphite and silicon dioxide.

Poly(ethene) consists of very long covalent molecules called macromolecules. Each chain contains many repeating units. Strong covalent bonds run along a chain, while intermolecular forces act between chains. Longer chains have more opportunities for intermolecular attraction than small molecules such as methane.

Long molecules, separate chains

Two uncrosslinked polyethene chainsTwo separate rows of six CH2 groups. Solid lines join groups along each chain and continue beyond the drawing. Dashed amber lines between the chains show weaker attractions, not covalent cross-links. Each labelled CH2 group contains one carbon and two hydrogens.CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2
Strong covalent bonds run along each chain. Weaker attractions act between the separate chains.

Example: uncrosslinked poly(ethene). Each CH2 label is a group, not one atom. The straight rows simplify the chain shapes. Cross-linked polymers have different structures.

Separate poly(ethene) chains: covalent bonds along each chain, intermolecular forces between chains. On softening, chains can move relative to one another.

Poly(ethene) needs more energy to separate its chains than methane needs to separate its small molecules. Heating can soften it without breaking its carbon backbone. It is an electrical insulator because it lacks mobile charged particles. A giant covalent structure, by contrast, has a continuous network of strong covalent bonds rather than separate molecules.

Diamond: four bonds per carbon

Diamond network fragment with four bonds at the central carbonThe central carbon bonds to four surrounding carbon atoms. Two plain bonds, a wedge towards the viewer and a dashed bond away from the viewer indicate a three-dimensional arrangement. Each outer carbon has three additional bond stubs continuing into the network.CCCCC
Strong covalent bonds extend through a three-dimensional network. There are no small diamond molecules.

Graphite: three bonds per carbon

One hexagon within a graphite sheetSix carbon atoms form a hexagon. Each has two bonds around the hexagon and a third bond continuing outwards into the sheet. These are covalent bonds. Each carbon also contributes one electron to delocalisation within the sheet; those electrons are not individually drawn here.CCCCCCGraphite sheets stacked with weaker attractions between themThree broad parallelograms represent whole sheets, not individual bonds. Dashed amber lines between sheets represent weaker attractions. The sheets can slide relative to each other.
Covalent bonds within sheets; weaker attractions between sheets. Each carbon supplies one delocalised electron within its sheet.

Silicon dioxide: a repeating network

Silicon dioxide coordination fragmentThe central silicon bonds to four oxygen atoms. Each of these oxygens bridges to a second silicon. Additional bonds from the four outer silicons continue beyond this fragment. The flat arrangement shows connectivity only; the real network is three-dimensional and the bonds are not at these drawn angles.SiSiSiSiSiOOOO
Each Si bonds to four O atoms; each O joins two Si atoms. Shared oxygen atoms give the ratio Si:O = 1:2.

Original network fragments, not to scale. Bond stubs continue beyond each drawing. Diamond and silica extend in three dimensions; graphite forms extended sheets. The drawings show connections, not measured bond angles.

Diamond: four bonds per carbon. Graphite: three bonds per carbon within layers. Silicon dioxide: a network of silicon and oxygen atoms. Learn to interpret the structures. Drawing the diamond and graphite structures is not required.
Use the arrangement to explain the property
StructureBonding and arrangementProperties and uses
DiamondEach C bonds to four others in a rigid three-dimensional network.Very hard: cutting tools. Very high melting point. No mobile electrons: does not conduct.
GraphiteEach C bonds to three others in a layer. One electron per C is delocalised. Weak forces act between layers.Conducts through mobile electrons. Layers slide: soft lubricant. Strong bonds within layers give a very high melting point.
Silicon dioxide, SiO2Each Si bonds to four O; each O bridges two Si. The formula is a ratio, not a separate molecule.Hard, high melting point; no mobile charged particles, so does not conduct. A principal constituent of sand.

Check your understanding

How can graphite be soft and still have a very high melting point?

  1. Its atoms are joined by weak bonds, but there are so many atoms that it takes a long time to melt.
  2. Its delocalised electrons make it soft, and the same electrons hold the layers together very tightly.
  3. Its layers slide over each other easily, but melting means breaking the strong covalent bonds within the layers.
Show the answer

Its layers slide over each other easily, but melting means breaking the strong covalent bonds within the layers.

Weak forces hold the layers together, so they slide and graphite feels soft. Melting breaks up the giant structure, which needs a lot of energy to break the covalent bonds in each layer.

07

Metals and alloys: properties from arrangement

Recognize a metal and an alloy, then connect structure to their different behaviour.

Metals generally have high melting and boiling points, conduct heat and electricity, and are malleable: they can be hammered into shape. Most are solids at room temperature. These are general patterns, not rules without exceptions.

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In a metal, positive ions occupy a lattice surrounded by delocalised electrons. These electrons are free to move throughout the metal. Metallic bonding is the strong electrostatic attraction between the positive ions and the delocalised electrons.

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Positive ions in delocalised electrons

A metallic structureTwelve positive metal ions form a regular arrangement. Twelve electron symbols lie between them, not attached to specific ions. Metallic bonding is the electrostatic attraction between the positive ions and delocalised electrons. Particle numbers and charges are schematic, not a particular metal formula.++++++++++++
Delocalised electrons can move through the structure. They are not confined to one atom or one bond.
  • Positive metal ion
  • Delocalised electron

A simplified two-dimensional fragment, not to scale. Plus signs indicate positive charge, not a specified ion charge. The electron count is illustrative.

Moving electrons carry charge. When layers of ions slide, attraction to the delocalised electrons continues, so the metal can change shape.
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Explaining metallic properties
PropertyStructural explanation
High melting pointMuch energy is needed to overcome the strong metallic bonding.
Electrical conductivityDelocalised electrons move and carry charge, in both solid and molten metal.
Thermal conductivityMobile electrons transfer energy rapidly through the metal; lattice vibrations also transfer energy.
MalleabilityLayers can slide while metallic attraction holds the structure together.

An alloy is a mixture of a metal with other elements. Brass contains copper and zinc. Stainless steel contains iron and chromium, with other elements depending on its composition. Unlike a compound, an alloy need not have a fixed ratio of elements.

Pure metal: regular layers

Equal-sized atoms in regular rowsThirteen equal-sized particle symbols are arranged in regular offset rows. This compares atomic sizes and arrangement; the delocalised electrons are omitted.
One type of atom, arranged in regular layers.

Alloy: disrupted layers

A different-sized atom distorts the regular arrangementThirteen particle symbols include one larger amber atom. The smaller neighbouring atoms are displaced, so the layers are less regular. This compares atomic sizes and arrangement; the delocalised electrons are omitted.
Different-sized atoms disrupt the regular arrangement.

A simplified substitutional alloy, not to scale. Other alloys may contain smaller atoms in gaps.

A pure metal contains one type of atom. An alloy contains more than one; differently sized circles represent its different constituent atoms.
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Different-sized atoms disrupt the regular arrangement. Layers slide less easily, so alloys are often harder and less malleable than the pure metal. Their melting behaviour can also differ. The explanation must fit the property: disrupted layers explain hardness, not every possible difference between an alloy and its constituents.

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Check your understanding

Why is "the larger atoms are harder" a poor explanation for an alloy being harder than the pure metal?

  1. The different-sized atoms disrupt the regular layers, so the layers cannot slide easily.
  2. The larger atoms form covalent bonds with the metal, and covalent bonds are stronger.
  3. The larger atoms fill the gaps, so the alloy has no delocalised electrons left.
Show the answer

The different-sized atoms disrupt the regular layers, so the layers cannot slide easily.

Hardness comes from the arrangement. Atoms of a different size distort the layers, which makes it harder for layers to slide. A single atom is not hard or soft.

Check your understanding

Four diagrams show particles in a solid. Which one represents an alloy?

  1. Identical circles packed in regular rows with no gaps between them
  2. Alternating circles marked + and - in a regular repeating pattern
  3. Small separate pairs of joined circles, spaced far apart from each other
  4. Regular rows of identical circles with a few larger, different circles in between
Show the answer

Regular rows of identical circles with a few larger, different circles in between

An alloy is a metal mixed with another element, so it has two kinds of atom, and the different sizes break up the regular rows.

Check your understanding

Copper is used for electrical wires and for the bases of cooking pots. Which properties suit both uses?

  1. It has a low melting point and is a poor conductor of heat
  2. It is soft, has a low density and reacts readily with cold water
  3. It is brittle and conducts electricity when molten but not when solid
  4. It conducts electricity and heat well, is malleable and has a high melting point
Show the answer

It conducts electricity and heat well, is malleable and has a high melting point

Wires need a good electrical conductor that can be drawn and bent. Pot bases need a good heat conductor that does not melt on a stove.

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Exam-style question

(a) Explain, in terms of structure and bonding, why copper has a high melting point and conducts electricity when solid. [3]
(b) Brass is an alloy of copper and zinc. Explain why brass is harder than pure copper. [2] [5]

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08

Work backwards from the evidence

Use several properties together, then explain the particles, attractions and charge carriers.

Worked exampleIdentify a likely structure

An unknown solid melts at a high temperature. It does not conduct when solid, but conducts after melting. What structure is most likely?

Try it first. Then show the worked stepsHide the worked steps
  1. The high melting point suggests strong attractions extending through the solid.
  2. The molten material has mobile charged particles.
  3. The solid lacks mobile charge carriers. That makes a metal unlikely.
  4. An ionic lattice fits both observations: fixed ions in the solid, mobile ions after melting.
Answer

A giant ionic structure. Strong electrostatic attractions require much energy to overcome; ions carry charge only when free to move.

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Worked examplePredict a compound from electron arrangement

Element X has electron arrangement 2,8,2. Predict its bonding and formula when it reacts with chlorine.

Try it first. Then show the worked stepsHide the worked steps
  1. X can lose two outer electrons to form X2+, with a full outer shell.
  2. Each chlorine atom gains one electron to form Cl-.
  3. Two chloride ions balance one X2+ ion.
Answer

XCl2, with ionic bonding between X2+ and Cl-. The electron arrangement helps predict chemical behaviour as well as physical properties.

Check your understanding

A pure substance melts at a low temperature and does not conduct as a solid or liquid. Which structure fits?

  1. Giant ionic: its ions are held weakly, so the lattice melts easily.
  2. Giant covalent: its few strong bonds break at a low temperature.
  3. Simple molecular: melting overcomes only weak forces between molecules.
Show the answer

Simple molecular: melting overcomes only weak forces between molecules.

Low melting point means little energy is needed, which fits weak intermolecular forces. The covalent bonds inside the molecules stay intact. No ions or free electrons means no conduction.

Exam-style question

Magnesium oxide melts at 2852 °C. Chlorine is a gas at room temperature. Explain this difference in terms of structure and bonding. [3]

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Exam definitions: learn these in mark-scheme words
TermDefinition to write
ElementA substance that cannot be broken down into simpler substances by chemical methods. It is made of only one type of atom.
CompoundA substance made of two or more elements chemically combined in a fixed ratio.
MixtureTwo or more substances that are not chemically combined. They can be separated by physical methods.
Ionic bondThe electrostatic attraction between oppositely charged ions.
Covalent bondThe electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.
AlloyA mixture of a metal with one or more other elements, usually other metals.
Pure only
Pure only: more exam definitions
TermDefinition to write
Metallic bondThe electrostatic attraction between a lattice of positive metal ions and a sea of delocalised electrons.

Bring it together

Quick revision

For every explanation: name the particles, identify the attraction, and say what can move.

Structure at a glance
Structure and particlesMeltingElectrical conductivity
IonicNaClPositive and negative ionsHigh: strong electrostatic attractions throughout the lattice.Solid: no. Molten or dissolved: yes, mobile ions.
Simple molecularMethane, iodineSeparate moleculesGenerally low: weak intermolecular forces are overcome, not covalent bonds.Pure solid or liquid: generally no mobile charged particles.
MacromolecularPoly(ethene)Long covalent moleculesPure onlySoftens as chains move past one another; covalent backbones remain intact.No mobile charged particles.
Giant covalentDiamond, graphite, SiO2Atoms in a continuous covalent networkPure onlyVery high: strong covalent bonds extend through the structure.Diamond and SiO2: no. Graphite: yes, delocalised electrons.
MetallicCopperPositive ions and delocalised electronsPure onlyGenerally high: strong attraction between ions and electrons.Solid and molten: yes, delocalised electrons.

Three useful corrections

Not conducting? Check whether charged particles can move, not just whether they exist.

Low melting point? Name the weak intermolecular forces, not "weak covalent bonds".

NaCl formula? It gives an ion ratio in a lattice, not a separate molecule.

Scope and references

Syllabus and sources

These notes cover Chemical Bonding and Structure (topic 3) for Pure Chemistry. Explanations, examples and diagrams are original, checked against the official learning outcomes below.

See how the learning outcomes map to these notes

  1. 3.1(a) Form ions by electron transfer

    • Electron loss produces a positive ion; electron gain produces a negative ion
    • Usual noble-gas electron arrangements and unchanged nuclei

    Ionic bonding: transfer, then attraction

  2. 3.1(b) Represent ionic bond formation

    • Metal and non-metal examples NaCl and MgCl2
    • Dot-and-cross electron accounting, ion charges and formula ratios

    Ionic bonding: transfer, then attraction

  3. 3.1(c) Describe a giant ionic lattice

    • NaCl as an extended arrangement of oppositely charged ions
    • Electrostatic attractions throughout the lattice; lattice drawings are not required

    Why salt melts high but conducts only when ions move

  4. 3.1(d) Explain ionic physical properties

    • Strong lattice attractions and high melting points
    • Fixed ions in a solid; mobile ions when molten or dissolved

    Why salt melts high but conducts only when ions move

  5. 3.2(a) Explain a covalent bond

    • A shared electron pair
    • Usual full outer shells, including the two-electron first shell

    Covalent bonding: count shared pairs

  6. 3.2(b) Draw the named covalent molecules

    • Dot-and-cross diagrams for H2, O2, H2O, CH4 and CO2
    • Shared pairs and lone pairs

    Covalent bonding: count shared pairs

  7. 3.2(c) Deduce another molecule from its electrons

    • Count valence electrons, place bonds and check outer shells
    • Apply the method to an unfamiliar simple molecule

    Covalent bonding: count shared pairs

  8. 3.2(d) Connect covalent structure and properties

    • Distinguish covalent bonds from intermolecular attractions
    • Melting, boiling and electrical conductivity from the actual particles and structure

    Strong bonds inside; weaker attractions between

  9. 3.3(a) Describe metallic bonding

    • Positive metal ions and delocalised electrons
    • Attraction between the ions and the electron sea

    Metals and alloys: properties from arrangement

  10. 3.3(b) Explain the general properties of metals

    • High melting and boiling points, with appropriate exceptions
    • Malleability and conduction of heat and electricity

    Metals and alloys: properties from arrangement

  11. 3.4(a) Distinguish an element, compound and mixture

    • Particle identity and chemical combination
    • A molecular element is still an element; a compound need not consist of molecules

    First, identify the particles

  12. 3.4(b) Recognise an alloy

    • A mixture containing a metal and another element
    • Brass and stainless steel

    Metals and alloys: properties from arrangement

  13. 3.4(c) Identify metals and alloys in diagrams

    • A regular arrangement in a pure metal
    • Different particles in an alloy representation

    Metals and alloys: properties from arrangement

  14. 3.4(d) Explain changed properties of alloys

    • Different-sized particles disrupt regular layers
    • Relate harder layer movement to strength or hardness

    Metals and alloys: properties from arrangement

  15. 3.4(e) Compare molecular and giant structures

    • Simple molecular methane and iodine
    • Macromolecular poly(ethene)
    • Giant covalent silicon dioxide, diamond and graphite

    Strong bonds inside; weaker attractions betweenA long molecule is different from a giant network

  16. 3.4(f) Compare diamond and graphite

    • Bonding, structure and electrical conductivity
    • Cutting action and lubrication; drawing the full structures is not required

    A long molecule is different from a giant network

  17. 3.4(g) Reason from properties to structure and back

    • Combine several physical observations rather than one clue
    • Use bonding and particle identity to explain properties and avoid unsupported chemical predictions

    A long molecule is different from a giant networkWork backwards from the evidence