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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

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 understandingA diagram contains only identical pairs of oxygen atoms. Element, compound or mixture?Think it through, then reveal the answer
An element. Each pair is a molecule, but every atom is oxygen. Two atoms do not necessarily mean 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.

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.
Check your understandingWhy is "solid salt has no charged particles" an incorrect explanation for its lack of conductivity?Think it through, then reveal the answer
Solid salt contains charged ions. They are held in the lattice and cannot move through it. Conductivity requires mobile charged particles.
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 example

Deduce an unfamiliar diagram: ammonia

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

  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.

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.

06

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.

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.
07

Work backwards from the evidence

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

Worked example

Identify 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?

  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.

Check your understandingA pure substance melts at a low temperature and does not conduct as a solid or liquid. Suggest a structure and explain the low melting point.Think it through, then reveal the answer
A simple molecular structure is likely. Relatively little energy is needed to overcome weak intermolecular forces. The covalent bonds within the molecules remain intact during melting.

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.

Metals: generally solid, with high melting and boiling points; conduct heat and electricity and are malleable. Alloys: mixtures containing a metal and other elements; identify more than one type of atom in the diagram.

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

Your syllabus, covered

These notes cover Chemical Bonding and Structure (topic 3) for Combined Science 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) 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 moveWork backwards from the evidence

  4. 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

  5. 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

  6. 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

  7. 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 betweenWork backwards from the evidence

  8. 3.3(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

  9. 3.3(b) Describe the general properties of metals

    • Generally high melting and boiling points
    • Malleability and good conduction of heat and electricity

    Metals and alloys: properties from arrangement

  10. 3.3(c) Recognise an alloy

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

    Metals and alloys: properties from arrangement

  11. 3.3(d) 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