Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.
Positive metal ions in a sea of delocalised electrons. Why metals conduct, can be hammered into shape, and have high m.p.
At a glance
Metallic bond: attraction between positive metal ions and a 'sea' of delocalised electrons.
Metal atoms lose their outer electrons; electrons free to move through the lattice.
Conducts — delocalised electrons carry charge.
Malleable and ductile — layers of ions slide without breaking the bond.
High m.p. (most metals) — strong attraction across the lattice.
Shiny — delocalised electrons reflect light.
What you’ll learn
Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).
2.4 — Describe the lattice of positive ions in a sea of electrons.
2.4 — Explain physical properties of metals using metallic bonding.
The metallic bond
Metal atoms donate outer electrons to a delocalised sea; positive ions sit in the lattice.
Setup. In a metal:
Each atom donates its outer-shell electrons to a 'sea' shared by all the metal ions.
The metal atoms become positive ions (cations), arranged in a regular lattice.
The delocalised electrons can move freely throughout the metal.
The bonding is the strong electrostatic attraction between the positive ions and the negative sea.
Two parts to remember:
Lattice of positive ions (where atoms once were).
Sea of delocalised electrons (the donated outer electrons, free to roam).
This bonding is in 3D. Layers of ions stacked together, with electrons moving in the spaces between.
Cambridge tip. When asked to "describe the bonding in a metal", say BOTH:
Lattice of positive ions.
Surrounded by a sea of delocalised electrons.
Strong electrostatic attraction between them.
Regular lattice of positive metal ions held together by a shared sea of delocalised electrons.
Positive metal ions in a regular lattice.
Sea of delocalised electrons.
Electrostatic attraction holds them together.
Strong, in 3D.
Properties explained
Conducts (delocalised e⁻), malleable (layers slide), high m.p. (strong attraction).
1. Electrical conductivity. Delocalised electrons can move through the lattice and carry charge. Metals conduct as solids AND in molten form (no need to melt them, unlike ionic compounds).
2. Thermal conductivity. Delocalised electrons also transfer kinetic energy quickly through the metal. That's why metals feel cold when you touch them — they conduct heat away from your skin.
3. Malleability and ductility.
Malleable: can be hammered into thin sheets.
Ductile: can be drawn into wires.
When force is applied, the layers of metal ions slide over each other. The delocalised electrons follow along — the metallic bonding is preserved. The metal deforms WITHOUT shattering. Compare with ionic compounds, which crack along cleavage planes when ions are displaced.
Layers of ions slide over one another; the electron sea keeps the bonding intact, so the metal bends instead of breaking.
4. High melting and boiling points. Strong electrostatic attraction across the entire lattice → lots of energy needed to break it. Most metals melt at >600°C.
Notable exception: mercury (m.p. −39°C) — only liquid metal at room temp.
5. Shiny appearance. Delocalised electrons absorb and re-emit photons of all visible wavelengths → silvery reflective lustre.
6. High density. Atoms tightly packed in the lattice.
Worked qualitative. Why is copper a good electrical wire material? It has many delocalised electrons (high conductivity), is ductile (can be drawn into wires), is malleable (easily worked), and is reasonably cheap. It also forms an oxide layer (verdigris) that protects underlying copper from further corrosion.
Worked qualitative. Why does an iron rod feel cold when you touch it but a wooden stick at the same temperature feels warm? Iron's delocalised electrons quickly conduct heat AWAY from your fingers. Wood is a poor conductor; little heat flows.
Conducts (electrons & heat).
Malleable & ductile (layers slide).
High m.p. (strong attraction).
Shiny, dense.
Alloys (extension)
A mixture of a metal with another element. Often harder than the pure metal because layers can't slide as easily.
Alloy. A mixture of a metal with one or more other elements (often another metal).
Why alloy? Pure metals are often too soft for engineering uses. Alloying makes them harder by DISRUPTING the regular layer arrangement: when atoms of different sizes are mixed in, the layers can't slide as easily.
Common alloys.
Brass = copper + zinc. Used for instruments, fittings.
Bronze = copper + tin. Used for statues, bearings.
Steel = iron + carbon (and trace others). Stronger than iron.
Stainless steel = iron + chromium + nickel. Resistant to corrosion.
Worked qualitative. Why is steel harder than pure iron? Carbon atoms are smaller than iron atoms; they fit in the gaps in the iron lattice and prevent layers from sliding past each other.
Cambridge tip. When asked "explain why an alloy is harder than the pure metal", mention DIFFERENT-SIZED atoms disrupting layer sliding.
Alloy: metal + other element(s).
Harder than pure metal.
Different atom sizes block layer sliding.
Examples: brass, bronze, steel.
Quick recap
Metallic bond: + ions in sea of delocalised electrons.
Conducts (electrons move).
Malleable / ductile (layers slide).
High m.p. (strong attraction).
Shiny.
Alloys: harder because layers can't slide as easily.
Memorise this
Verbatim phrases and definitions Cambridge mark schemes credit.
Metallic bond — electrostatic attraction between a lattice of positive ions and a sea of delocalised electrons.
Delocalised electron — outer-shell electron not associated with one specific atom; free to move through the lattice.
Malleable — can be hammered into shape without breaking.
Ductile — can be drawn into wires.
Alloy — mixture of a metal with one or more other elements.
How it’s examined
Metallic bonding appears every Paper 2 (3-4 marks: describe the bond, properties) and Paper 4 (4-6 marks: explain a property, alloy hardness). Examiner reports flag students saying 'metallic bond is between metals and non-metals' (that's IONIC bonding).
Step-by-step solutions to past-paper-style questions on metallic bonding, written exactly the way a tutor would explain them at the board.
1Describe the metallic-bonding model
Getting started• definition, model
▼
Question
Describe the structure and bonding in a metal using the precise Cambridge wording.
Step-by-step solution
Step 1
A metal is a giant lattice (regular arrangement) of positively charged metal ions.
Step 2
The outer-shell electrons are delocalised — they are free to move and form a 'sea' of electrons that surrounds the ions.
Step 3
The bonding is the strong electrostatic attraction between the positive ions and the sea of delocalised electrons, which holds the lattice together.
Answer
A lattice of positive metal ions in a sea of delocalised electrons; the metallic bond is the electrostatic attraction between the positive ions and the delocalised electrons.
Examiner tip
Three marks need all three ideas: positive ions, delocalised electrons, and electrostatic attraction between them. 'Sea of electrons' alone is not enough — name the attraction.
2Why metals conduct electricity
Getting started• Adapted from 0620/42 May/Jun 2024 Q7• conductivity
▼
Question
Explain why metals are good conductors of electricity.
Step-by-step solution
Step 1
Metals contain delocalised electrons that are free to move throughout the whole lattice.
Step 2
When a potential difference is applied, these mobile electrons drift through the metal.
Step 3
Moving electrons carry electrical charge, so a current flows — this is what makes the metal a good electrical conductor.
Answer
The delocalised electrons are free to move and carry charge through the lattice when a voltage is applied, so a current flows.
Examiner tip
The mark is for 'delocalised/free electrons that MOVE and carry charge'. Just saying 'it has electrons' scores nothing — every substance has electrons.
3Why metals are malleable and ductile
Building confidence• malleability, ductility
▼
Question
Explain, in terms of structure and bonding, why metals can be hammered into shape (malleable) and drawn into wires (ductile).
Step-by-step solution
Step 1
The metal ions are arranged in layers within the lattice.
Step 2
When a force is applied, layers of positive ions can slide over each other into new positions.
Step 3
The bonding is non-directional — the sea of delocalised electrons surrounds the ions wherever they move, so the metallic bond is not broken. The ions remain held together in their new shape.
Step 4
Because the layers slide without the structure shattering, the metal can be reshaped (malleable) or pulled into a wire (ductile).
Answer
Layers of positive metal ions slide over one another when a force is applied; the delocalised electrons keep the ions bonded after sliding, so the metal changes shape without breaking.
Examiner tip
Examiners reject 'the bonds break, then re-form'. The bonds DON'T break — layers slide while the delocalised electrons keep them held together.
4Why metals conduct heat
Building confidence• thermal conductivity
▼
Question
Explain why metals are good conductors of heat (thermal energy).
Step-by-step solution
Step 1
The delocalised electrons are free to move through the whole metal lattice.
Step 2
When one end is heated, those electrons gain kinetic energy and move faster.
Step 3
The mobile electrons quickly transfer this kinetic energy as they travel through the metal and collide with ions, spreading thermal energy rapidly from the hot end to the cold end.
Answer
The delocalised electrons gain kinetic energy at the hot end and, being free to move, transfer that energy rapidly through the lattice, so heat is conducted quickly.
Examiner tip
Link the SAME delocalised electrons to thermal conduction — they carry kinetic (thermal) energy, not just electrical charge.
5Comparing metallic melting points
Stretch• properties, reasoning
▼
Question
Magnesium (m.p. 650°C) has a much higher melting point than sodium (m.p. 98°C). Explain this difference in terms of metallic bonding.
Step-by-step solution
Step 1
Both are giant metallic lattices of positive ions in a sea of delocalised electrons, so both have high melting points compared with simple molecules.
Step 2
Each magnesium atom loses two outer electrons to form Mg2+, whereas each sodium atom loses one to form Na+.
Mg→Mg2++2e−
Step 3
Magnesium therefore has more delocalised electrons per ion and a higher charge on each ion, giving a stronger electrostatic attraction between the ions and the electron sea.
Step 4
Stronger metallic bonding means more energy is needed to overcome it, so magnesium has the higher melting point.
Answer
Magnesium forms Mg2+ ions and releases two delocalised electrons per atom, giving stronger electrostatic attraction than sodium's Na+ and one electron — so more energy is needed to melt it.
Examiner tip
The discriminator is 'more delocalised electrons AND higher ionic charge → stronger attraction'. Avoid vague 'bigger atom' answers.
6Why alloys are harder than pure metals
Stretch• Links to Metals topic (alloys)• alloys, reasoning
▼
Question
A pure metal is bent easily, but an alloy of the same metal is harder. Use the metallic-bonding model to explain why, and explain why the alloy still conducts electricity.
Step-by-step solution
Step 1
In a pure metal the ions are the same size and arranged in regular layers, so the layers can slide over each other easily — the metal is soft and bends.
Step 2
An alloy contains atoms/ions of different sizes. The different-sized ions distort the regular layers so the layers can no longer slide easily past one another.
Step 3
Because sliding is disrupted, the alloy is harder and stronger than the pure metal.
Step 4
The alloy still has a sea of delocalised electrons that are free to move, so it still conducts electricity — disrupting the layers does not remove the delocalised electrons.
Answer
Different-sized ions in an alloy distort the layers, so they cannot slide easily, making the alloy harder; the delocalised electrons are still free to move, so it still conducts electricity.
Examiner tip
Two ideas score: (1) different-sized atoms disrupt/distort the layers so they can't slide; (2) delocalised electrons remain, so conduction is unchanged.
Model Answers — Metallic Bonding
High-scoring sample answers for metallic bonding on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Question 1
Paper 4 short-answer style1 mark
State the type of bonding present in a sample of pure copper. (1 mark)
Model answer
Metallic bonding (metallic bonds).
Why this scores
One mark for 'metallic'. A metal element on its own always has metallic bonding — not ionic or covalent.
Question 2
Paper 4 short-answer style2 marks
Name the two types of particle present in the structure of a metal. (2 marks)
Model answer
Positively charged metal ions (positive ions), and delocalised electrons (a 'sea' of free outer-shell electrons).
Why this scores
Two marks: (1) positive metal ions; (2) delocalised/free electrons. 'Atoms' is not accepted — the atoms have lost their outer electrons to become ions.
Question 3
Paper 4 structured style3 marks
Describe the bonding in a metal. (3 marks)
Model answer
A metal consists of a giant lattice of positively charged metal ions surrounded by a 'sea' of delocalised electrons. The metallic bond is the strong electrostatic attraction between the positive ions and the delocalised electrons, and this attraction holds the lattice together.
Why this scores
Three marks: (1) lattice of positive ions; (2) delocalised/'sea' of electrons; (3) electrostatic attraction between them. All three keywords must appear.
Question 4
Paper 4 structured style4 marks
Explain why metals are good conductors of both electricity and heat. (4 marks)
Model answer
Metals contain delocalised electrons that are free to move throughout the lattice. When a potential difference is applied, these electrons drift through the metal and carry charge, so a current flows — this makes the metal a good conductor of electricity. When the metal is heated, the same delocalised electrons gain kinetic energy and, being free to move, transfer this energy quickly through the structure, so the metal is also a good conductor of heat.
Why this scores
Four marks: free/delocalised electrons; they move and carry charge (electricity); they gain kinetic energy when heated; they transfer that energy through the lattice (heat).
Question 5
Paper 4 (Extended) structured style5 marks
Explain, in terms of its structure and bonding, why a metal is malleable and ductile. (5 marks)
Model answer
A metal is made of layers of positive ions in a sea of delocalised electrons. When a force is applied, the layers of ions can slide over each other into new positions. Because the metallic bonding is non-directional, the delocalised electrons surround the ions wherever they move, so the metallic bond is not broken — the ions stay held together. As a result the metal can be hammered or rolled into thin sheets (malleable) and drawn out into wires (ductile) without shattering.
Why this scores
Five marks: layers of ions; layers slide over each other; bonding is non-directional / electrons still surround the ions; bond is not broken; hence malleable and ductile. Stating that 'bonds break' loses marks.
Question 6
Paper 4 (Extended) structured style6 marks
Aluminium has a much higher melting point than sodium. Both are metals. Using the metallic-bonding model, explain why aluminium has the higher melting point, and explain why aluminium and its alloys are widely used. (6 marks)
Model answer
Both metals have a giant lattice of positive ions in a sea of delocalised electrons, held by electrostatic attraction. Each aluminium atom loses three outer electrons to form Al3+, while each sodium atom loses only one to form Na+. Aluminium therefore has more delocalised electrons per ion and a higher charge on each ion, giving a stronger electrostatic attraction between the ions and the electron sea. Because the metallic bonding is stronger, more energy is needed to overcome it, so aluminium has the much higher melting point. Aluminium and its alloys are widely used because they are good conductors, malleable/ductile (layers of ions slide), and strong for their low density — and alloying with different-sized atoms distorts the layers so they cannot slide easily, making the alloy harder and stronger while the delocalised electrons keep it conducting.
Why this scores
Six marks across: lattice of ions + electron sea + electrostatic attraction; Al loses 3 e⁻ vs Na 1 e⁻; more electrons and higher charge → stronger attraction; more energy to overcome → higher m.p.; useful properties (conduction/malleability/low density); alloying distorts layers → harder while still conducting.
Key Definitions and Keywords — Metallic Bonding
Definitions to memorise and the exact keywords mark schemes credit for metallic bonding answers — sharpened from recent examiner reports for the 2026 0620 sitting.
Metallic bond
Examiner keyword
The strong electrostatic attraction between a lattice of positive metal ions and the surrounding 'sea' of delocalised outer-shell electrons.
Delocalised electrons
Examiner keyword
Outer-shell electrons that are not bound to a single atom and are free to move throughout the whole metal lattice (the 'sea' of electrons).
Malleable
Examiner keyword
Able to be hammered or rolled into thin sheets without breaking, because layers of ions can slide over each other.
Ductile
Examiner keyword
Able to be drawn out into wires, because layers of ions can slide while the metallic bond is maintained.
Giant metallic lattice
The regular, repeating three-dimensional arrangement of positive metal ions held in place by the sea of delocalised electrons.
Common Mistakes and Misconceptions — Metallic Bonding
The traps other students keep falling into on metallic bonding questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.
✕Saying electrons are 'shared' in metallic bonding.
0620/42 — recurring
▼
Why it happens
Confusing metallic bonding with covalent bonding.
How to avoid it
Use the word DELOCALISED — the outer electrons are free to move, not shared between two atoms and not transferred to another atom.
✕Saying metals are malleable because the metallic bonds break and re-form.
▼
Why it happens
Misunderstanding how the layers move.
How to avoid it
The bonds DON'T break — layers of ions slide over each other while the delocalised electrons keep them bonded throughout.
✕Describing the lattice as made of metal atoms rather than positive ions.
▼
Why it happens
Forgetting that the atoms have lost their outer electrons to the electron sea.
How to avoid it
The particles are positive ions (e.g. Na+, Mg2+, Al3+) because their delocalised electrons have left them.
✕Explaining conduction by just saying 'metals have electrons'.
▼
Why it happens
Knowing electrons are involved but not stating the key idea.
How to avoid it
Every substance has electrons. The mark is for delocalised/free electrons that MOVE and carry charge through the lattice.
Metallic Bonding — frequently asked questions
The things students keep getting wrong in this sub-topic, answered.