Chemical Bonds

The three types of strong chemical bond — ionic, covalent, and metallic. The framework that runs through every question in Module 2, and the one fatal error that costs marks every single year.

SPEC 4.2.1.1 • BONDING

The Three Types of Bond

Every substance in GCSE Chemistry is held together by one of three types of strong chemical bond. Get these three names and what they mean, and you have the framework for the entire module.

Ionic

Metal + Non-metal

Electrons are transferred from the metal to the non-metal.

Example: NaCl

Covalent

Non-metal + Non-metal

Electrons are shared between two non-metal atoms.

Example: CO₂

Metallic

Metal only

Electrons become delocalised across all the metal ions.

Example: Cu

The Core Rule

Look at the elements in the substance. Metal + non-metal → ionic. Non-metals only → covalent. Metal(s) only → metallic. This decision tree handles every substance you'll meet at GCSE.

Definition: Ionic Bond

The mark-scheme definition. AQA tests this directly as a 1-mark "define" question — learn it word for word.

Ionic Bond

A strong electrostatic force of attraction between oppositely charged ions.

Where it happens

Ionic bonding occurs in compounds formed from metals combined with non-metals. The metal atom loses electrons to form a positive ion. The non-metal atom gains electrons to form a negative ion. The opposite charges then attract each other — that attraction is the ionic bond.

Mark-scheme phrase

The exact words AQA wants: "electrostatic force of attraction" and "oppositely charged ions". "Strong attraction between + and −" alone may not get the mark — the word electrostatic matters.

Definition: Covalent Bond

The shortest definition of the three — but the easiest one to get wrong by writing too vaguely.

Covalent Bond

A shared pair of electrons between two non-metal atoms.

Where it happens

Covalent bonding occurs in most non-metallic elements (e.g. O₂, N₂, Cl₂) and in compounds of non-metals (e.g. H₂O, CO₂, CH₄). Each atom in the bond contributes one electron to the shared pair, so both atoms count the pair towards their outer shell.

Mark-scheme phrase

"Shared pair of electrons" — not just "shared electrons", not "shared bond". The word pair matters: one shared pair = one covalent bond. Two shared pairs between the same two atoms = a double bond.

Definition: Metallic Bond

The third bond type. Different mechanism, different language — and the one students most often forget exists.

Metallic Bond

The attraction between positive metal ions and a sea of delocalised electrons.

Where it happens

Metallic bonding occurs in metallic elements (e.g. Cu, Fe, Mg) and in alloys (mixtures of metals, e.g. brass, steel). Each metal atom releases its outer electron(s) into a shared "sea" that runs through the whole structure. What remains are positive ions, held in place by their attraction to that electron sea.

Mark-scheme phrase

The key word is "delocalised" — the term used in the spec. AQA mark schemes also accept "free electrons" (real example: P1 Jun22 Q01.5 says "allow free electrons"). Both score the mark, but "delocalised" is the technical term and the safer choice in extended answers.

How to Identify the Bond Type

Given any substance, work out the bond type from the elements involved. This decision is asked directly in the exam — usually as a 1-mark question per substance.

Elements involvedBond typeWhat happens to electrons

Metal + Non-metalIonicTransferred from metal to non-metal

Non-metal + Non-metalCovalentShared between the two atoms

Metal only / Metals togetherMetallicDelocalised — released into a shared sea

Ionic TRANSFERRED

Covalent SHARED

Metallic DELOCALISED

Memorise these three pairings

Three bond types. Three verbs. Match them correctly and you protect every mark in this module. Mix them up and you've described a different bond.

Visualising the Three Bonds

One picture for each bond. Different mechanisms, all driven by the same goal: stable electron arrangements.

One picture, one verb

Ionic: the electron leaves one atom and goes to the other — transfer. Covalent: the electron pair stays between the two atoms — share. Metallic: the electrons leave their atoms entirely and roam — delocalised.

01Worked Example 3 MARKS

Name the type of bonding in each of the following substances:

(a) NaCl (b) CO₂ (c) Cu

(a)

NaCl — Na is a metal (Group 1), Cl is a non-metal (Group 7). Metal + non-metal → ionic bonding.

(b)

CO₂ — both C and O are non-metals. Non-metal + non-metal → covalent bonding.

(c)

Cu — copper is a metal on its own (no non-metal). → metallic bonding.

Mark scheme note: 1 mark per correct identification. AQA generally accepts minor / phonetic spelling errors where the meaning is clear, but write the words carefully — "ionic", "covalent", "metallic" — so there's no ambiguity for the examiner.

Why Do Atoms Bond?

One reason underpins all three bond types: atoms bond to reach a stable electron arrangement — usually a full outer shell.

The driving force

Most atoms have incomplete outer shells, which makes them reactive. By bonding, they end up with a full outer shell — the same electron arrangement as a noble gas. That's the goal. Each bond type is just a different mechanism for getting there.

Bond typeHow the full outer shell is achieved

IonicMetal loses outer electrons (revealing a full shell underneath). Non-metal gains electrons to fill its outer shell. Both ions end up with full outer shells.

CovalentBoth atoms share electrons. The shared pair counts towards both atoms' outer shells, so both end up with a full outer shell.

MetallicMetal atoms release their outer electrons into a shared sea. The remaining positive ions have full inner shells; the delocalised electrons hold the structure together.

Examiner watch-out

In ionic bonding, the language matters: atoms react, ions form. After transfer, refer to ions, not atoms. Saying "the atoms have full outer shells" is wrong — the atoms are gone, replaced by ions.

Examiner Alert: The Fatal Error

P1 Jun24 Q05.1 — the most-penalised error in Module 2

Every year, students write "atoms share electrons in ionic bonding". Every year, the mark is lost. The examiner is unforgiving: if you describe a bond using the wrong verb, you have described a different bond.

Weak student

"In NaCl, the sodium and chlorine atoms share electrons to form an ionic bond."

Result: 0 marks. ✘

Used "share" for ionic. The verb describes covalent bonding — so the answer doesn't describe ionic bonding at all.

Strong student

"In NaCl, sodium transfers an electron to chlorine. Na⁺ and Cl⁻ ions form, held together by electrostatic attraction."

Result: Full marks. ✔

Used "transfer" for ionic. Mentioned the resulting ions and the electrostatic attraction.

The contradiction trap

Even worse: students who correctly write "transferred" and then add "…and the atoms share the electron pair" have contradicted themselves. AQA cancels the mark. Once you've said "transferred", never say "shared" anywhere in the same answer.

PEE Worked Example 3 MARKS02

Sodium chloride (NaCl) contains ionic bonds. Explain how the bond between sodium and chlorine forms. Refer to electrons.

•

PPoint. NaCl is ionic because it contains a metal (Na) and a non-metal (Cl).

•

EEvidence. Sodium loses one outer electron to form Na⁺. Chlorine gains that electron to form Cl⁻. Both ions have full outer shells.

•

EExplain. The oppositely charged Na⁺ and Cl⁻ ions attract each other by electrostatic forces, forming an ionic bond.

Mark-scheme keywords: "transferred / lost / gained" (1 mark) "Na⁺ and Cl⁻ ions form" (1 mark) "electrostatic attraction between oppositely charged ions" (1 mark). The word shared appears nowhere — if it did, the answer would be self-contradictory.

Common Errors

Students often writeCorrect versionWhy it matters

"Atoms share electrons in ionic bonding""Electrons are transferred from the metal to the non-metal"The verb defines the bond type. Wrong verb = wrong bond.

"Covalent bonds form between metals""Covalent bonds form between non-metals "Metal + metal → metallic. Covalent needs two non-metals.

"Metallic bonding only happens in pure metals""Metallic bonding occurs in metals and alloys "Spec 4.2.1.1 explicitly includes alloys (e.g. brass, steel)

"Strong attraction between + and −" (for ionic)" Electrostatic force of attraction between oppositely charged ions"Missing "electrostatic" can lose the mark on a 1-mark definition.

Describing transfer, then adding "…and they share"Pick one verb and stick with it.Contradicting yourself cancels the mark.

Practice Question 1

Name the type of bonding in each of the following substances.4 MARKS

(a) MgO (b) H₂O (c) Fe (d) brass (a mixture of Cu and Zn)

(Style based on AQA 8462/1H bond-identification questions)

(a)

MgO — Mg is a metal, O is a non-metal → ionic.

(b)

H₂O — H and O are both non-metals → covalent.

(c)

Fe — iron on its own, a metal → metallic.

(d)

Brass — a mixture of two metals (Cu and Zn) → metallic (alloys are metallic).

Common slip: students sometimes label H₂O as ionic because "water dissolves things". The bond inside the H₂O molecule is covalent. What dissolves is unrelated to its bonding.

Practice Question 2

What is meant by a covalent bond?1 MARK

A

A shared pair of electrons between two non-metal atoms.

Mark scheme: "shared pair of electrons" earns the mark. "Shared electrons" alone is risky — AQA wants the word pair, because the bond is one pair. Do not accept: "atoms share atoms", "shared bond", or anything mentioning transfer.

Practice Question 3

Magnesium chloride (MgCl₂) is an ionic compound. Describe how the bonds in MgCl₂ form. Refer to electrons in your answer.3 MARKS

•

P MgCl₂ is ionic — Mg is a metal, Cl is a non-metal.

•

E Each magnesium atom transfers two electrons — one to each of two chlorine atoms. Mg becomes Mg²⁺; each Cl becomes Cl⁻.

•

E The Mg²⁺ and Cl⁻ ions are held together by strong electrostatic forces of attraction between the opposite charges.

Mark-scheme breakdown: 1 mark for electrons transferred from Mg to Cl 1 mark for the ions formed (Mg²⁺ and Cl⁻, with correct charges) 1 mark for electrostatic attraction between oppositely charged ions. Watch: the ratio (1 Mg to 2 Cl) is given by the formula MgCl₂; getting that wrong loses the second mark.

Summary

Three Bond Types — Three Verbs

Ionic (metal + non-metal): electrons transferred — electrostatic attraction between oppositely charged ions

Covalent (non-metal + non-metal): a shared pair of electrons between two atoms

Metallic (metal only / alloys): positive ions held by a sea of delocalised electrons

The Identification Rule

Metal + non-metal → ionic

Non-metal + non-metal → covalent

Metal(s) only → metallic

Examiner Alert — the #1 trap

Writing "share" for ionic bonding is the most-penalised error in Module 2. Three bond types, three verbs — transferred / shared / delocalised — never mix them up, and never contradict yourself within the same answer.

Why atoms bond

To achieve a stable electron arrangement — usually a full outer shell. Ionic, covalent, and metallic are three different mechanisms for the same goal.

Ready for 2.02

Next up: Ionic Bonding and Ionic Compounds — the formation mechanism in detail, dot-and-cross diagrams for Group 1 / 2 / 6 / 7 elements, and the giant ionic lattice.