Metallic Bonding

Why does copper conduct electricity? Most students answer “because it’s a metal”, which is exactly the zero-mark circle Paper 4 is built to catch. The Supplement model behind the real answer takes one sentence: a lattice of positive ions in a sea of delocalised electrons. Learn that sentence and its three applications (conduction, malleability, melting point) and this subtopic becomes 3-4 reliable marks on the Extended papers.

The model (Supplement)

In a solid metal, the atoms release their outer-shell electrons into a shared pool. What remains is a regular lattice of positive metal ions, surrounded by a “sea” of delocalised electrons: electrons that belong to no single atom and move freely throughout the structure. Metallic bonding is the electrostatic attraction between these positive ions and the delocalised electrons.

Three pieces, all required in a definition: (1) positive ions arranged in a lattice, (2) delocalised (free-moving) electrons, (3) electrostatic attraction between them. The model completes the bonding trilogy alongside ionic bonds and giant covalent structures, and exam questions enjoy making you compare all three.

Properties explained by the model

Property	Explanation from the model
Conduct electricity (solid and molten)	Delocalised electrons are free to move through the structure and carry charge
Conduct heat well	Free electrons transfer kinetic energy quickly through the metal
Malleable and ductile	Layers of positive ions slide over each other; the delocalised electrons move with them, so the bonding is not broken
Mostly high melting points	Strong electrostatic attraction between positive ions and the electron sea needs a large amount of energy to overcome

The malleability explanation is the one examiners use to separate grades. In an ionic crystal, shifting a layer brings like charges face to face and the crystal shatters. In a metal, the electron sea is everywhere, so a shifted layer is still bonded. The metal bends instead of breaking. This idea returns in topic 9, where alloys are harder than pure metals because differently sized atoms disrupt the regular layers and stop them sliding.

Note the charge carriers. In a metal, current is carried by electrons. In a molten or dissolved ionic compound, it is carried by ions. Writing the wrong carrier is a classic crossed wire between the two models, and electrolysis questions in topic 4 test exactly this distinction at the electrodes.

A worked comparison

Questions rarely ask for the model in isolation; they ask why magnesium conducts but magnesium oxide (solid) does not, or why copper bends but sodium chloride crystals crack. Answer template: name the particles present, state which (if any) are free to move, conclude. Magnesium: positive Mg ions in a sea of delocalised electrons; electrons free to move; conducts. Solid MgO: ions fixed in a lattice; no mobile charged particles; does not conduct.

Worked exam question

Aluminium is used for overhead power cables. (a) Describe the bonding in aluminium. (2) (b) Explain why aluminium conducts electricity. (1) (c) Explain why aluminium can be drawn into wires without breaking. (2)

Model answer: (a) A lattice of positive aluminium ions (1) surrounded by a sea of delocalised electrons, with electrostatic attraction between the ions and the electrons (1). (b) The delocalised electrons are free to move through the metal and carry charge (1). (c) The layers of positive ions can slide over each other (1); the delocalised electrons continue to attract the ions, so the metallic bonds do not break (1).

Mark-by-mark: (a) splits into the ions point and the electrons-plus-attraction point; “atoms in a sea of electrons” loses the first mark because the particles must be positive ions. (b) needs free-moving electrons carrying charge, one clean sentence. (c) the slide mark and the bonding-maintained mark are separate; most candidates earn the first and forget the second.

The mistakes that cost marks

1. “Positive atoms” or just “atoms” in the lattice. Once the outer electrons are delocalised, the particles are positive ions, and the word ions is checked.
2. Explaining conduction with moving ions. The ions vibrate but stay in the lattice; the electrons do the conducting. (Save mobile ions for molten ionic compounds.)
3. Omitting “electrostatic attraction”. Ions plus electrons with no named force describes ingredients, not a bond.
4. Circular answers: “metals conduct because they are good conductors”. Every explanation must reach particle level or it scores nothing.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”Metal atoms float in electrons"	"A lattice of positive ions surrounded by a sea of delocalised electrons"
"Electricity flows through easily"	"Delocalised electrons are free to move and carry charge through the structure"
"Metals bend because they’re flexible"	"Layers of positive ions slide over each other while the delocalised electrons maintain the attraction"
"The bond is strong"	"Strong electrostatic attraction between the positive ions and the delocalised electrons”

The Malaysia note

Metallic bonding is the shortest Supplement subtopic in topic 2, and Malaysian Extended candidates sitting May/June tend to lose its marks not to difficulty but to imprecision: “atoms” for ions, “electricity passes” for electrons carrying charge. Mark schemes are unforgiving on this vocabulary. One focused session of phrasing drills usually locks in all of it; that is the level of specific fix a free 1-hour trial lesson with a Chemistry specialist is designed to find and deliver.

Test yourself

All three are Supplement level. Answer first, then click; the answers stay hidden until you do.

Q1 (2 marks). Describe the bonding in solid copper.

Show answer

• A (regular) lattice of positive copper ions [1] • Surrounded by a sea of delocalised electrons, with electrostatic attraction between the ions and the electrons [1]

Q2 (2 marks). Explain, in terms of its bonding, why magnesium has a high melting point.

Show answer

• There is strong electrostatic attraction between the positive (magnesium) ions and the delocalised electrons [1] • A large amount of energy is needed to overcome this attraction [1]

Q3 (3 marks). A copper rod bends when struck, but a crystal of copper(II) oxide shatters. Explain this difference in terms of structure and bonding.

Show answer

• In copper, the layers of positive ions can slide over each other [1] • The delocalised electrons move with the layers, so the metallic bonding is not broken and the metal bends [1] • In ionic copper(II) oxide, a shifted layer brings ions of like charge next to each other; they repel and the crystal shatters [1]