Metallic bond — ions in an electron sea
A metal is a regular 3-D lattice of positive ions surrounded by a sea of delocalised electrons that move throughout the structure. The positive ions are metal atoms that have lost their outer electrons (for example Na+ or Cu2+). The metallic bond is the strong electrostatic attraction between the positive ions and the electron sea. For full marks name both the lattice of positive ions AND the sea of delocalised electrons.
Metals conduct via delocalised electrons
Metals conduct because the sea of delocalised electrons is free to move. When a potential difference is applied, these electrons drift through the lattice towards the positive terminal, and this movement of electrons is the current. For full credit use "delocalised" or "mobile" (not just "free"), and say electrons, not ions, carry the charge. Metals conduct in both the solid and liquid state.
Malleability — layers slide, sea holds
Metals are malleable and ductile because the positive ions lie in layers that slide over one another when a force is applied. As the layers move, the sea of delocalised electrons stays around the ions, so the electrostatic attraction is maintained and the bond is not broken; the metal changes shape instead of shattering. Contrast ionic solids, where a shift brings like charges together, so they repel and the crystal shatters.
Drawn from real examiner reports.
"Free electrons" scores nothing
Across several sittings candidates described the bonding electrons as "free electrons" without "delocalised" or "mobile". Cambridge does not credit "free electrons" alone. The required term is delocalised electrons — not bound to any single ion and able to move through the whole lattice. Write "a sea of delocalised electrons".
W22 Paper 41 Q2(a); S22 Paper 41 Q3(a)
Describing ionic bonding by mistake
When asked about metallic bonding, many candidates describe ionic bonding instead. The models differ: metallic bonding has only positive metal ions in a sea of delocalised electrons; ionic bonding has positive AND negative ions and no electron sea. A pure metal (copper, iron, sodium) is one element, so its bonding is metallic; ionic bonding needs at least two elements.
S22 Paper 31 Q3(a)(i)(ii)
Malleability: "bonds break" is wrong
Explaining malleability by saying "the metallic bonds break and reform when the metal is hammered" does not gain the mark. The accepted answer is that layers of positive ions slide over one another while the sea of delocalised electrons maintains the electrostatic attraction, so the bonds are not broken and the metal bends rather than shatters.
S22 Paper 31 Q3(a)(ii)
Naming only one part of the bond
The mark-scheme definition of metallic bonding has two parts: a lattice of positive ions AND a sea of delocalised electrons, held by electrostatic attraction. Candidates who give the electrons but not the lattice of positive ions (or the reverse) score at most one mark of two. Always state both particles and the attraction.
S22 Paper 31 Q3(a)(i)
Ions do not carry charge in a solid metal
In a solid metal the charge is carried by the delocalised electrons, not the ions. The positive ions are fixed in the lattice and only vibrate. Writing "the ions move to carry the current" describes a molten ionic compound or solution, not a metal. Use "delocalised electrons carry charge" for metals.
They are positive ions, not "atoms"
In the metallic bonding model the lattice particles are positive ions (cations), because each metal atom has lost its outer electrons to the electron sea. Labelling them "metal atoms" or "neutral atoms" is a common error — write "lattice of positive ions". Show the electrons spread throughout, not orbiting individual ions, and never as negative ions.
State the structure, then link the property
Start every metallic-bonding "Explain" with the structure — "positive ions in a sea of delocalised electrons" — then add the link. Conductivity: electrons move and carry charge. Malleability: layers of ions slide while the electron sea keeps the attraction.
Use the exact mark-scheme words
Cambridge credits exact words: "lattice of positive ions" (not "metal atoms"), "delocalised electrons" (not "free electrons"), "electrostatic attraction" for the bond, "layers slide" (not "bonds break") for malleability. The loose word usually costs the mark.
Draw the metal structure correctly
For "draw a metal", show positive ions in a regular lattice (label "positive ions", not "atoms") and delocalised electrons spread between them (label "delocalised electrons"). Do not draw electrons orbiting single ions or add negative ions.
Scope: This sub-topic is Extended (Supplement) only. All exam questions on metallic bonding at Cambridge IGCSE 0620 are for Extended candidates.
Metallic bond (mark-scheme definition): The electrostatic attraction between a lattice of positive ions and a sea of delocalised electrons. Both components must be stated for full marks.
Delocalised electrons ( reminder: these are outer-shell electrons released by the metal atoms): Electrons that are not bound to any single ion; they can move freely throughout the entire metallic structure. "Delocalised" or "mobile" are both accepted; "free" alone is not.
Lattice of positive ions: When metal atoms form a metallic solid, their outermost electrons leave the atom, creating positive ions (, ). These ions pack into a regular, repeating three-dimensional arrangement (a lattice).
Summary in one sentence: A metal consists of a regular lattice of positive ions held together by a sea of delocalised electrons that move throughout the structure; the metallic bond is the electrostatic attraction between the two.
(Extended) Define a metallic bond.
(Extended)
Draw and label a diagram to represent the structure of a metal, showing the metallic bonding.
Your diagram should show:
