Metallic Bonding Worked ExamplesIGCSE Chemistry 0620

(a) Calcium is a metal element, so the bonding is metallic. (It is not ionic: there are no negative ions.)

(b) P is one of the regular, repeating particles with a $2+$ charge: a positive calcium ion, $\text{Ca}^{2+}$. Each calcium atom has given up its two outer-shell electrons.

Q is one of those released electrons, which are no longer attached to any one ion: a delocalised electron.

(c) The ions are held in fixed positions in the lattice, so they cannot carry the current. The delocalised electrons move through the lattice when a voltage is applied.

A metal is a giant lattice of positive ions surrounded by a 'sea' of delocalised electrons. The bond is the electrostatic attraction between the two. That is statement B.

A describes ionic bonding: a metal contains no negative ions. C describes a covalent bond. D describes the intermolecular forces in a simple molecular substance — a metal is not made of molecules.

(a) The mark scheme looks for three separate ideas. Marking point 1: a giant lattice of positive ions (potassium ions).

Marking point 2: a 'sea' of delocalised electrons — electrons free to move through the whole lattice.

Marking point 3: the bonding is the electrostatic attraction between the positive ions and the delocalised electrons. Saying only "strong forces" or "electrostatic bonds" does not score — the answer must say what attracts what.

(b) Each potassium atom has one electron in its outer shell. This electron becomes delocalised, leaving an ion with one more proton than electrons: $\text{K}^+$, charge $1+$.

A substance conducts when it contains charged particles that are free to move. Look at each diagram for a mobile charged particle.

(a) Silver contains delocalised electrons. When a voltage is applied, these electrons move (flow) through the lattice, carrying the charge. The positive silver ions stay in their lattice positions.

(b) Silver chloride is ionic. It has no delocalised electrons, and its $\text{Ag}^+$ and $\text{Cl}^-$ ions are held in fixed positions in the solid lattice, so they cannot move.

(c) Yes — when it melts, the lattice breaks down and the ions are free to move, so the molten compound conducts.

(a) The positive ions in gold are arranged in regular layers.

When a force is applied, the layers of ions slide over each other into new positions — compare the top two rows in the two diagrams.

The metal does not shatter because the delocalised electrons move with the ions and still attract the positive ions in their new positions. Metallic bonding is not between particular pairs of particles, so it is not broken by the sliding.

(b) Ductile means it can be drawn out into wires. The same explanation applies: layers of ions slide over one another while the attraction to the delocalised electrons holds the structure together.

A solid that contains ions and conducts must have delocalised electrons: E is a metal. A solid that contains ions but conducts only when molten is an ionic compound: F.

Statement 1 is correct: a metal such as E is an element. Statement 2 is wrong: F is a compound of a metal and a non-metal.

Statement 3 is correct: the ions in a metallic lattice are all positive; the negative charge is carried by the delocalised electrons. Statement 4 is wrong: if F had delocalised electrons it would conduct as a solid.

So the answer is A. Options C and D swap the two solids round; B gives F delocalised electrons, which contradicts the data.

Decide the bonding type from the elements: a metal alone → metallic; a metal with a non-metal → ionic; non-metals only → covalent.

Aluminium: metallic. The particles attracted are the positive aluminium ions, $\text{Al}^{3+}$, and the delocalised electrons.

Calcium oxide: ionic. The attraction is between the oppositely charged ions, $\text{Ca}^{2+}$ and $\text{O}^{2-}$.

Hydrogen: covalent. In a covalent bond the shared pair of electrons is attracted to the nuclei of the two hydrogen atoms.

Diamond: covalent (given). The shared pairs of electrons are attracted to the nuclei of the carbon atoms on either side of each bond.

(a) Graphite lit the lamp but it is not a metal — it is a form of the non-metal carbon (it has delocalised electrons between its layers). Conducting shows mobile charged particles are present, not that the substance is a metal.

(b) Zinc has metallic bonding: a lattice of positive ions in a sea of delocalised electrons. The delocalised electrons move through the structure when the circuit is connected.

(c) The dull grey surface is most likely a layer of zinc oxide (a coating from reaction with air), which does not conduct and stops current reaching the metal. The student should rub the strip with sandpaper (emery paper) until it is shiny.

(d) A lamp only shows on/off. Replace (or add to) the lamp with an ammeter and compare the current, using strips of the same length and thickness.

In the pure metal all the atoms are the same size, so the rows are straight and layers slide easily. In the alloy the larger magnesium atoms disrupt the regular layers, so the layers cannot slide over each other as easily. The alloy is harder and less malleable: A.

B is wrong: a metal or alloy is not made of molecules, so intermolecular forces are not involved. C is wrong: an alloy is a mixture of metals, not an ionic compound. D is wrong: the alloy still has delocalised electrons — it still conducts electricity.

(a) Mercury is a metal, so its bonding is still metallic when liquid: positive ions and delocalised electrons. The delocalised electrons can move through the liquid and carry the current.

(b) Bromine is a simple molecular substance made of $\text{Br}_2$ molecules. It has no ions and no delocalised electrons, so there are no charged particles that can move.

(c) Yes, in both states. Melting only lets the ions move past each other; gallium keeps its delocalised electrons as a solid and as a liquid, so it conducts in both.

Error 1: "negative ions". Metal atoms lose their outer-shell electrons, so the lattice is made of positive ions.

Error 2: "intermolecular forces". A metal contains no molecules. The ions are held by metallic bonding: attraction to the delocalised electrons.

Error 3: "ions move". In a solid metal the ions stay in fixed positions; the delocalised electrons move and carry the current.

(b) Metallic bonding is the electrostatic attraction between the positive ions in a giant metallic lattice and a sea of delocalised electrons.

(a) W is the metal: it conducts when solid (only a metal or graphite does this) and it bends and flattens when hammered — it is malleable. Its high melting point is consistent with this but is not enough on its own, because Y also has a high melting point.

(b) X conducts only when molten and shatters: a giant ionic lattice. Z has a very low melting point and never conducts: a simple molecular substance.

(Y — very high melting point, never conducts — is a giant covalent structure.)

(c) W has delocalised electrons in both states, and they can move, so it conducts solid or molten. X has ions, which are fixed in the lattice in the solid but free to move when molten.