Metallic and ionic bonding: VCE Chemistry Unit 1

What this dot point is asking

VCAA wants you to describe the metallic bonding model and use it to explain the properties of metals and alloys, to describe the ionic bonding model and use it to explain the properties of ionic compounds, and to write correct names and formulas of binary and ternary ionic compounds.

The answer

Metallic bonding model

A metal is a regular three-dimensional lattice of positive metal cations held together by a sea of delocalised valence electrons. Each atom contributes its valence electrons to a shared pool that is free to move throughout the lattice. The non-directional electrostatic attraction between the positive cations and the negative electron sea is the metallic bond.

The model explains the standard suite of metallic properties:

Alloys

An alloy is a mixture of a metal with one or more other elements (often other metals). Examples: bronze (Cu + Sn), brass (Cu + Zn), stainless steel (Fe + Cr + Ni + C), solder (Sn + Pb).

Alloying typically makes a metal harder and stronger but less malleable. The model explanation: the foreign atoms are a different size and disrupt the regular planes of cations. Layers cannot slide past one another as easily, so the alloy resists deformation. Most pure metals are too soft for structural use; alloying is the standard fix.

Ionic bonding model

An ionic compound is a 3D lattice of alternating positive and negative ions held together by the strong electrostatic attraction between opposite charges (the ionic bond). It is non-directional: every cation is attracted to every nearby anion.

Ionic compounds form when a metal (low ionisation energy) transfers electrons to a non-metal (high electronegativity), producing cations and anions in whole-number ratios so the lattice is electrically neutral.

Properties:

Writing names and formulas

Binary ionic compound = a metal cation + a non-metal anion. The anion takes the -ide suffix (chloride, oxide, sulfide, nitride).

Ternary ionic compound = uses a polyatomic ion (carbonate CO3^2-, sulfate SO4^2-, nitrate NO3^-, phosphate PO4^3-, hydroxide OH^-, ammonium NH4^+, hydrogencarbonate HCO3^-).

Procedure for the formula:

1. Identify the charges of the cation and anion.
2. Cross over the charges (without the signs) to get the subscripts.
3. Reduce to the lowest whole-number ratio.
4. Use brackets around a polyatomic ion if you need more than one of it.

Examples:

For transition-metal cations with more than one common charge (Fe^2+/Fe^3+, Cu^+/Cu^2+), the charge is written as a Roman numeral in brackets.

Examples in context

Example 1. Aluminium electrolysis at Portland Aluminium. The Portland smelter in Victoria draws roughly $500 \, \text{MW}$ from the Loy Yang power station to electrolyse molten $\text{Al}_2 \text{O}_3$ at $950^{\circ}\text{C}$. Aluminium oxide is an ionic lattice: $\text{Al}^{3+}$ and $\text{O}^{2-}$ in a $2:3$ ratio, held by very large electrostatic forces (lattice energy near $15{,}900 \, \text{kJ}$ per mole), giving a melting point above $2000^{\circ}\text{C}$. Dissolving $\text{Al}_2 \text{O}_3$ in molten cryolite $\text{Na}_3 \text{AlF}_6$ lowers the operating temperature dramatically. The product aluminium is a metallic lattice: $\text{Al}^{3+}$ cations in a sea of $3$ delocalised electrons per atom, accounting for its low density, high conductivity and excellent malleability.

Example 2. Bronze coinage at the Royal Australian Mint. The Mint in Canberra produces five-cent through fifty-cent coins from a copper-nickel alloy with about $25\%$ nickel. Pure copper is too soft because identical cation layers can slide easily. Adding nickel atoms (similar size, different electron count) distorts the lattice and pins layer movement, increasing hardness by roughly a factor of three. The alloy is still metallic: delocalised electrons remain free to move, so the coin conducts and reflects light with the typical silvery lustre. The same principle is used for the gold coin's aluminium-bronze alloy, which combines copper's good machining properties with aluminium's surface oxide layer that resists tarnishing.

Try this

Q1. Explain, using the bonding model, why magnesium is a good electrical conductor but solid magnesium chloride is not. [3 marks]

• Cue. Mg has delocalised valence electrons free to move and carry current. $\text{MgCl}_2$ has ions locked in lattice; charges cannot move (conducts only when molten or dissolved).

Q2. Write the formula and name for the ionic compound formed between (a) calcium and phosphate, (b) chromium(III) and sulfate. [4 marks]

• Cue. (a) $\text{Ca}_3 (\text{PO}_4)_2$ calcium phosphate (cross $2 \times 3 = 6$ charge each side). (b) $\text{Cr}_2 (\text{SO}_4)_3$ chromium(III) sulfate.

Q3. Consider sodium and bronze. (a) Describe the bonding in each. (b) Predict and explain which has the higher melting point. (c) Explain why bronze is harder than pure copper. [2+2+2 marks]

• Cue. (a) Both metallic; sodium pure, bronze copper-tin alloy. (b) Bronze higher: alloy elements increase electrostatic attraction and disrupt layer movement; sodium $98^{\circ}\text{C}$, bronze near $950^{\circ}\text{C}$. (c) Tin atoms of different size disrupt the regular copper lattice; layers can no longer slide.