Structure and bonding: 2.62

Ionic bonding takes place between metals and non-metals creating a giant structure of repeating ions. In this section we take a look at the nature of the ionic bond.

Syllabus reference

Structure 2.1.2 - The ionic bond is formed by electrostatic attractions between oppositely charged ions.

Deduce the formula and name of an ionic compound from its component ions, including polyatomic ions.
Binary ionic compounds are named with the cation first, followed by the anion. The anion adopts the suffix “ide”.
Interconvert names and formulas of binary ionic compounds.

Guidance

The following polyatomic ions should be known by name and formula: ammonium NH₄⁺, hydroxide OH^–, nitrate NO₃^–, hydrogencarbonate HCO₃^–, carbonate CO₃^2–, sulfate SO₄^2–, phosphate PO₄^3–.

Tools and links

Reactivity 3.2 - Why is the formation of an ionic compound from its elements a redox reaction?
AHL Structure 2.2 - How is formal charge used to predict the preferred structure of sulfate?
AHL Reactivity 3.1 - Polyatomic anions are conjugate bases of common acids. What is the relationship between their stability and the conjugate acid’s dissociation constant, Ka?

Ionic or covalent?
Consequences of covalent character

Ionic or covalent?

An ionic bond is an electrostatic force between positive and negative ions. This electrostatic force in non-directional, it acts in all directions. The strength of the force depends on the magnitude of the charges on the ions and the sum of the ionic radii.

But what determines whether a compound is ionic or covalent?

If the bonding atoms have a large difference in electronegativity then this causes transfer of electrons and the formation of ionic compounds. As the difference in electronegativity decreases, the bond develops covalent character until eventually it becomes essentially covalent.

Notice that this process is not black and white. The bond type changes gradually from pure ionic to pure covalent, passing through all degrees.

Pure ionic >>> ionic with covalent character >>> polarised covalent >>> pure covalent

Pure ionic compounds are formed by group 1 metals when combining with non-metals. These are highly electropositive, having electronegativity values of between 0.7 and 1.0.

However, when the electronegativity value of the metal is higher, the bonding has a degree of covalent character. This means that the negative ion electron density is distorted and drawn towards the metal ion. There is some electron density between the two particles, typical of covalent bonding. This can be seen on electron density maps produced in X-ray crystallography. Instead of the electron density being symmetrical around the ions, it is distorted towards the positive ions.

The classic example is aluminium chloride. Aluminium is a metal from group 13 and consequently forms Al³⁺ ions. However, it is not very electropositive and the high charge density of the small Al³⁺ ion allows it to polarise the negative charge cloud on negative ions formed from atoms of lesser electronegativity.

Aluminium oxide is an ionic compound, but aluminium chloride is only ionic in the solid state at low temperatures. At higher temperatures it becomes covalent. This is because the high charge density Al³⁺ ion can polarise the Cl- charge cloud, making an ionic bond with a high degree of covalent character, so much so that AlCl₃ is usually considered to be covalent.

The difference in electronegativity between aluminium (1.5) and chlorine (3.0) is 1.5 units. This could be taken as a rough guide for the limit between ionic and covalent bonding.

When metals bond to non-metals if the difference in electronegativity is greater than 1.5, then the compound would be expected to be ionic, less that 1.5 and covalency is expected. It should be stressed that this is only an approximation and it is easy to find exceptions.

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Consequences of covalent character

Covalent substances are molecular and usually have low melting and boiling points. They tend to dissolve in non-polar solvents, forming solutions that do not conduct electricity. Many covalent chorides are hydrolysed by water, forming hydrogen chloride. All of these facts can be used as pointers towards covalent character in ionic compounds.

Aluminium chloride is a covalent substance whose characteristics reflect this nature.

compound	sodium chloride	aluminium chloride	silicon chloride
bonding	ionic	covalent	covalent
melting point /ºC	801	190 (sublimes)	-69
reaction with water	dissolves and dissociates into ions	hydrolysed HCl	hydrolysed HCl

Magnesium chloride is an ionic substance. However, the small size and double charge of the magnesium ion causes a degree of covalent character. For example, evaporation of a solution of magnesium chloride produces 'basic' magnesium chloride - Mg(OH)Cl - showing that the magnesium chloride has been partially hydrolysed by water.

Even lithium chloride has a small degree of covalent character, being soluble in non-polar solvents.

Example: Magnesium chloride and silicon(IV) chloride have very different properties.

Give the formula and physical state at room temperature of each compound.
State the conditions under which each compound conducts electricity (if at all).
Each chloride is added to water in separate experiments. Suggest an approximate pH value for the solution formed and give an equation for the reaction.

1. The valency comes from the group number, hence MgCl₂ and SiCl₄.

2. Magnesium chloride is ionic. It conducts electricity when molten or in aqueous solution. Silicon chloride does not conduct electricity.

3. Magnesium chloride is ionic, it dissolves in water and the ions dissociate. The pH is about 5 as magnesium chloride is weakly acidic by hydrolysis (it is the salt of a weak base and a strong acid)

MgCl₂ + xH₂O → Mg²⁺(aq) + 2Cl^-(aq)

Silicon(IV) chloride is hydrolysed by water: The solution formed has a pH of 1 as the HCl formed is a strong acid.

SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl

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