Ionic Bonding and Lattices

Learning Objectives (OCR Spec 2.2.2)

By the end of this lesson you should be able to:

• Describe ionic bonding as the electrostatic attraction between oppositely charged ions
• Predict the charges of common ions from their position in the Periodic Table
• Draw dot-and-cross diagrams for simple ionic compounds
• Describe a giant ionic lattice and explain the physical properties of ionic compounds
• Explain trends in melting point, solubility and electrical conductivity

What Is an Ionic Bond?

An ionic bond is the electrostatic force of attraction between oppositely charged ions. It forms when electrons are transferred from a metal to a non-metal so that each atom achieves a noble gas electron configuration.

For example, in sodium chloride:

Na (1s2 2s2 2p6 3s1) -> Na+ (1s2 2s2 2p6) + e- [loses 1 electron] Cl (1s2 2s2 2p6 3s2 3p5) + e- -> Cl- (1s2 2s2 2p6 3s2 3p6) [gains 1 electron]

Both ions now have the stable configuration of a noble gas (Ne for Na+; Ar for Cl-).

Ionic bonding is typically found between elements with a large difference in electronegativity (usually a Group 1, 2 or 3 metal reacting with a Group 5, 6 or 7 non-metal).

Predicting Ionic Charges

Group	Common Ion	Example
1	+1	Na+, K+
2	+2	Mg^2+, Ca^2+
13 (III)	+3	Al^3+
14 (IV)	+/-4 (rare)	-
15 (V)	-3	N^3-, P^3-
16 (VI)	-2	O^2-, S^2-
17 (VII)	-1	F-, Cl-, Br-, I-

You should also know common polyatomic ions:

Ion	Formula	Charge
Ammonium	NH4+	+1
Hydroxide	OH-	-1
Nitrate	NO3-	-1
Hydrogencarbonate	HCO3-	-1
Carbonate	CO3^2-	-2
Sulfate	SO4^2-	-2
Phosphate	PO4^3-	-3

Dot-and-Cross Diagrams

Dot-and-cross diagrams show the transfer of electrons in ionic bonding. Show only the outer shell. Use dots for one element and crosses for the other. Enclose each ion in square brackets with its charge on the upper right.

Sodium Chloride (NaCl)

   [ Na ]+       [ .. .. ]-
                [  :Cl: x ]
                [  .. .. ]

Na loses 1 outer electron (empty brackets with + charge); Cl gains 1 electron (shown as a cross alongside its seven dots) to reach a full octet.

Magnesium Oxide (MgO)

   [ Mg ]^2+      [ :O: xx ]^2-
                 (8 outer electrons)

Mg loses its two 3s electrons (shown as no dots in the Mg bracket); O gains two electrons (2 crosses added to its 6 dots) to reach 8 outer electrons.

Calcium Chloride (CaCl2)

Ca has 2 outer electrons to lose; each Cl gains 1. So one Ca donates to TWO Cl atoms:

   [ Ca ]^2+    2 x  [ :Cl: x ]-

Sodium Oxide (Na2O)

Two Na atoms donate 1 electron each to one O:

   2 x [ Na ]+     [ :O: xx ]^2-

Tip: Always balance total electrons transferred. If Ca gives 2 and Cl takes 1, you need 2 Cl per Ca.

Giant Ionic Lattices

In the solid state, ionic compounds form giant ionic lattices in which each cation is surrounded by anions and vice versa in a regular, repeating 3D pattern. The forces between ions extend throughout the whole crystal - it is not a molecule but a single giant structure.

Sodium Chloride Structure

• Each Na+ is surrounded by 6 Cl- (octahedral coordination)
• Each Cl- is surrounded by 6 Na+
• Coordination number = 6:6
• The lattice is face-centred cubic (fcc)

Bonding Strength

The strength of ionic bonding (often quantified as lattice enthalpy) depends on:

1. Ionic charge - higher charges mean stronger attraction (Q1 x Q2 in Coulomb's law)
2. Ionic radius - smaller ions allow closer packing, so larger force (1/r in Coulomb's law)

Compare:

• NaCl: +1/-1, medium ions, m.p. 801 degC
• MgO: +2/-2, small ions, m.p. 2852 degC (much higher)

Physical Properties of Ionic Compounds