1.6.3Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Ionic Bond Strength — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 19 of 36 Unit quiz
Section 1 of 6What Controls the Strength
The force between two ions follows Coulomb's law, rising with the size of the charges and falling with the distance between them.
Ionic charge
- Ions with higher charges such as Mg²⁺ and O²⁻ attract each other much more strongly than ions with a single charge.
Ionic radius
- Smaller ions can approach more closely, and the electrostatic force increases as the distance between the centres decreases.
Charge density
- The ratio of charge to size combines both factors, so small highly charged ions give the strongest attractions.
Lattice enthalpy
- The enthalpy change when a mole of solid ionic compound forms from its gaseous ions is a measure of this strength, and is exothermic.
Section 2 of 6Comparing Real Compounds
Comparing Real Compounds
NaCl and MgO
- MgO melts at about 2852 °C against 801 °C for NaCl because both of its ions carry double the charge and are smaller.
Group 1 halides
- Melting points fall from NaF at about 993 °C to NaCl at 801 °C, NaBr at about 747 °C and NaI at about 661 °C as the halide ion gets larger.
MgO and CaO
- CaO melts at about 2613 °C, lower than MgO, because the Ca²⁺ ion is larger than the Mg²⁺ ion.
Ionic radii
- Mg²⁺ is about 0.072 nm, Na⁺ about 0.102 nm, Cl⁻ about 0.181 nm and O²⁻ about 0.140 nm.
| Compound | Ion charges | Melting point / °C |
|---|---|---|
| NaCl | 1+, 1− | 801 |
| NaF | 1+, 1− | 993 |
| CaO | 2+, 2− | 2613 |
| MgO | 2+, 2− | 2852 |
Section 3 of 6Building an Explanation
1
Name the type of bonding and say that melting means overcoming the electrostatic attraction between oppositely charged ions.
2
Compare the charges on the ions of the two compounds and say which has the larger charges.
3
Compare the ionic radii and say which compound has the smaller ions and so a shorter distance between charge centres.
4
Conclude that the compound with higher charge density has stronger ionic bonding.
5
State that more energy is needed to separate the ions, so it has the higher melting point and larger lattice enthalpy.
Section 4 of 6Links to Other Properties
Links to Other Properties
Hardness
- Compounds with strong ionic bonds are hard but brittle, because the layers cannot move without bringing like charges together.
Solubility
- Very strongly bonded compounds such as MgO are often insoluble because the hydration energy cannot pay for breaking the lattice.
Conductivity when molten
- All ionic compounds conduct once molten, but the temperature needed varies with the bond strength.
Trend in radius
- Ionic radius grows down a group because there are more filled shells, and isoelectronic ions shrink as nuclear charge increases.
Section 5 of 6Worked Example
Question Explain why magnesium oxide has a higher melting point than sodium chloride.
- Both are giant ionic lattices, so melting needs the electrostatic attractions between ions to be overcome.
- Mg²⁺ and O²⁻ carry charges of 2+ and 2−, whereas Na⁺ and Cl⁻ carry 1+ and 1−.
- Mg²⁺ and O²⁻ are also smaller than Cl⁻, so their centres are closer together.
Answer Higher charges and smaller ionic radii give stronger attractions in MgO, so more energy is needed to melt it.
Section 6 of 6Exam Checklist & Tips
1.6.3 – Key Points for the Exam
- Charge density The charge on an ion divided by its volume or radius, high for small highly charged ions.
- Lattice enthalpy The enthalpy change when one mole of an ionic solid forms from its gaseous ions.
- Ionic radius The size of an ion, measured from the nucleus to the edge of its electron cloud.
- Electrostatic attraction The force between opposite charges that holds ions together.
- Isoelectronic Having the same number of electrons, as in Na⁺, Mg²⁺ and O²⁻.
Common Mistakes
- Saying the ions in MgO have a stronger bond only because of charge, ignoring the smaller radius.
- Writing that ionic bonds are broken when sodium chloride dissolves without mentioning the new ion-water attractions.
- Using the idea of intermolecular forces for ionic compounds.
- Believing a larger ion gives stronger bonding because it has more electrons.
Last updated:
\n