Ionic Bonds and Naming Ionic Compounds
Quick Notes
- Ionic bonds are formed by electrostatic attraction between oppositely charged ions (cations and anions).
- To name binary ionic compounds:
- Name the cation first, then the anion.
- The anion ends in “-ide”.
- Example: NaCl = sodium chloride
- Polyatomic ions are groups of atoms that carry a charge and behave as a single particle in compounds.
- Common polyatomic ions you should know:
- Ammonium: NH4+
- Hydroxide: OH−
- Nitrate: NO3−
- Hydrogencarbonate: HCO3−
- Carbonate: CO32−
- Sulfate: SO42−
- Phosphate: PO43−
- Always ensure the total positive and negative charges balance when writing formulas.
Full Notes
What Is an Ionic Bond?
An ionic bond is the electrostatic attraction between oppositely charged ions (cations and anions).
A compound that is made up of ions and held together by ionic bonding is called an ionic compound.
Ionic bonding usually occurs between metals and non-metals (with the metal existing as a positively charged ion and the non-metal a negatively charged ion).
Writing Formulas for Ionic Compounds
The formula for an ionic compound shows the simplest ratio the ions combine in to give a total charge of zero.
To write the correct formula:
- Write the symbols and charges of the ions.
- Balance the charges so the total charge equals zero.
- Use subscripts to show how many of each ion are needed.
Example Magnesium chloride
Mg2+ and Cl− → MgCl2
Here, Mg has a 2+ charge and Cl a 1− charge. To give no overall charge, we need 1 × Mg (2+) and 2 × Cl (2−). The 2+ and 2− cancel overall to give zero overall charge.
Example Aluminium oxide
2 × Al3+ and 3 × O2− → Al2O3
Example Calcium nitrate
Ca2+ and 2 × NO3− → Ca(NO3)2
Example Ammonium sulfate
2 × NH4+ and 1 × SO42− → (NH4)2SO4
Naming Binary Ionic Compounds
Binary compounds contain two elements: a metal (cation) and a non-metal (anion).
We use a standard naming system to avoid confusion:
- Name the metal first.
- Name the non-metal second, changing the ending to “-ide”.
Example Sodium chloride
NaCl = sodium chloride
Example Magnesium oxide
MgO = magnesium oxide
Example Potassium bromide
KBr = potassium bromide
Polyatomic Ions – Must Know
Polyatomic ions are made up of small groups of atoms that share an overall charge. You are expected to recognise and recall the following common polyatomic ions:
| Ion | Formula |
|---|---|
| Ammonium | NH4+ |
| Hydroxide | OH− |
| Nitrate | NO3− |
| Hydrogencarbonate | HCO3− |
| Carbonate | CO32− |
| Sulfate | SO42− |
| Phosphate | PO43− |
Example Formulas using polyatomic ions
- NaOH = sodium hydroxide
- Ca(NO3)2 = calcium nitrate
- (NH4)2SO4 = ammonium sulfate
- MgCO3 = magnesium carbonate
Summary
- Ionic bonds form between metal cations and non-metal or polyatomic anions.
- We write ionic formulas by balancing charges of the ions.
- We name compounds by cation first, then anion with “-ide” or polyatomic name.
Linked Questions
Why is the formation of an ionic compound from its elements a redox reaction?
When an ionic compound forms, one element loses electrons while another gains electrons. The metal atom is oxidised as it transfers electrons to the non-metal, which is reduced. For example, in sodium chloride formation, sodium atoms lose one electron each (oxidation) while chlorine atoms gain one electron each (reduction). Because both oxidation and reduction occur simultaneously, the process is classified as a redox reaction, resulting in oppositely charged ions held together by electrostatic attraction.
How is formal charge used to predict the preferred structure of sulfate?
Formal charge helps identify the most stable Lewis structure by comparing the distribution of electrons across atoms. For sulfate (SO₄²⁻), several resonance forms are possible. Calculating formal charge for each structure shows that the most stable arrangement minimises charge separation — usually with two sulfur–oxygen double bonds and two sulfur–oxygen single bonds, giving an average S–O bond order of 1.5. This resonance-delocalised form reduces formal charge magnitudes on individual atoms, indicating greater stability and accurately reflecting experimental bond length data.
Polyatomic anions are conjugate bases of common acids. What is the relationship between their stability and the conjugate acid’s dissociation constant, Ka?
The stability of a conjugate base (polyatomic anion) is inversely related to the acid’s strength, expressed by its Ka value. A strong acid has a large Ka and dissociates readily, producing a conjugate base that is weak and very stable. Conversely, a weak acid has a small Ka and forms a stronger, less stable conjugate base. Greater anion stability arises from charge delocalisation, resonance, and electronegativity of the atoms bearing the charge — all of which reduce the base’s tendency to accept a proton.