Compounds, Formulae and Ionic Charges

Learning Objectives (OCR Spec 2.1.3)

• Write formulae of ionic compounds from ionic charges
• Predict charges of ions from a group of the periodic table
• Recall the names and formulae of common polyatomic ions
• Construct formulae of compounds including those of transition metals (where charge is given)
• Name ionic compounds correctly using IUPAC conventions
• Distinguish between ionic and covalent nomenclature

From Atoms to Compounds

Elements rarely exist as single atoms. Most combine chemically to form compounds — substances in which atoms of different elements are held together by chemical bonds. Compounds are either:

• Ionic — composed of cations (+) and anions (−) held together by electrostatic attraction
• Covalent (molecular) — composed of molecules held together by shared electron pairs

OCR A-Level A first focuses on writing correct formulae for ionic compounds because this underpins all subsequent stoichiometry. Getting a formula wrong propagates the error through every subsequent mole calculation, so learning this thoroughly early is essential.

Predicting Ionic Charges from the Periodic Table

Main-group elements form ions by gaining or losing electrons to achieve a full outer shell (a "noble gas configuration"). The group number predicts the charge:

Group	Typical Ion	Electrons Lost/Gained	Examples
1	+1	Lose 1	Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺
2	+2	Lose 2	Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺
13 / 3	+3	Lose 3	Al³⁺ (B rarely forms ions)
14 / 4	Rarely ionic	—	C, Si mostly form covalent compounds
15 / 5	−3	Gain 3	N³⁻, P³⁻ (mostly in nitrides/phosphides)
16 / 6	−2	Gain 2	O²⁻, S²⁻, Se²⁻
17 / 7	−1	Gain 1	F⁻, Cl⁻, Br⁻, I⁻
18 / 0	Normally no ions	Already full shell	—

Pattern: Metals (left of periodic table) form cations; non-metals (right) form anions. The charge reflects the most stable noble gas configuration.

Why do these charges make sense? Consider sodium: it has configuration [Ne] 3s¹. Losing one electron gives [Ne] — the same as the noble gas neon, a very stable arrangement. Gaining 7 electrons to reach argon's configuration would require enormous energy and produce a highly unstable Na⁷⁻. Losing 1 electron is overwhelmingly preferred.

Similarly chlorine has configuration [Ne] 3s² 3p⁵. Gaining one electron gives [Ar] configuration — stable. Losing 7 electrons is energetically prohibitive. So Cl⁻ is the natural ion.

Transition Metals

Transition metals (Fe, Cu, Cr, Mn, Ni, Co, Zn, etc.) can form multiple stable ions, e.g. Fe²⁺ and Fe³⁺. This is because their d-orbitals have similar energies to the s-orbitals, allowing variable numbers of electrons to be lost. OCR will provide the charge in the name using Roman numerals:

• Iron(II) = Fe²⁺
• Iron(III) = Fe³⁺
• Copper(I) = Cu⁺
• Copper(II) = Cu²⁺
• Chromium(III) = Cr³⁺
• Chromium(VI) = Cr⁶⁺ (found in CrO₄²⁻ and Cr₂O₇²⁻)
• Manganese(II) = Mn²⁺
• Manganese(IV) = Mn⁴⁺ (as in MnO₂)
• Manganese(VII) = Mn⁷⁺ (in MnO₄⁻)

Note Zn always forms Zn²⁺ and is often excluded from the definition of a "true" transition metal because it has a full d-subshell.

Common Polyatomic Ions — LEARN THESE

Polyatomic ions act as a single charged unit. You must memorise these for the exam; they are not given on the data sheet.

Name	Formula	Charge
Ammonium	NH₄⁺	+1
Hydroxide	OH⁻	−1
Nitrate	NO₃⁻	−1
Nitrite	NO₂⁻	−1
Hydrogencarbonate	HCO₃⁻	−1
Hydrogensulfate	HSO₄⁻	−1
Carbonate	CO₃²⁻	−2
Sulfate	SO₄²⁻	−2
Sulfite	SO₃²⁻	−2
Phosphate	PO₄³⁻	−3
Manganate(VII)	MnO₄⁻	−1
Dichromate(VI)	Cr₂O₇²⁻	−2
Chromate(VI)	CrO₄²⁻	−2
Peroxide	O₂²⁻	−2
Cyanide	CN⁻	−1

Memory trick: -ate ions usually have more oxygens than -ite ions (nitrate NO₃⁻ vs nitrite NO₂⁻; sulfate SO₄²⁻ vs sulfite SO₃²⁻).

Writing Ionic Formulae — The Cross-Over Rule

For an ionic compound to exist, total positive charge must equal total negative charge (the compound is overall neutral). The quickest method:

1. Write the cation and anion with their charges
2. Cross the magnitude of each charge to become the subscript of the other ion
3. Simplify if possible
4. Use brackets around polyatomic ions when the subscript is greater than 1