Formulas

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Write formulas of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral), including:

    • (a) Predict ionic charge from the position of an element in the Periodic Table
    • (b) Recall the names and formulas for these ions: NO₃⁻, CO₃²⁻, SO₄²⁻, OH⁻, NH₄⁺, Zn²⁺, Ag⁺, HCO₃⁻, PO₄³⁻

  2. (a) Write and construct balanced equations, including ionic equations (which should not include spectator ions) (b) Use appropriate state symbols in equations

  3. Define and use the terms empirical and molecular formula

  4. Understand and use the terms anhydrous, hydrated and water of crystallisation

  5. Calculate empirical and molecular formulas, using given data

1. Writing Formulas of Ionic Compounds

https://cambridgeassistant.com

What are Ionic Compounds?

Ionic compounds are formed when a metal reacts with a non-metal. The metal loses electrons to become a positively charged ion (called a cation), and the non-metal gains electrons to become a negatively charged ion (called an anion). These oppositely charged ions are held together by strong forces of attraction called ionic bonds.

A key rule: Ionic compounds are always electrically neutral. This means the total positive charge must equal the total negative charge.

Predicting Ionic Charges from the Periodic Table

You can predict the charge on an ion by looking at which group (column) an element is in on the Periodic Table:

Positive ions (metals):

  • Group 1 elements (e.g., Li, Na, K) form ions with a 1+ charge
  • Group 2 elements (e.g., Be, Mg, Ca) form ions with a 2+ charge
  • Group 13 elements (e.g., Al, Ga) form ions with a 3+ charge

Negative ions (non-metals):

  • Group 15 elements (e.g., N) form ions with a 3− charge (they gain 3 electrons)
  • Group 16 elements (e.g., O, S) form ions with a 2− charge (they gain 2 electrons)
  • Group 17 elements (e.g., F, Cl, Br, I) form ions with a 1− charge (they gain 1 electron)

Special cases:

  • Hydrogen can form H⁺ (positive ion)
  • Transition elements can have variable charges (different charges in different compounds)

Important Ions You Must Memorize

You need to know the names and formulas of these common ions:

Ion NameFormulaCharge
NitrateNO₃⁻1−
CarbonateCO₃²⁻2−
SulfateSO₄²⁻2−
HydroxideOH⁻1−
AmmoniumNH₄⁺1+
ZincZn²⁺2+
SilverAg⁺1+
Hydrogen carbonateHCO₃⁻1−
PhosphatePO₄³⁻3−

Tip: Ions made up of more than one type of atom (like NO₃⁻ or SO₄²⁻) are called compound ions or polyatomic ions.

Using Roman Numerals to Show Charge

For transition elements (like iron, copper, chromium), the charge can vary. We use Roman numerals in the name to show which charge the metal has:

  • Iron(II) means the iron has a 2+ charge (Fe²⁺)
  • Iron(III) means the iron has a 3+ charge (Fe³⁺)
  • Copper(II) means the copper has a 2+ charge (Cu²⁺)

Writing Formulas of Ionic Compounds

To work out the formula of an ionic compound, follow these steps:

Step 1: Write down the ions and their charges.

Step 2: Work out how many of each ion you need so that the positive and negative charges cancel out (the compound is neutral).

Step 3: Write the formula. If you need more than one of a compound ion, put it in brackets.

Example 1: Magnesium chloride

Step 1: Magnesium is in Group 2 → Mg²⁺
Chlorine is in Group 17 → Cl⁻

Step 2: To balance the charges, you need two chloride ions (each with 1− charge) for every one magnesium ion (with 2+ charge).

Step 3: Formula = MgCl₂

Example 2: Iron(III) oxide

Step 1: The Roman numeral tells us iron has a 3+ charge → Fe³⁺
Oxygen is in Group 16 → O²⁻

Step 2: To balance the charges:

  • 2 iron ions give: 2 × (3+) = 6+
  • 3 oxide ions give: 3 × (2−) = 6−
  • Total charge = 0 ✓

Step 3: Formula = Fe₂O₃

Example 3: Aluminium nitrate

Step 1: Aluminium is in Group 13 → Al³⁺
Nitrate is a compound ion → NO₃⁻

Step 2: To balance the charges, you need three nitrate ions for every one aluminium ion.

Step 3: Because nitrate is a compound ion and you need more than one, put it in brackets:
Formula = Al(NO₃)₃

Sign in to view full notes