Writing Chemical Formulae
This topic covers the fundamental skill of writing and recognising the chemical formulae for common substances. It is the basic language of chemistry, essential for understanding chemical reactions and stoichiometry.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The formula of an ionic compound is determined by balancing the positive and negative charges of its ions to achieve a neutral overall charge.
- Charges for simple ions can often be predicted from their group in the Periodic Table (e.g., Group 1 ions are +1, Group 17 ions are -1).
- Formulae for common covalent compounds (like H2O, CO2, NH3, CH4) and common acids (HCl, H2SO4, HNO3) must be memorised.
- When a formula requires more than one polyatomic ion (e.g., OH-, SO42-), the ion is placed in brackets with a subscript outside, like in Ca(OH)2.
- Several non-metal elements exist as diatomic molecules and their formulae must be known: H2, N2, O2, and the halogens (F2, Cl2, Br2, I2).
Definitions
- Ionic Compound
- A substance formed from the electrostatic attraction between positively charged ions (cations, usually metals) and negatively charged ions (anions, usually non-metals or polyatomic ions).
- Covalent Compound
- A substance formed when atoms, typically non-metals, share electrons to form strong covalent bonds.
- Polyatomic Ion
- A group of covalently bonded atoms that has an overall positive or negative charge, such as the ammonium ion (NH4+) or the carbonate ion (CO32-).
Worked example
What is the correct chemical formula for aluminium sulfate?
- 1
Identify the constituent ions and their charges.
Aluminium is in Group 13, so it forms an Al3+ ion.
The sulfate ion is a common polyatomic ion with the formula SO42-.
- 2
To create a neutral compound, the total positive charge must equal the total negative charge.
- 3
The lowest common multiple of the charge magnitudes (3 and 2) is 6.
- 4
To get a total positive charge of +6, two Al3+ ions are needed (2 × +3 = +6).
- 5
To get a total negative charge of -6, three SO42- ions are needed (3 × -2 = -6).
- 6
Combine the ions in this ratio.
Since there is more than one polyatomic sulfate ion, enclose it in brackets.
- 7
The final formula is Al2(SO4)3.
Answer: Al2(SO4)3
Common mistakes
- ×Forgetting to use brackets for multiple polyatomic ions, for instance writing MgNO32 instead of the correct Mg(NO3)2.
- ×Incorrectly balancing charges, leading to formulae like NaS for sodium sulfide instead of Na2S.
- ×Confusing similar-sounding ions, such as sulfide (S2-) with sulfate (SO42-), or nitride (N3-) with nitrate (NO3^-).
- ×Mixing up the formulae for ammonia (NH3) and the ammonium ion (NH4+).
No-calculator tips
- ✓Use the 'swap and drop' method for simple ionic compounds: the numerical value of an ion's charge becomes the subscript for the other ion. For Al3+ and O2-, swap the 3 and 2 to get Al2O3.
- ✓Anchor your memory of ion charges to the Periodic Table: Group 1 = +1, Group 2 = +2, Group 13 = +3, Group 16 = -2, Group 17 = -1. This reduces the amount you need to memorise.
- ✓For memorising polyatomic ions, group them by charge: e.g., OH- and NO3- are both -1; SO42- and CO32- are both -2.