Chemical formulae become easier when students stop crossing numbers mechanically and start balancing electrical charge and atom ratios. In Punggol Secondary Chemistry, valency, ions, formulae and compound names sit underneath equations, stoichiometry, acids, salts and bonding. A weak formula foundation makes almost every later Chemistry topic harder.
Parents searching for chemical formulae, valency, ions and charges, writing ionic formulae, naming compounds, polyatomic ions or Secondary Chemistry basics are usually trying to help a student understand why magnesium chloride is MgCl₂ while aluminium oxide is Al₂O₃.
This upgraded Science Improvements In Punggol owner connects to Atoms, Elements, Compounds and the Periodic Table, Ionic, Covalent and Metallic Bonding and Moles, Molar Mass and Stoichiometry.
The formula-writing reasoning system
- Identify whether the compound is ionic or molecular.
- Identify the elements or ions present.
- Write the correct charges or valencies.
- Choose the smallest whole-number ratio that gives the correct total charge or bonding pattern.
- Write subscripts, not charges, in the final formula.
- Use brackets around polyatomic ions when more than one is needed.
- Check that the formula is chemically and electrically sensible.
Ions form when atoms gain or lose electrons
A neutral atom has equal numbers of protons and electrons.
If it loses electrons, it becomes a positive ion. If it gains electrons, it becomes a negative ion.
Common main-group charges follow electron structure
- Group 1 metals commonly form 1+ ions;
- Group 2 metals commonly form 2+ ions;
- aluminium commonly forms 3+ ions;
- halogens commonly form 1− ions;
- oxygen commonly forms 2− ions;
- nitrogen can form 3− in simple ionic models.
These patterns arise because atoms tend toward more stable outer-electron arrangements.
Transition metals can have more than one common charge
Iron can form Fe²⁺ and Fe³⁺. Copper can form Cu⁺ and Cu²⁺. Other transition metals also show variable oxidation states.
This is why names such as iron(II) chloride and iron(III) chloride include Roman numerals.
An ionic formula must be electrically neutral overall
For magnesium chloride:
- Mg²⁺ contributes +2;
- Cl⁻ contributes −1;
- two chloride ions are needed to balance one magnesium ion.
The formula is therefore MgCl₂.
Charge balance explains aluminium oxide
Aluminium forms Al³⁺ while oxide is O²⁻.
The smallest whole-number combination with equal positive and negative charge is:
- 2 × Al³⁺ = +6;
- 3 × O²⁻ = −6.
The formula is Al₂O₃.
The criss-cross method is a shortcut, not the explanation
Students often cross charge numbers into subscripts. This can work, but only if the ratio is simplified and the charges were correct to begin with.
The real principle is total charge balance.
Always simplify ionic subscripts
Calcium ions are Ca²⁺ and oxide ions are O²⁻. Crossing blindly might suggest Ca₂O₂.
The simplest whole-number ratio is 1:1, so the correct formula is CaO.
Subscripts tell atom or ion ratios
In Al₂O₃, the subscript 2 means two aluminium atoms or ions are present per formula unit, while the subscript 3 means three oxygen atoms or oxide ions.
A subscript does not represent the charge itself.
Charges are written as superscripts
For an ion:
- Na⁺ has charge +1;
- Ca²⁺ has charge +2;
- Cl⁻ has charge −1;
- SO₄²⁻ has charge −2.
Students should keep superscript charge and subscript quantity visually separate.
Polyatomic ions act as charged groups
Common examples include:
- NH₄⁺ ammonium;
- OH⁻ hydroxide;
- NO₃⁻ nitrate;
- SO₄²⁻ sulfate;
- CO₃²⁻ carbonate;
- PO₄³⁻ phosphate at higher levels.
These groups stay together when writing many ionic formulae.
Brackets are needed when more than one polyatomic ion is present
Calcium hydroxide contains one Ca²⁺ and two OH⁻ ions.
The correct formula is Ca(OH)₂, not CaOH₂, because the subscript applies to the whole hydroxide ion.
Ammonium sulfate shows the same rule
Ammonium is NH₄⁺ and sulfate is SO₄²⁻.
Two ammonium ions are needed per sulfate ion, giving (NH₄)₂SO₄.
Ionic compound names usually list cation then anion
- NaCl → sodium chloride;
- MgO → magnesium oxide;
- CaCO₃ → calcium carbonate;
- Al₂(SO₄)₃ → aluminium sulfate.
Monatomic negative ions usually end in -ide. Polyatomic ions keep their established names.
Roman numerals specify variable metal charge
FeCl₂ is iron(II) chloride because two Cl⁻ ions require Fe²⁺.
FeCl₃ is iron(III) chloride because three Cl⁻ ions require Fe³⁺.
Molecular compounds follow a different logic
Covalent molecules are not built from charge-balancing cations and anions in the same way.
The formula reflects the actual number of atoms bonded in each molecule.
Prefixes can name simple molecular compounds
- CO → carbon monoxide;
- CO₂ → carbon dioxide;
- N₂O₄ → dinitrogen tetroxide.
The exact naming conventions used depend on syllabus depth and compound type.
Valency describes combining capacity
Valency is a useful school concept describing how many bonds an atom tends to form or how many electrons it tends to lose, gain or share in simple compounds.
Oxidation state and formal charge provide more advanced descriptions, but valency remains useful for introductory formula writing.
Hydrogen has valency 1 in many simple compounds
Oxygen commonly has valency 2, nitrogen 3 and carbon 4 in basic covalent models.
This explains formulae such as H₂O, NH₃ and CH₄ without using ionic charge balance.
Empirical formula is the simplest whole-number ratio
The empirical formula describes the simplest whole-number ratio of atoms in a compound.
For example, glucose has molecular formula C₆H₁₂O₆ but empirical formula CH₂O.
Molecular formula gives actual atom numbers in a molecule
A molecular formula can be a whole-number multiple of the empirical formula.
This connects formula writing directly to the stoichiometry owner.
Chemical equations require correct formulae before balancing
Students should never change chemical subscripts simply to balance an equation.
Subscripts define the substance. Coefficients change how many formula units or molecules participate.
Example: balancing water formation
The correct water formula is H₂O.
To balance:
2H₂ + O₂ → 2H₂O
Changing H₂O into H₂O₂ would create a different chemical substance rather than balance the equation.
Acid formulae encode ionisable hydrogen
- HCl → hydrochloric acid;
- HNO₃ → nitric acid;
- H₂SO₄ → sulfuric acid.
The formulas matter for neutralisation stoichiometry because different acids can supply different numbers of H⁺ ions per formula unit in the simplified model.
Salt formulae connect acids to ions
Chlorides contain Cl⁻, nitrates contain NO₃⁻ and sulfates contain SO₄²⁻.
This makes formula knowledge essential for Salt Preparation.
Secondary G1, G2 and G3: depth changes, ratio logic remains
Different Chemistry levels may require common-ion formulae, covalent naming, oxidation states or empirical/molecular formula calculations.
The transferable core remains identity → charge or valency → smallest valid ratio → correct notation.
A 30-minute formula drill
- Write charges for ten common ions.
- Write formulae for five binary ionic compounds.
- Simplify one crossed-charge formula.
- Write three compounds using polyatomic ions.
- Name five ionic compounds.
- Determine iron charge from FeCl₂ and FeCl₃.
- Write four simple covalent formulae from valency.
- Separate empirical from molecular formula.
Common formula misconceptions
- charges become subscripts without checking simplification;
- Ca²⁺ and O²⁻ produce Ca₂O₂ rather than CaO;
- subscripts can be changed to balance equations;
- polyatomic ions do not need brackets;
- all compounds are named using the same prefix system;
- Roman numerals show how many metal atoms are present;
- empirical and molecular formula are always identical;
- valency, oxidation state and ionic charge are always interchangeable concepts.
How to diagnose a formula error
If an ionic formula is wrong, calculate total positive and negative charge explicitly. If brackets are missing, identify whether the subscript applies to one atom or a whole polyatomic ion. If equation balancing changes subscripts, reset the correct substance formula before using coefficients.
When Science tuition in Punggol adds value
Formula writing improves when students explain every subscript rather than cross numbers automatically. In eduKate Punggol’s three-student Science tutorials, one learner can identify ions, another balance charge and another audit naming and notation.
Parents can review Science Tuition Punggol, Secondary 3 Chemistry Tuition Punggol, or the Science Article Index.
Conclusion: formulae encode ratios, charges and structure
Chemical formulae are compressed chemical information. Ionic formulae balance charge, molecular formulae show bonded atom numbers and names encode composition. Once students understand what each symbol and subscript means, formula writing becomes reasoning rather than guesswork.
