Chemical formulae become easier when students stop treating subscripts as decorations and start asking what particle ratio makes the compound electrically and chemically consistent. In Punggol Secondary Chemistry, valency, ion charge, compound naming and formula writing sit underneath almost every later topic: equations, stoichiometry, acids and bases, salts, electrolysis and qualitative analysis.

Parents searching for chemical formulae, valency, ionic charges, compound naming, empirical formula, molecular formula or Secondary Chemistry formula writing are usually trying to help a student understand why magnesium chloride is MgCl₂ rather than MgCl, or why calcium nitrate needs brackets in Ca(NO₃)₂.

This upgraded Science Improvements In Punggol owner extends Atoms, Elements, Compounds and the Periodic Table, Ionic, Covalent and Metallic Bonding and Moles, Molar Mass and Stoichiometry.

The formula-writing reasoning system

1. Identify the elements or ions present.
2. Write the correct symbol for each.
3. Write the charge or combining capacity.
4. Find the smallest whole-number ratio that gives the required balance.
5. Use brackets around a polyatomic ion only when more than one is needed.
6. Reduce the ratio if a common factor exists.
7. Check the final formula against charge neutrality or known molecular structure.

Chemical symbols must be exact

An element symbol begins with a capital letter; a second letter, if present, is lowercase.

• Co = cobalt;
• CO = carbon monoxide;
• Cl = chlorine;
• Ca = calcium;
• Na = sodium.

Capitalisation changes chemical meaning, so formula writing begins with symbol accuracy.

Atoms and ions are not interchangeable

An atom is electrically neutral overall. An ion has gained or lost electrons and carries net charge.

• Na → sodium atom;
• Na⁺ → sodium ion;
• Cl → chlorine atom;
• Cl⁻ → chloride ion.

Writing a formula for an ionic compound therefore means combining ions in a ratio that produces zero total charge.

Valency and ionic charge are related but not identical ideas

In school Chemistry, valency is often used to describe an atom’s combining capacity, while ionic charge tells us the actual electrical charge on an ion.

For many simple main-group ions, the numerical values line up neatly, which makes valency a useful introductory shortcut. But charge includes sign; valency is usually treated as an unsigned combining number.

Ionic compounds must be electrically neutral overall

Consider magnesium chloride:

• Mg²⁺ contributes +2;
• Cl⁻ contributes −1.

Two chloride ions are needed to balance one magnesium ion:

MgCl₂

The 2 belongs to chloride because there are two chloride ions for every magnesium ion in the formula unit.

Charge balance is the real method

The cross-over method can be a convenient shortcut, but students should understand what it is doing.

For aluminium oxide:

• Al³⁺;
• O²⁻.

The smallest neutral combination is two Al³⁺ ions (+6) with three O²⁻ ions (−6):

Al₂O₃

Thinking in total charge prevents mechanical cross-over mistakes.

Always reduce to the simplest ratio

If calcium ions Ca²⁺ combine with oxide ions O²⁻, crossing the numbers blindly might produce Ca₂O₂.

But the simplest whole-number ratio is 1:1:

CaO

Ionic formula units are written in the lowest whole-number ratio unless a specific molecular or structural reason dictates otherwise.

Polyatomic ions behave as units

A polyatomic ion is a charged group of covalently bonded atoms that acts as one ion in many reactions.

• NH₄⁺ = ammonium;
• OH⁻ = hydroxide;
• NO₃⁻ = nitrate;
• SO₄²⁻ = sulfate;
• CO₃²⁻ = carbonate;
• PO₄³⁻ = phosphate.

The atoms inside the ion stay grouped when writing the compound formula.

Brackets are used when more than one polyatomic ion is needed

Calcium nitrate contains Ca²⁺ and NO₃⁻. Two nitrate ions are required:

Ca(NO₃)₂

Without brackets, CaNO₃₂ would imply a completely different atom count.

Do not use brackets when only one polyatomic ion is present

Sodium nitrate is:

NaNO₃

Writing Na(NO₃) is unnecessary because there is only one nitrate ion.

Transition metals can have more than one common charge

Iron can form Fe²⁺ and Fe³⁺ ions. Copper can commonly form Cu⁺ and Cu²⁺ ions.

This is why names such as iron(II) chloride and iron(III) chloride include Roman numerals.

Roman numerals specify oxidation state or ion charge

• iron(II) chloride → FeCl₂;
• iron(III) chloride → FeCl₃;
• copper(I) oxide → Cu₂O;
• copper(II) oxide → CuO.

The Roman numeral belongs to the metal’s oxidation state, not the number of atoms in the compound.

Naming simple ionic compounds

For a simple binary ionic compound:

1. name the positive ion first;
2. name the negative ion second;
3. for a single-element negative ion, use the -ide ending.

• NaCl → sodium chloride;
• MgO → magnesium oxide;
• CaBr₂ → calcium bromide.

Polyatomic ion names are retained

• Na₂SO₄ → sodium sulfate;
• CaCO₃ → calcium carbonate;
• NH₄NO₃ → ammonium nitrate;
• KOH → potassium hydroxide.

Do not convert nitrate, sulfate or carbonate into -ide endings.

Covalent compounds use a different naming logic

Simple molecular compounds made from non-metals are often named using prefixes that indicate atom numbers.

• CO → carbon monoxide;
• CO₂ → carbon dioxide;
• N₂O₄ → dinitrogen tetroxide;
• PCl₃ → phosphorus trichloride.

The exact naming conventions depend on the compound family and syllabus, so students should follow the naming system appropriate to the course.

Prefixes describe molecular composition

• mono- = 1;
• di- = 2;
• tri- = 3;
• tetra- = 4;
• penta- = 5;
• hexa- = 6.

The first element often omits mono- in conventional naming.

Molecular formula and empirical formula are different

A molecular formula gives the actual number of each type of atom in one molecule.

An empirical formula gives the simplest whole-number ratio of atoms.

• glucose molecular formula: C₆H₁₂O₆;
• glucose empirical formula: CH₂O.

Ionic compounds are usually described by formula units

NaCl does not mean one isolated sodium atom permanently bonded to one isolated chlorine atom as a molecule.

It represents the simplest ratio in the extended ionic lattice: one Na⁺ for every one Cl⁻.

Subscripts belong to the substance identity

Changing a subscript changes the compound.

• CO ≠ CO₂;
• H₂O ≠ H₂O₂;
• FeCl₂ ≠ FeCl₃.

This is why balancing a chemical equation must never be done by changing subscripts.

Coefficients balance equations; subscripts define compounds

To balance:

H₂ + O₂ → H₂O

write:

2H₂ + O₂ → 2H₂O

Do not write H₂O₂ unless hydrogen peroxide is genuinely the product.

Hydrates include water in the crystal structure

Some ionic crystals contain a fixed ratio of water molecules within the crystal lattice.

For example:

CuSO₄·5H₂O

represents copper(II) sulfate pentahydrate.

Water of crystallisation is not simply surface moisture

The water is incorporated in a definite stoichiometric relationship within the crystal structure.

Heating can remove this water and alter colour or structure, depending on the substance.

Empirical formula from composition data

The calculation pathway is:

1. convert each element’s mass to moles;
2. divide all mole values by the smallest;
3. convert to the simplest whole-number ratio;
4. write the empirical formula.

This connects formula writing directly to the mole concept.

Percent composition can be converted to empirical formula

If percentages are given, imagine a 100 g sample. Each percentage then becomes a mass in grams.

Convert those masses into moles and determine the simplest ratio.

Fractional ratios must be converted carefully

After dividing by the smallest mole value, ratios such as 1:1.5 should not simply be rounded to 1:2.

Multiply all values by a suitable whole number:

• 1 : 1.5 → ×2 → 2 : 3;
• 1 : 1.33 → often ×3 → about 3 : 4;
• 1 : 1.25 → ×4 → 4 : 5.

Molecular formula requires molar mass

Once the empirical formula is known:

1. calculate empirical formula mass;
2. divide molecular molar mass by empirical formula mass;
3. use the whole-number multiplier;
4. multiply every empirical subscript by that number.

Formulae predict molar ratios

Al₂(SO₄)₃ contains:

• 2 aluminium atoms/ions in the formula ratio;
• 3 sulfate groups;
• 3 sulfur atoms;
• 12 oxygen atoms.

Reading nested brackets correctly is essential for molar-mass and stoichiometry calculations.

Formula writing connects to reaction prediction

If the formula of a salt is wrong, every later equation, mole ratio and concentration calculation built from it can also be wrong.

This is why formula writing is foundational rather than a minor introductory skill.

Secondary G1, G2 and G3: depth changes, ratio logic remains

Different Chemistry levels may require simple ionic formulae, oxidation states, empirical/molecular formulas or more advanced nomenclature.

The transferable core remains identity → charge/valency → ratio → correct symbols and brackets.

A 30-minute formula drill

1. Write ten common ion symbols and charges.
2. Form five binary ionic compounds.
3. Form five compounds with polyatomic ions.
4. Use brackets correctly.
5. Name five ionic compounds from formulae.
6. Write formulas from five names.
7. Distinguish coefficients from subscripts.
8. Calculate one empirical formula from masses.
9. Convert it to a molecular formula from molar mass.

Common formula-writing misconceptions

• ionic compounds can carry net charge as written;
• crossed charge numbers never need reducing;
• brackets are always required around polyatomic ions;
• Roman numerals give the number of metal atoms;
• coefficients and subscripts can be changed interchangeably;
• molecular and empirical formula are always identical;
• NaCl represents one discrete molecule in the same way as CO₂;
• 1:1.5 can be rounded directly to 1:2.

How to diagnose a formula error

If an ionic formula is wrong, calculate total positive and negative charge. If brackets fail, ask whether more than one polyatomic ion is present. If naming fails, decide whether the compound is ionic or molecular before choosing the naming convention.

When Science tuition in Punggol adds value

Chemical formulae improve when students justify every subscript rather than apply a cross-over mechanically. In eduKate Punggol’s three-student Science tutorials, one learner can identify ions, another balance charge and another audit naming and brackets.

Parents can review Science Tuition Punggol, Secondary 3 Chemistry Tuition Punggol, or the Science Article Index.

Conclusion: every subscript should have a reason

Correct formulae come from chemical identity and ratio. Ionic compounds balance charge, polyatomic ions stay grouped, molecular names encode composition and empirical formulas reduce ratios to simplest terms. Once students can explain every symbol and subscript, later Chemistry becomes much more secure.