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How to Perform in the new G3 SEC Examinations | Learner’s Guide Vol 0132 | Chemistry: Ionic Formula Workshop — Balance the Charges Before Writing Subscripts

G3 SEC Chemistry · Learner’s Guide Vol 0132. Balance cation and anion charges, preserve polyatomic groups, and verify neutral ionic formulae.

A pupil combines magnesium ions, Mg²⁺, with chloride ions, Cl⁻, and writes MgCl. The mistake is easy to understand: one magnesium symbol and one chlorine symbol look tidy together. But an ionic compound must have no overall electrical charge. One Mg²⁺ contributes +2; one Cl⁻ contributes -1. To balance +2, two chloride ions are needed. The simplest formula is MgCl₂.

A chemical formula is therefore not a decoration made by putting symbols side by side. It is a compact record of which ions are present and the simplest whole-number ratio that makes the compound electrically neutral. When the ions have equal and opposite charges, the ratio can be 1:1. When their charge magnitudes differ, the numbers of ions must compensate.

This G3 SEC Chemistry Learner’s Guide Vol 0132 develops one reliable routine: identify each ion and its charge, find the lowest whole-number combination whose charges sum to zero, write the formula with the correct subscripts and verify the result. You will practise single-atom and polyatomic ions, distinguish formula subscripts from equation coefficients, and connect the formula to ionic bonding and lattice properties. All worked problems are original educational examples, not official examination questions.

Chapter index

1. Begin with ions, not with the finished compound

An ion is an atom or group of atoms with an overall electrical charge. A positively charged ion is a cation; a negatively charged ion is an anion. A metal atom may form a positive ion by losing one or more electrons, while a non-metal atom may form a negative ion by gaining electrons. The actual ion type and charge depend on the element and context.

Sodium commonly forms Na⁺ by losing one electron. Chlorine commonly forms Cl⁻ by gaining one electron. The charges are equal in magnitude and opposite in sign, so one sodium ion and one chloride ion form the simplest neutral ratio 1:1, represented by NaCl.

Magnesium commonly forms Mg²⁺ by losing two electrons. Chloride remains Cl⁻ in the familiar ionic compound. One magnesium ion therefore requires two chloride ions to give total charge +2 + 2(-1) = 0. The formula MgCl₂ records that 1:2 ratio.

Do not confuse the charge on an ion with a chemical formula subscript. The superscript 2+ on Mg²⁺ means charge of positive two elementary charge units. The subscript 2 in MgCl₂ tells us that the formula unit contains two chloride ions for each magnesium ion in the simplest ratio. They perform different jobs.

A neutral formula is an accounting statement about the total charges of its ions. You can read it without drawing an atom. But to explain why the ions form, the electron-transfer model and ionic bonding are also important.

Start every problem with the ion symbols and charges on a separate line. If you begin by guessing a formula from the element names, you may silently choose an incorrect ratio and then search for a reason afterwards.

2. Check the actual 2027 G3 SEC Chemistry syllabus

The official 2027 G3 school-candidate syllabus index lists combined Science (Physics/Chemistry) K326 and combined Science (Chemistry/Biology) K328, both of which include Chemistry. Combined Science K327 is Physics/Biology and does not contain the Chemistry disciplinary paper. Pure Chemistry K324 is a separate syllabus.

The combined Science K326–K328 syllabus explicitly includes formation of ions by electron loss and gain, ionic bonding between metals and non-metals including dot-and-cross diagrams, and deduction of ionic compound formulae from ion charges and vice versa. Formula and equation writing also appears under Chemical Calculations. This is a syllabus-based skill, not a speculative advanced topic.

For combined Science, Paper 1 lasts one hour, is worth 40 marks and contributes 20%. The relevant Chemistry structured-and-free-response paper is Paper 3, lasting 1 hour 15 minutes, worth 65 marks and weighted at 32.5%. A candidate takes the other disciplinary paper relevant to K326 or K328, and the practical Paper 5 lasts 1 hour 30 minutes, carries 30 marks and contributes 15%. These are formats, not promises about a particular ionic compound on the paper.

A candidate taking pure Chemistry should consult K324 for its own complete assessment structure and requirements rather than assuming combined Science papers apply unchanged. The ion charge and compound examples below serve the broad Chemistry skill but do not replace either full syllabus.

For related learning, use the Science Hub and G3 SEC Learner’s Guide. Volume 0104 on matter and energy boundaries discusses conservation broadly; this volume’s specific job is balancing ionic charge and writing unambiguous formulae.

3. An ionic compound has a neutral overall charge

Consider Na⁺ and O²⁻. A single sodium ion contributes +1 and an oxide ion -2. Their sum is -1, not zero. Two sodium ions contribute +2, which exactly balances one oxide ion -2. Therefore the simplest formula is Na₂O.

You can show the balance numerically: 2(+1)+1(-2)=0. This is a reliable check when a student is uncertain whether the subscript should be attached to the metal or the non-metal. The formula says two Na⁺ for each O²⁻, not one sodium and two oxide ions.

Now consider calcium, Ca²⁺, and oxide, O²⁻. Their charges already cancel in a 1:1 ratio: +2−2=0. The correct formula is CaO. Writing Ca₂O₂ represents the same relative ratio but is not the simplest whole-number formula, so reduce it. Writing CaO₂ would give +2+2(-2)=-2 in the simple ion model and is wrong for calcium oxide.

When the positive and negative charges have equal magnitudes, the formula often needs no visible subscript. The numeral one is conventionally omitted from chemical formulae. MgO contains one magnesium and one oxide ion per simplest formula unit. The absence of a numeral does not mean the compound has no ions.

A strong learner tests neutrality directly instead of relying solely on a shortcut. Ask, “How many positive charge units are there in this formula, and how many negative charge units?” If the sums differ, the ionic formula is not electrically neutral under the stated ions.

The principle is simple but general. The compound’s charge is not chosen from how its name sounds. It follows from the charges and the simplest ratio.

4. The least common multiple gives a method

Suppose aluminium ions Al³⁺ combine with oxide ions O²⁻. One aluminium contributes +3; one oxide contributes -2. Neither a 1:1 nor a 1:2 ratio is neutral. The least common multiple of charge magnitudes three and two is six. Two aluminium ions provide +6, while three oxide ions provide -6. The formula is Al₂O₃.

The charge check is 2(+3)+3(-2)=+6−6=0. A formula Al₃O₂ reverses the ratio and gives 3(+3)+2(-2)=+9−4=+5, not neutral. A learner who copies the other ion’s charge as a subscript without remembering which symbol it belongs to can make precisely that mistake.

An often taught shortcut is to exchange the numerical charge magnitudes as subscripts, then reduce to the smallest whole numbers where necessary. This can be useful, but it should never replace the neutrality check. The common-multiple method makes the underlying reasoning explicit.

For Mg²⁺ and O²⁻, copying magnitudes mechanically would produce Mg₂O₂. You must reduce that to MgO. For Al³⁺ and Cl⁻, three chloride ions are needed for one aluminium: AlCl₃. In each case the final formula must have the simplest neutral ratio.

When the charge magnitudes are four and two in a hypothetical model, the ratio of the corresponding ion counts may reduce by a common factor. Do not leave a 2:4 ratio if the least combination is 1:2. Chemistry convention normally uses simplest formula units for ionic compounds.

This method also reinforces Volume 0131 on inequality operations only as a broader reminder that a symbolic operation must preserve its underlying meaning; the relevant mathematics here is whole-number ratios, not an inequality procedure.

5. A formula subscript is not an ion charge

Look closely at MgCl₂. Magnesium appears without a subscript, so there is one magnesium ion in the simplest formula unit. Chlorine appears with a subscript two, meaning two chloride ions for each magnesium ion in that ratio. The charge on magnesium is 2+, not a subscript two on magnesium.

In written explanations you can describe the ions as Mg²⁺ and Cl⁻ and the compound as MgCl₂. The first notation includes charges on separate ions. The second is a neutral formula showing the ratio of ions in the solid. The notation serves two different purposes.

Another example is calcium chloride, CaCl₂. It is not correct to say the chloride ion has charge -2 simply because two chlorides appear. Each chloride ion is -1; there are two of them. Similarly, two sodium ions in Na₂O do not make each sodium ion 2+. Each is +1.

The number outside a chemical formula in an equation is a coefficient, not a subscript. In 2NaCl, the coefficient two represents two formula units of sodium chloride in a particle-counting representation, or two moles of formula units in a molar equation ratio. It does not change NaCl into NaCl₂.

Students who lose track of whether a numeral belongs as a coefficient, subscript or superscript may balance equations incorrectly or misinterpret particles. Label these three places before solving: charge belongs high on an ion, count within a formula belongs low after the symbol or group, and number of formula units belongs in front.

A good formula should be read as a complete message. Changing the placement of a two changes that message even when the same two letters remain.

6. Polyatomic ions stay together as groups

Some ions contain several atoms bonded together and have one overall ionic charge. Examples used in school Chemistry include nitrate, NO₃⁻; sulfate, SO₄²⁻; carbonate, CO₃²⁻; hydroxide, OH⁻; and ammonium, NH₄⁺. The charge belongs to the whole ion, not independently to each atom within it.

Consider sodium sulfate. Sodium is Na⁺ and sulfate is SO₄²⁻. Two Na⁺ ions are needed to balance one SO₄²⁻ ion. The formula is Na₂SO₄. The sulfate group remains intact, and because only one sulfate group is needed, parentheses are unnecessary.

Now take magnesium nitrate. Magnesium is Mg²⁺ and nitrate is NO₃⁻. Two nitrate ions are required per magnesium ion, so the formula is Mg(NO₃)₂. Parentheses show that the subscript two applies to the entire NO₃ group, not merely to the oxygen atom.

If you wrote MgNO₃₂ without correct grouping, the notation would be ambiguous or invalid. Mg(NO₃)₂ clearly means two nitrate ions, giving a total of two nitrogen atoms and six oxygen atoms per simplest formula unit, along with one magnesium.

Try aluminium sulfate, Al³⁺ and SO₄²⁻. The least common multiple is six, so use two aluminium ions and three sulfate ions. The formula is Al₂(SO₄)₃. The charge check is 2(+3)+3(-2)=0. The parentheses protect the identity of the repeated polyatomic group.

A safe rule is to preserve each named polyatomic ion as a single unit during the charge-balance step. Only after finding the group ratio should you write subscripts and parentheses. Do not split nitrate into nitrogen and oxygen charges to guess the compound.

7. More than one possible charge requires more information

Some metals can form ions with different charges, and the compound name or problem statement must identify the relevant charge. Iron, for example, can form Fe²⁺ or Fe³⁺ in common compounds. The formulas with oxide therefore differ: Fe²⁺ with O²⁻ gives FeO, while Fe³⁺ with O²⁻ gives Fe₂O₃.

The two formulas are not interchangeable. FeO has a 1:1 iron-to-oxide ratio consistent with Fe²⁺; Fe₂O₃ has a 2:3 ratio consistent with Fe³⁺. The formula depends on which iron ion the question specifies or asks you to deduce from other information.

If a Chemistry task simply gives “iron ion” without a charge or additional context, do not automatically assume the one you used last week. Identify which ion is intended from the name, oxidation-state notation or supplied information. If there is genuinely insufficient information for one unique formula, say what is missing.

This is a useful example of scientific precision. Two compounds may involve the same element names yet contain different ions and have different properties. The formula is a statement about a specific substance, not a generic combination of a metal word and a non-metal word.

A pupil might claim that Fe₂O₃ contains two iron ions each with charge +2 because the subscript is two. That confuses the count with charge. Two Fe³⁺ ions give +6; three O²⁻ ions give -6. The subscript numbers and superscript charge numbers play complementary but separate roles.

The method remains the same: know the ion, count total charges and write the simplest neutral ratio. When the ion identity is uncertain, solving the ratio without clarifying it is premature.

8. Ionic bonding is the attraction between oppositely charged ions

In the familiar formation of sodium chloride, a sodium atom loses one electron to form Na⁺ while a chlorine atom gains an electron to form Cl⁻. The resulting oppositely charged ions attract electrostatically. Ionic bonding is the strong electrostatic attraction between positive and negative ions in the lattice.

A dot-and-cross diagram is a representation of electrons from different starting atoms. It can show that an electron was transferred and that the ions have the resulting arrangements under the model. Do not mistake the diagram for two isolated ions holding hands in a permanent molecule-like pair. Solid sodium chloride forms an extended ionic lattice rather than discrete NaCl molecules.

For magnesium chloride, a magnesium atom loses two electrons in the simple ionic model, while two chlorine atoms each gain one electron. The resulting ions are Mg²⁺ and two Cl⁻. Their charges balance overall. This gives the formula MgCl₂ and helps explain its ratio.

The ionic formula tells you the simplest relative number of ions, not the precise number of each ion in a visible crystal. A macroscopic piece contains vast numbers of ions arranged in a lattice. The formula unit is a convenient accounting ratio for the compound.

A strong explanation identifies electron transfer to form ions, their charges and the electrostatic attraction. A bare statement such as “metal plus non-metal makes salt” can be a helpful starting clue but is not a complete answer when the question asks how the bonding forms.

The SEAB combined Science syllabus explicitly includes dot-and-cross representations of NaCl and MgCl₂, and properties related to lattice structure. This is why the structure behind the formula matters.

9. Why solid and molten ionic compounds behave differently

Solid ionic compounds contain positive and negative ions held in fixed positions within a giant ionic lattice. The ions can vibrate but are not free to migrate through the solid as mobile charge carriers in the ordinary conduction model. This explains why many ionic solids do not conduct electricity.

When an ionic compound is molten or suitably dissolved in water, the ions may be mobile and able to carry charge. The ability to conduct then differs from the solid state. The key physical reason is mobility of charged particles, not the idea that the compound becomes “more charged” upon melting.

All these states can contain the same types of ions. The charge of a sodium ion does not change from +1 to +2 merely because the ionic compound melts. The arrangement and freedom of movement change.

A learner might say solid NaCl does not conduct because it contains no charged particles. That is incorrect. It contains ions, but in the solid lattice they cannot move freely to transport charge through the material. The corrected statement names both the particles and their mobility.

Likewise, do not claim that every solution of every substance conducts simply because it is a liquid. A solution’s conductivity depends on whether it contains appropriate mobile charge carriers. Sugar dissolved in water does not behave like a solution containing many mobile ions under ordinary conditions.

The connection from formula to properties is valuable: the formula identifies ions and ratio, bonding identifies attraction, lattice arrangement explains some solid properties, and ion mobility helps explain electrical conduction.

10. Subscripts cannot be changed to balance an equation

A chemical equation needs equal numbers of each type of atom on both sides. To balance an equation, change coefficients in front of formulas where appropriate, not the internal subscripts of substances. Changing MgCl₂ into MgCl changes the substance’s stated ion ratio and does not merely adjust how many units participate.

For example, sodium reacts with chlorine in the familiar representation 2Na + Cl₂ → 2NaCl. The coefficient two in front of Na and NaCl balances the two chlorine atoms in Cl₂ and the sodium atoms. The formula NaCl remains unchanged because it represents sodium chloride itself.

If you wrote NaCl₂ to make the chlorine count appear balanced without using coefficients, you would invent a different formula inconsistent with the ionic charges Na⁺ and Cl⁻ in the intended compound. The balance of atoms across the equation and the balance of charge within an ionic formula are related but separate checks.

Try magnesium reacting with chlorine: Mg + Cl₂ → MgCl₂. The equation is already balanced with one magnesium atom and two chlorine atoms on each side. The ionic formula MgCl₂ emerges from charge balance and is not a coefficient trick.

In a reaction involving aluminium and oxygen to form aluminium oxide, the correct product formula is Al₂O₃ from Al³⁺ and O²⁻. Only then balance the full equation: 4Al + 3O₂ → 2Al₂O₃. Each side now contains four aluminium atoms and six oxygen atoms.

The order is important. First establish the correct chemical formula for each substance; then use coefficients to balance the equation. Skipping that distinction can produce a balanced-looking expression about the wrong chemical substances.

11. A full worked ratio audit

Suppose a task gives cation Al³⁺ and anion SO₄²⁻ and asks for the compound formula. Start with the charge magnitudes three and two. Their lowest common multiple is six. Two aluminium ions provide +6; three sulfate ions provide -6. Write Al₂(SO₄)₃.

Now count what the formula contains. There are two aluminium ions. There are three sulfate groups. Each sulfate group contains one sulfur and four oxygen atoms, so the formula unit representation contains three sulfur and twelve oxygen atoms in its groups. This atom-count check is useful when the formula appears later in a balanced equation.

Perform the charge check separately: 2×(+3)+3×(-2)=0. Counting atoms and counting charges are different operations. Both need to be correct.

A student proposes Al₃(SO₄)₂. That has three aluminium ions giving +9 and two sulfate ions giving -4, for net +5. The formula does not describe a neutral compound of the named ions. Another proposes Al₂SO₄₃, omitting the needed parentheses and producing unclear grouping. Both can be repaired by returning to the ion identities first.

Consider whether the final formula is in the simplest ratio. Two aluminium to three sulfate is already simplest because two and three have no common factor greater than one. A doubled expression Al₄(SO₄)₆ would preserve a 2:3 ratio but is not the conventional simplest formula unit.

The best examination working is compact: “Al³⁺, SO₄²⁻; 2×(+3)+3×(-2)=0; Al₂(SO₄)₃.” Three steps show the complete reasoning without extraneous writing.

12. Original practice: fifteen formula decisions

A. Combine Na⁺ and Cl⁻ in the simplest neutral formula.

B. Combine Mg²⁺ and Cl⁻.

C. Combine Ca²⁺ and O²⁻.

D. Combine Al³⁺ and O²⁻.

E. Combine Na⁺ and O²⁻.

F. Combine Ca²⁺ and Cl⁻.

G. Combine Na⁺ and SO₄²⁻.

H. Combine Mg²⁺ and NO₃⁻.

I. Combine Al³⁺ and SO₄²⁻.

J. Combine NH₄⁺ and Cl⁻.

K. Combine Ca²⁺ and CO₃²⁻.

L. Combine K⁺ and PO₄³⁻.

M. Combine NH₄⁺ and SO₄²⁻.

N. Combine Fe²⁺ and O²⁻, then Fe³⁺ and O²⁻. Explain why the formulas differ.

O. A student writes Mg₂O₂ from Mg²⁺ and O²⁻. Explain what should be changed.

P. Explain the difference between the subscript two in MgCl₂ and the coefficient two in 2MgCl₂.

Q. What is wrong with balancing a chemical equation by turning NaCl into NaCl₂?

R. Explain why solid NaCl does not conduct in the ordinary school model while molten NaCl can.

S. State what a dot-and-cross diagram contributes to an explanation of magnesium chloride formation.

T. The task supplies an unspecified iron ion with no charge. Can you always identify one unique iron oxide formula? Explain.

Write the ion charges above your working before choosing a formula. For polyatomic ions, keep the group intact and add parentheses only if the entire group is repeated.

13. Full answer clinic with neutral-charge verification

A. NaCl. +1−1=0.

B. MgCl₂. +2+2(-1)=0.

C. CaO. +2−2=0, a 1:1 ratio.

D. Al₂O₃. 2(+3)+3(-2)=0.

E. Na₂O. 2(+1)−2=0.

F. CaCl₂. +2+2(-1)=0.

G. Na₂SO₄. Two Na⁺ ions balance one sulfate ion of -2.

H. Mg(NO₃)₂. One Mg²⁺ requires two nitrate ions each -1.

I. Al₂(SO₄)₃. Two Al³⁺ ions balance three sulfate ions.

J. NH₄Cl. Ammonium +1 balances chloride -1.

K. CaCO₃. Calcium +2 balances carbonate -2.

L. K₃PO₄. Three potassium ions each +1 balance phosphate -3.

M. (NH₄)₂SO₄. Two ammonium ions each +1 balance sulfate -2.

N. FeO for Fe²⁺ with O²⁻; Fe₂O₃ for Fe³⁺ with O²⁻. The ion charge determines the neutral ratio.

O. Reduce the 2:2 ratio to 1:1, giving MgO.

P. The subscript two gives the number of chloride ions per magnesium in the formula ratio. The coefficient two gives two whole formula units in a particle-count representation.

Q. It changes the formula and substance rather than the amount of the existing compound. Balance the equation with suitable coefficients while retaining chemically correct formulas.

R. Solid ionic ions are fixed in the lattice and cannot migrate freely; molten ions are mobile and carry electric charge.

S. It can show the two electrons lost by magnesium, one electron gained by each of two chlorine atoms, and the resulting ions with their electronic arrangements.

T. Not without the relevant charge or other information. Fe²⁺ and Fe³⁺ lead to different oxide formulas.

Every numerical check is based on the named ions. Memorising one line without charge reasoning makes the next unfamiliar formula harder than it needs to be.

14. How to handle unfamiliar ion combinations

A formula question may supply unfamiliar ion names and charges. You do not need to pretend that you have memorised every possible compound. If the ion symbols and charges are given, use them directly. Identify the positive ion, identify the negative ion, and search for the smallest whole-number ratio yielding total zero.

Suppose a hypothetical cation is X²⁺ and anion Y³⁻. The lowest common charge multiple is six. Three X²⁺ ions provide +6 and two Y³⁻ ions -6. The simplest neutral formula is X₃Y₂. This is a ratio exercise using fictional symbols, not a claim about a real substance called X₃Y₂.

If an ion is polyatomic, treat it as one charged group. For example, X²⁺ and ZO₃⁻ would combine as X(ZO₃)₂ in a fictional charge-balancing exercise. Parentheses preserve the repetition of the whole group; the internal atoms are not separately rebalanced by charge.

Do not reverse the formula order casually. In common ionic compound notation, the cation is usually written before the anion. Retaining conventional order helps readers recognise the compound and makes the formula easier to audit.

If you suspect a formula, test its charge sum. This step is quick even with unfamiliar symbols. A formula with nonzero overall charge cannot be the neutral ionic compound requested under that specified combination.

The routine transfers to real examples because it is built on a conserved principle: the sum of positive and negative charges in a neutral formula unit must be zero.

15. Seven days of practice from ions to properties

Day 1: identify ions. Distinguish atom, cation, anion and neutral ionic compound. Practise reading charge superscripts without confusing them with subscripts.

Day 2: equal charge magnitudes. Work with Na⁺/Cl⁻, Ca²⁺/O²⁻ and other 1:1 neutral ratios. Explain why no numeral one appears in the formula.

Day 3: unequal charges. Use +2/-1 and +3/-2 cases. Find the least common multiple, write the ion count ratio and verify neutral total charge.

Day 4: polyatomic groups. Practise nitrate, sulfate, carbonate, hydroxide and ammonium where relevant. Protect a repeated group using parentheses.

Day 5: formation and bonding. Describe electron loss and gain, electrostatic attraction and the ionic lattice. Explain why a formula is a ratio rather than a discrete molecule count.

Day 6: equations and properties. Distinguish subscript from coefficient, then explain why solid and molten ionic compounds differ in electrical conductivity.

Day 7: independent clinic. Answer Questions A–T with no notes, then classify errors as wrong charge, wrong ratio, missing parentheses, wrong formula order, wrong coefficient or incorrect property explanation.

Parents can ask, “Do the positive and negative charges cancel?” That one question is often more productive than asking a child to memorise a long list of formulas without understanding.

16. A formula is a charge-balanced promise

Writing the formula is the last step of a reasoning process, not the first. Every cation contributes positive charge, every anion negative charge, and the simplest formula ratio must balance them. A correct subscript follows from that neutral total, while a correct explanation links the ions to electron transfer, electrostatic attraction and lattice structure.

Continue with the Science Hub, the G3 SEC Learner’s Guide and Volume 0116 on paper chromatography. That earlier Chemistry guide owns mixture separation; this one owns compound formula construction. They should not be confused because the distinction between compounds and mixtures matters throughout Chemistry.

Official learning outcomes and assessment details come from the 2027 SEAB G3 school-candidate syllabus index and combined Science K326/K327/K328 syllabus. The exercises are original teaching examples and not official SEC questions.