ChemistryICSE Class 8 · Language of Chemistry

Symbols, Valency, Radicals and Writing Chemical Formulae by the Criss-Cross Method (ICSE Class 8)

Updated 17 October 2026 · Fact-checked

A symbol is one or two letters for an element (first capital). A formula shows the elements in a compound and the atoms of each. Write it by the criss-cross method: put the valencies above the symbols, exchange them as subscripts, reduce to the simplest ratio, and bracket a radical if more than one is needed (Al₂(SO₄)₃).

Understand Symbols, Valency, Radicals and Writing Formulae

Symbols. One or two letters; the first is capital, the second small (C, Ca, Cl, Co). Some come from Latin names: Na (natrium, sodium), K (potassium), Fe (iron), Cu (copper), Ag (silver), Pb (lead).

Valency is the combining capacity.

Valency 1	Valency 2	Valency 3
H, Na, K, Ag; Cl, Br; OH, NO₃, NH₄	O, Mg, Ca, Zn, Cu, Fe (ferrous); SO₄, CO₃	Al, N (in NH₃), Fe (ferric); PO₄

A group of atoms acting as a unit with a combining capacity (SO₄, CO₃, NO₃, OH, NH₄, PO₄) is a radical.

Criss-cross method.
1. Write the symbols (metal or positive part first).
2. Write the valency above each.
3. Criss-cross: the valency of each becomes the subscript of the other.
4. Reduce to the simplest whole-number ratio; leave out subscript 1.
5. Put a radical in brackets if more than one is needed.

Examples:

Al (3) and O (2): $A l_{2} O_{3}$ .
Ca (2) and Cl (1): $C a C l_{2}$ .
Mg (2) and O (2): $M g_{2} O_{2} \to M g O$ .
Ca (2) and OH (1): $C a (O H)_{2}$ .
Al (3) and SO₄ (2): $A l_{2} (S O_{4})_{3}$ .
Ca (2) and PO₄ (3): $C a_{3} (P O_{4})_{2}$ .
NH₄ (1) and SO₄ (2): $(N H_{4})_{2} S O_{4}$ .

Ferrous and ferric. Iron has valency 2 (ferrous: FeCl₂) and 3 (ferric: FeCl₃).

Reading a formula. A subscript multiplies the symbol before it (or the bracket before it); a number in front multiplies the whole formula: $3 A l_{2} (S O_{4})_{3}$ has $3 \times 2 = 6$ Al, $3 \times 3 = 9$ S and $3 \times 12 = 36$ O atoms.

Symbols, Valency, Radicals and Writing Formulae: formulas and key facts

Criss-cross: $A^{x} B^{y} \to A_{y} B_{x}$; Reduce the subscripts; bracket radicals.
Examples: $A l_{2} O_{3} ∣ C a C l_{2} ∣ M g O ∣ C a (O H)_{2} ∣ A l_{2} (S O_{4})_{3} ∣ C a_{3} (P O_{4})_{2} ∣ (N H_{4})_{2} S O_{4}$; From valencies.
Latin symbols: Na sodium
K potassium
Fe iron
Cu copper
Ag silver
Pb lead; From Latin names.

How to solve Symbols, Valency, Radicals and Writing Formulae questions

Write symbols, add valencies, criss-cross, simplify, and check the charges balance.

1Write the positive part first and the negative part second.
2Put the valency of each above its symbol or radical.
3Criss-cross the valencies as subscripts.
4Reduce to the lowest ratio; drop subscript 1.
5Put brackets around a radical that needs a subscript.
6Check: total valency of positive part $=$ total valency of negative part.

Symbols, Valency, Radicals and Writing Formulae in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Toothpaste to fertiliser

Suppose a fertiliser bag lists ammonium sulphate.

Write its formula.

Try it first, then show the solutionSolution

NH₄ has valency 1; SO₄ has valency 2.
Criss-cross: $(N H_{4})_{2} S O_{4}$ (brackets for the radical).

Answer: (NH₄)₂SO₄

Memory hook: Two ammoniums for one sulphate.

Rust and salt

Imagine a chemistry label shows ferric chloride.

Write its formula.

Try it first, then show the solutionSolution

Ferric means iron with valency 3; chlorine 1.
Criss-cross: $F e C l_{3}$ .

Answer: FeCl₃

Memory hook: Ferric: three.

Tricks to remember Symbols, Valency, Radicals and Writing Formulae

Short memory aids. Say each one aloud once, then use it on a question the same day.

Swap and drop
Swap the valencies, drop the 1, divide by common factor.
Use it when: Criss-cross.
Bracket for two
If you need two or more of a radical, put it in brackets.
Use it when: Radicals.
Ferrous 2, ferric 3
Ferrous is the smaller valency (2); ferric is the larger (3): "fewer" and "more".
Use it when: Iron valencies.

Symbols, Valency, Radicals and Writing Formulae: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Formula of magnesium oxide · 1 mark · MCQBoard level

The formula of magnesium oxide is:
(a) Mg₂O
(b) MgO
(c) MgO₂
(d) Mg₂O₂

Try it first, then show the solutionSolution

Mg (2), O (2): $M g_{2} O_{2} \to M g O$ .

Answer: (b) MgO

Example 2: Four formulae · 2 marks · Short answerBoard level

Write the formulae of (a) sodium chloride, (b) calcium chloride, (c) aluminium oxide, (d) calcium hydroxide.

Try it first, then show the solutionSolution

(a) NaCl; (b) CaCl₂; (c) Al₂O₃; (d) Ca(OH)₂. (½ each)

Answer: (a) NaCl (b) CaCl₂ (c) Al₂O₃ (d) Ca(OH)₂

Example 3: Phosphate and sulphate · 3 marks · Short answerHarder board level

Using the criss-cross method, write the formulae of (a) calcium phosphate and (b) aluminium sulphate. Show the steps.

Try it first, then show the solutionSolution

(a) Ca (2), PO₄ (3): $C a_{3} (P O_{4})_{2}$ . (1½)
(b) Al (3), SO₄ (2): $A l_{2} (S O_{4})_{3}$ . (1½)

Answer: Ca₃(PO₄)₂; Al₂(SO₄)₃

Example 4: Ferrous or ferric · 3 marks · Short answerHarder board level

Iron forms FeCl₂ and FeCl₃. (a) Find the valency of iron in each. (b) Name each compound. (c) Write the formula of iron oxide in which iron has valency 3.

Try it first, then show the solutionSolution

(a) 2 and 3. (1)
(b) Ferrous chloride and ferric chloride. (1)
(c) Fe (3), O (2): $F e_{2} O_{3}$ . (1)

Answer: (a) 2, 3 (b) ferrous, ferric chloride (c) Fe₂O₃

Example 5: Spot the errors · 4 marks · Long answerHOTS / competency

The following formulae are wrong: (i) Mg₂O₂ (as a final answer), (ii) CaCl, (iii) Al₃O₂, (iv) NaCO₃. Correct each and give the reason.

Try it first, then show the solutionSolution

(i) MgO: reduce the subscripts. (1)
(ii) CaCl₂: Ca has valency 2, Cl 1. (1)
(iii) Al₂O₃: criss-cross of 3 and 2 gives Al₂O₃ (the numbers were exchanged wrongly). (1)
(iv) Na₂CO₃: Na is 1, CO₃ is 2. (1)

Answer: (i) MgO (ii) CaCl₂ (iii) Al₂O₃ (iv) Na₂CO₃

Where students lose marks in Symbols, Valency, Radicals and Writing Formulae

The slips examiners see most often, and what to write instead.

Mistake: Not reducing the subscripts
The final formula must be in the simplest ratio.
Wrong working
Mg₂O₂
Right working
MgO
Do this: Always check for a common factor.
Mistake: Missing brackets
A radical needs brackets if repeated.
Wrong working
CaOH₂
Right working
Ca(OH)₂
Do this: Brackets for two or more radicals.
Mistake: Changing valency in a subscript
Subscripts come from the exchange.
Wrong working
Al₃O₂
Right working
Al₂O₃
Do this: Exchange: the valency of Al becomes O's subscript.

Exam tips

Write the valency above each symbol before criss-crossing.
Use brackets for radicals needing more than one.
Learn the table of valencies and radicals.
Verify the formula by balancing charges.

Practice questions on Symbols, Valency, Radicals and Writing Formulae

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
Give the symbol of potassium and state its origin.
Hint
Latin name.
Answer
K, from the Latin name kalium (the symbol Na comes from natrium for sodium).
Q2 · 3 marks · Short answerHarder board level
Write the formulae of (a) zinc nitrate, (b) aluminium hydroxide, (c) ammonium sulphate.
Hint
Brackets for radicals.
Answer
(a) Zn(NO₃)₂; (b) Al(OH)₃; (c) (NH₄)₂SO₄.
Q3 · 3 marks · Short answerHarder board level
Deduce the valency of the metal in each: (a) Al₂O₃, (b) CaCl₂, (c) FeCl₃. (O = 2, Cl = 1)
Hint
Reverse the criss-cross.
Answer
(a) Al: 3; (b) Ca: 2; (c) Fe: 3.
Q4 · 3 marks · Short answerHarder board level
Name the valency of each radical: SO₄, CO₃, NO₃, OH, NH₄, PO₄, and write the formula of sodium carbonate.
Hint
Table.
Answer
SO₄ 2, CO₃ 2, NO₃ 1, OH 1, NH₄ 1, PO₄ 3. Sodium carbonate: Na (1), CO₃ (2): Na₂CO₃.
Q5 · 4 marks · Long answerHOTS / competency
How many atoms of each element are shown by (a) Ca(OH)₂, (b) 3Al₂(SO₄)₃, (c) 2H₂O? Give the total atoms for (b).
Hint
Multiply brackets and coefficients.
Answer
(a) Ca 1, O 2, H 2. (b) Al $3 \times 2 = 6$ ; S $3 \times 3 = 9$ ; O $3 \times 12 = 36$ ; total $6 + 9 + 36 = 51$ . (c) H 4, O 2.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Symbol for iron?Show answer
Fe.
2.Valency of Al?Show answer
3.
3.Formula of calcium hydroxide?Show answer
Ca(OH)₂.
4.Valency of SO₄?Show answer
2.
5.What is a radical?Show answer
A group of atoms acting as one unit.

Symbols, Valency, Radicals and Writing Formulae: frequently asked questions

What is the criss-cross method?

A way to write formulae: write the valencies above the symbols, exchange them as subscripts, and reduce to the simplest ratio.

What is a radical?

A group of atoms that behaves as a single unit in reactions, such as SO₄, CO₃, NO₃, OH and NH₄.

Why do some symbols come from Latin names?

Because the elements were named in Latin long ago: Na from natrium, K from kalium, Fe from ferrum, Cu from cuprum, Ag from argentum and Pb from plumbum.