A formula is a word. These are the words the boards set.

• Aluminium Sulphate

  Aluminium Al · valency 3

  Sulphate SO₄ · valency 2

  Al₂(SO₄)₃

• Calcium Hydroxide

  Calcium Ca · valency 2

  Hydroxide OH · valency 1

  Ca(OH)₂

• Sodium Chloride

  Sodium Na · valency 1

  Chloride Cl · valency 1

  NaCl

Aluminium sulphate is the one that shows the bracket doing its work: the sulphate radical is a group, three of them are needed, and the subscript sits outside the bracket so it multiplies the whole group rather than the oxygen alone.

The 8 the rule does not build

Knowing which formulas the rule does not reach is part of knowing the rule, and an aggregated formula list never tells you. These are found by running the rule rather than from a list kept beside it: each one below is a formula a board sets that no pair of parts produces.

• CO , NO₂ , SO₂ and SO₃ are covalent oxides, in which sulphur is not the divalent sulphide.

• Fe₃O₄ carries iron in two states at once, ferrous and ferric together.

• CH₃COOH and CH₃OH are written as organic formulas, by the carbon chain rather than by two parts.

• CaOCl₂ is bleaching powder, a mixed salt rather than one positive part and one negative one.

None of them is an error. They are the places where a formula records something the two-valency rule does not model — a covalent bond, a mixed oxidation state, a carbon chain — and a student who has been told the rule is universal meets them as a contradiction rather than as a boundary.

Which board sets what

The Set by column above is the short answer. The long one is on each board's own page, chapter by chapter, with the equations as well as the formulas:

• ICSE — Classes 6 to 9. ICSE introduces chemistry in Class 6 and the language of chemistry with it — the names and symbols of the first twenty elements. Word equations follow in Class 7 and formulas in Class 8, but valency, the formulas of radicals and the balancing of equations are not taught until Class 9.
• CBSE — Classes 7 to 10. A CBSE student meets the chemical formulas and equations listed below between Class 7 and Class 10. Writing a chemical formula is not explained until Class 9, and writing and balancing a chemical equation not until Class 10 — by which point both have been set in exercises for two years.
• SSLC — Classes 8 to 10. A Kerala SSLC student meets 52 chemical formulas and 51 chemical equations between Class 8 and Class 10. Balancing is explained first, in Class 8, before a chemical formula or a chemical equation has been explained at all; writing a formula follows in Class 9.

Questions

How do you write a chemical formula?

Write the positive part first and the negative part second, give each one the other’s valency as its subscript, and reduce to the lowest terms. Calcium is divalent and hydroxide is monovalent, so calcium hydroxide is Ca(OH)₂. Magnesium and oxide are both divalent, so magnesium oxide is MgO and never Mg₂O₂.

What is the criss-cross rule?

The name for that swap: each part takes the other’s valency as its subscript. It is a rule about arithmetic, not about chemistry, and it only works once both valencies are known — which is why the valency chart comes first.

When does a formula need brackets?

When a compound radical — one that is a group of elements rather than a single one — appears more than once. Hydroxide is OH, so two of them is Ca(OH)₂ and never CaOH₂, which would say something different about the oxygen.

Why does the same metal give two different formulas?

Because some metals combine in more than one ratio, and the name has to say which. Iron is divalent as ferrous and trivalent as ferric, so “Iron Oxide” is not an answerable question: FeO is Iron(II) Oxide and Fe₂O₃ is Iron(III) Oxide.

Do all chemical formulas follow the criss-cross rule?

No. Of the 94 formulas the ICSE, CBSE and Kerala SSLC syllabuses set, 86 are built by it and 8 are not — the covalent oxides, one iron oxide carrying two states at once, two organic formulas and one mixed salt. All eight are named on this page with the reason.

This page is Step II. One comes before it, one after.

A table is something to look up. Chapter II of The Language of Chemistry, Playbook® is where a student stops needing to: the parts come as punch-out tokens, the formula is built with the hands before the pen, and the chapter does not open onto equations until the score is made.