Reference · ICSE / CBSE / SSLC · Classes 7 to 10
Valency, and the radicals worth learning first.
Every element and radical that appears in a formula your board actually sets, with its valency — and the rule that turns two valencies into a formula. Nothing here is a trick; it is the alphabet the rest of chemistry is written in, and it is the first thing The Language of Chemistry, Playbook® teaches.
These valencies are checked rather than asserted. Applied through the criss-cross rule they rebuild 86 of the 94 chemical formulas the ICSE, CBSE and Kerala SSLC syllabuses set — every run of this site’s build tests it. The eight they do not rebuild are named at the bottom of the page, with the reason.
The elements, and what each combines as
23 elements — every one that appears in a formula the three boards set. A metal with two valencies gets both, with the classical name for each state, because those names are what the boards print and do not explain.
| Symbol | Element | Valency | Called | Note |
|---|---|---|---|---|
| H | Hydrogen | 1 | Monovalent | — |
| C | Carbon | 4 | Tetravalent | — |
| N | Nitrogen | 3 | Trivalent | Trivalent in ammonia. In the boards’ formulas nitrogen otherwise appears inside a radical — nitrate or nitrite — rather than on its own. |
| O | Oxygen | 2 | Divalent | — |
| F | Fluorine | 1 | Monovalent | — |
| Na | Sodium | 1 | Monovalent | — |
| Mg | Magnesium | 2 | Divalent | — |
| Al | Aluminium | 3 | Trivalent | — |
| P | Phosphorus | 3 · 5 | Trivalent | Appears in the boards’ formulas only inside the phosphate and phosphite radicals. |
| S | Sulphur | 2 | Divalent | Divalent as the sulphide. Sulphur shows 4 in SO2 and 6 in SO3; those are covalent oxides and are not built by the criss-cross rule. |
| Cl | Chlorine | 1 | Monovalent | — |
| K | Potassium | 1 | Monovalent | — |
| Ca | Calcium | 2 | Divalent | — |
| Mn | Manganese | 2 · 4 | Divalent | Divalent in MnCl2, tetravalent in MnO2. In KMnO4 it is inside the permanganate radical. |
| Fe | Iron | 2 · 3 | Ferrous · Ferric | — |
| Cu | Copper | 1 · 2 | Cuprous · Cupric | — |
| Zn | Zinc | 2 | Divalent | — |
| Br | Bromine | 1 | Monovalent | — |
| Ag | Silver | 1 | Monovalent | — |
| I | Iodine | 1 | Monovalent | — |
| Ba | Barium | 2 | Divalent | — |
| Hg | Mercury | 1 · 2 | Mercurous · Mercuric | — |
| Pb | Lead | 2 · 4 | Plumbous · Plumbic | — |
The radicals, simple and compound
They are split because the author’s note on the CBSE Class 9 chapter says they should be: “If the Radicals were classified into Simple and Compound Radicals, it would help the Students to write the Chemical Formula correctly.” A simple radical is one element carrying a charge. A compound radical is a group of elements that combines as a single unit and keeps its own valency — which is why it takes a bracket when more than one of it appears.
| Radical | Formula | Valency | Called |
|---|---|---|---|
| Hydride | H | 1 | Monovalent |
| Fluoride | F | 1 | Monovalent |
| Chloride | Cl | 1 | Monovalent |
| Bromide | Br | 1 | Monovalent |
| Iodide | I | 1 | Monovalent |
| Oxide | O | 2 | Divalent |
| Sulphide | S | 2 | Divalent |
| Radical | Formula | Valency | Called |
|---|---|---|---|
| Ammonium | NH4 | 1 | Monovalent |
| Hydroxide | OH | 1 | Monovalent |
| Nitrate | NO3 | 1 | Monovalent |
| Nitrite | NO2 | 1 | Monovalent |
| Bicarbonate | HCO3 | 1 | Monovalent |
| Chlorate | ClO3 | 1 | Monovalent |
| Permanganate | MnO4 | 1 | Monovalent |
| Acetate | CH3COO | 1 | Monovalent |
| Aluminate | AlO2 | 1 | Monovalent |
| Carbonate | CO3 | 2 | Divalent |
| Sulphate | SO4 | 2 | Divalent |
| Sulphite | SO3 | 2 | Divalent |
| Zincate | ZnO2 | 2 | Divalent |
| Phosphate | PO4 | 3 | Trivalent |
| Phosphite | PO3 | 3 | Trivalent |
How two valencies become a formula
Write the positive part first and the negative part second. Give each the other’s valency as its subscript, then reduce to the lowest terms. That is the whole rule, and it is Step II of the method.
-
Sodium 1 · Chloride 1
NaCl
-
Calcium 2 · Hydroxide 1
Ca(OH)2
-
Aluminium 3 · Sulphate 2
Al2(SO4)3
-
Iron (Ferric) 3 · Oxide 2
Fe2O3
Magnesium and oxide are both divalent, so magnesium oxide reduces to MgO and never Mg2O2. That reduction is the step most often skipped.
Where the rule stops
Eight of the formulas the three boards set are not built this way, and knowing which is part of knowing the rule. CO, NO2, SO2 and SO3 are covalent oxides, in which sulphur is not the divalent sulphide. Fe3O4 carries iron in two states at once. CH3COOH and CH3OH are written as organic formulas. And CaOCl2, bleaching powder, is a mixed salt rather than one positive part and one negative one.
The metals with two valencies
These are where marks are lost, and where a textbook question is sometimes unanswerable as printed. “Copper Sulphate” names no valency, so no formula can be worked out from it; Copper(II) Sulphate, or cupric sulphate, can be. The author’s notes on all three boards return to this.
| Element | Lower | Called | Higher | Called |
|---|---|---|---|---|
| Iron Fe | 2 | Ferrous | 3 | Ferric |
| Copper Cu | 1 | Cuprous | 2 | Cupric |
| Mercury Hg | 1 | Mercurous | 2 | Mercuric |
| Lead Pb | 2 | Plumbous | 4 | Plumbic |
Questions
- What is valency?
- The combining capacity of an element, measured by the number of atoms of hydrogen one atom of it can combine with. It is the number you need before you can write a chemical formula, and it is what the criss-cross rule uses.
- What is the difference between a simple radical and a compound radical?
- A simple radical is one element carrying a charge — chloride, oxide, sulphide. A compound radical is a group of elements that combines as a single unit and keeps its own valency — sulphate, nitrate, ammonium, hydroxide. Splitting them apart is the fastest way to stop guessing at formulas, and it is what the boards’ own textbooks do not do.
- Why do iron and copper have two valencies?
- Some metals combine in more than one ratio. Iron is divalent as ferrous and trivalent as ferric; copper is monovalent as cuprous and divalent as cupric. This is why “Iron Oxide” and “Copper Sulphate” are not answerable questions — without the state, the formula cannot be worked out. Fe2O3 is Iron(III) Oxide, or ferric oxide, and CuSO4 is Copper(II) Sulphate, or cupric sulphate.
- What is the valency of a permanganate radical?
- Permanganate, MnO4, is monovalent. It is a compound radical, so it takes a bracket when more than one of it appears in a formula.
- How do you write a chemical formula from valencies?
- Write the positive part first and the negative part second, then give each 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.
This page is Step I. There are two more.
A table is something to look up. The Playbook is where a student stops needing to: thirty elements and twenty-five radicals come as punch-out tokens, and a chapter does not open onto the next one until the score is made. Alphabets, then words, then sentences.