ChemistryClass 9ICSE

Iron Has Two Valencies and Both Chlorides Are Real Compounds

Learn the symbols of common elements and how valence electrons fix valency, classify elements as mono- to tetravalent and handle variable valency, build formulae by the criss-cross method, and sort radicals into acidic, basic and compound.

Why does iron have two different chlorides?

Sodium forms exactly one chloride:

NaCl

. Magnesium forms exactly one:

MgCl_{2}

.

Iron forms two —

FeCl_{2}

and

FeCl_{3}

— and both are perfectly genuine compounds with different colours and different reactions.

That happens because iron can lose either two electrons or three, giving

Fe^{2 +}

Fe^{3 +}

. An element that can do this is said to show variable valency, and iron, copper and lead are the standard examples.

So the two chlorides need two names. The lower valency takes the ending -ous and the higher takes -ic:

FeCl_{2} = ferrous chloride FeCl_{3} = ferric chloride

Writing iron chloride is therefore incomplete — it does not say which compound is meant.

That is the kind of precision this chapter is about. Chemistry has its own written language: symbols for the elements, valency to say how many bonds each forms, formulae built from valencies, and radicals as the words those formulae are made of.

This page covers the first part of the ICSE Class 9 Chemistry chapter on the language of chemistry — symbols and valency, variable valency, the criss-cross method, and radicals.

How do valence electrons decide an element's valency?

A metal's valency is the number of valence electrons it loses; a non-metal's valency is the number it must gain to complete its octet.

metal: valency = number of valence electrons

non-metal: valency = 8 - number of valence electrons

Why the two rules differ. Every atom is aiming for a full outer shell of eight. An atom with one, two or three valence electrons finds it easier to lose them; an atom with five, six or seven finds it easier to gain the few it lacks.

Worked determinations for metals.

- Sodium,

2, 8, 1

— loses

1

, so valency 1
- Magnesium,

2, 8, 2

— loses

2

, so valency 2
- Aluminium,

2, 8, 3

— loses

3

, so valency 3
- Calcium,

2, 8, 8, 2

— loses

2

, so valency 2

Worked determinations for non-metals.

- Chlorine,

2, 8, 7

— gains

1

, so

8 - 7 =

valency 1
- Oxygen,

2, 6

— gains

2

, so

8 - 6 =

valency 2
- Nitrogen,

2, 5

— gains

3

, so

8 - 5 =

valency 3
- Carbon,

2, 4

— shares

4

, so

8 - 4 =

valency 4

The symbols of the common elements. The first letter is always a capital and the second, if there is one, is always small — so cobalt is

Co

while carbon monoxide is

CO

, two entirely different things.

- Hydrogen H, helium He, lithium Li, beryllium Be, boron B, carbon C, nitrogen N, oxygen O, fluorine F, neon Ne
- Sodium Na, magnesium Mg, aluminium Al, silicon Si, phosphorus P, sulphur S, chlorine Cl, argon Ar, potassium K, calcium Ca
- Iron Fe, copper Cu, zinc Zn, silver Ag, gold Au, lead Pb, tin Sn, mercury Hg

Several symbols come from Latin names rather than English ones — sodium from natrium, potassium from kalium, iron from ferrum, copper from cuprum, silver from argentum, gold from aurum, lead from plumbum, tin from stannum, mercury from hydrargyrum. That is why the letters often look unrelated to the English word, and it is the reason Na cannot be guessed from "sodium".

Valency is a number with no sign. The charge on an ion carries a sign —

Na^{+}

Cl^{-}

O^{2 -}

— but the valency of sodium is simply

1

and of oxygen simply

2

. **So a valency is never written as

+ 1

- 2

**, and confusing the two makes the criss-cross method in the third section produce nonsense.

What does mono-, di-, tri- and tetravalent mean, and why is valency variable?

Elements are named after their valency, and a few elements have more than one.

- Monovalent — valency

1

H

Na

K

Ag

, and the non-metals

Cl

Br

I

F

- Divalent — valency

2

Mg

Ca

Zn

Ba

, and

O

S

- Trivalent — valency

3

Al

B

, and

N

P

- Tetravalent — valency

4

C

Si

- Zero valency — the noble gases

He

Ne

Ar

, which already have a complete outer shell and therefore do not normally combine at all

Variable valency. Some elements can lose different numbers of electrons and so show two valencies. The lower one takes -ous and the higher one takes -ic.

Iron — valency

2

3

:

-

Fe^{2 +}

, ferrous:

FeCl_{2}

ferrous chloride,

FeSO_{4}

ferrous sulphate,

FeO

ferrous oxide
-

Fe^{3 +}

, ferric:

FeCl_{3}

ferric chloride,

Fe_{2} (SO_{4})_{3}

ferric sulphate,

Fe_{2} O_{3}

ferric oxide

Copper — valency

1

2

:

-

Cu^{+}

, cuprous:

Cu_{2} O

cuprous oxide,

CuCl

cuprous chloride
-

Cu^{2 +}

, cupric:

CuO

cupric oxide,

CuSO_{4}

cupric sulphate

Lead — valency

2

4

:

-

Pb^{2 +}

, plumbous:

PbO

plumbous oxide,

PbCl_{2}

plumbous chloride
-

Pb^{4 +}

, plumbic:

PbO_{2}

plumbic oxide,

PbCl_{4}

plumbic chloride

Two more worth knowing. Mercury is mercurous at valency

1

and mercuric at valency

2

; tin is stannous at

2

and stannic at

4

.

The modern naming uses Roman numerals instead, which removes all ambiguity: iron(II) chloride for

FeCl_{2}

and iron(III) chloride for

FeCl_{3}

, copper(I) oxide and copper(II) oxide. The Roman numeral gives the valency directly, whereas -ous and -ic only say which of two is meant and require you to know both.

The endings are relative, not absolute. Cuprous is valency

1

and ferrous is valency

2

— so -ous does not mean "two"; it means "the lower of this element's two". Reading -ous as a fixed number is the standard error, and it is why

Cu_{2} O

catches people out: cuprous copper has valency

1

, so two coppers are needed for one divalent oxygen.

Zero valency is not the absence of valency. The noble gases have a complete outer shell and therefore no tendency to combine, which is a positive chemical fact about them rather than a gap in the data. That is why helium fills balloons safely and why argon is used inside a light bulb — a valency of zero is exactly what those jobs require.

Formula

How does the criss-cross method give you a formula?

Write the two radicals side by side with their valencies above, cross the valencies down as subscripts, then simplify.

- Step 1 — write the positive radical first and the negative one second
- Step 2 — write each valency above its radical
- Step 3 — cross them over, so each becomes the other's subscript
- Step 4 — simplify the subscripts to their lowest whole-number ratio
- Step 5 — put brackets round any compound radical that carries a subscript greater than one

Worked example 1 — sodium oxide.

Na

has valency

1

and

O

has valency

2

. Crossing gives

Na_{2} O_{1}

, and a subscript of

1

is never written:

Na_{2} O

Worked example 2 — calcium chloride.

Ca

2

and

Cl

1

CaCl_{2}

Worked example 3 — aluminium oxide.

Al

3

and

O

2

Al_{2} O_{3}

Worked example 4 — where simplification is needed.

Mg

2

and

O

2

. Crossing gives

Mg_{2} O_{2}

, and dividing both subscripts by

2

MgO

Skipping the simplification is the commonest error in the whole chapter.

Mg_{2} O_{2}

is not a formula anyone writes, and the same applies to calcium carbonate:

Ca

2

and

CO_{3}

2

, giving

Ca_{2} (CO_{3})_{2}

, which must be reduced to

CaCO_{3}

.

Worked example 5 — a compound radical needing brackets.

Al

3

and

SO_{4}

2

Al_{2} (SO_{4})_{3}

The brackets say that the whole sulphate group is taken three times — so the formula contains

3

sulphur atoms and

12

oxygen atoms. **Writing

Al_{2} SO_{43}

would be meaningless, which is why the brackets are compulsory.

Worked example 6 — calcium phosphate.**

Ca

2

and

PO_{4}

3

Ca_{3} (PO_{4})_{2}

Worked example 7 — ammonium sulphate.

NH_{4}

1

and

SO_{4}

2

(NH_{4})_{2} SO_{4}

Here the ammonium group takes the subscript, so it gets the brackets and the sulphate does not.

Worked example 8 — ferric sulphate.

Fe

3

in the ferric state and

SO_{4}

2

Fe_{2} (SO_{4})_{3}

Worked example 9 — zinc nitrate.

Zn

2

and

NO_{3}

1

Zn (NO_{3})_{2}

Naming a compound from its formula runs the method backwards. Given

Ca_{3} (PO_{4})_{2}

, the positive radical is calcium and the negative one is phosphate, so the name is calcium phosphate — and the subscripts are not mentioned in the name at all, because the valencies already fix them.

A bracket is needed only when the subscript exceeds one.

Na_{2} SO_{4}

needs no brackets round the sulphate, because the sulphate is taken once. So brackets are not decoration on compound radicals — they appear exactly when a group is multiplied, and their absence in

Na_{2} SO_{4}

is as correct as their presence in

Al_{2} (SO_{4})_{3}

What are acidic, basic and compound radicals?

A radical is an atom or a group of atoms carrying a charge that behaves as a single unit in a chemical reaction. It survives a reaction intact and moves from one compound to another as one piece.

Basic radicals are positively charged and come from metals, or from ammonia:

- Valency

1

H^{+}

Na^{+}

K^{+}

Ag^{+}

NH_{4}^{+}

(ammonium),

Cu^{+}

- Valency

2

Mg^{2 +}

Ca^{2 +}

Zn^{2 +}

Ba^{2 +}

Fe^{2 +}

Cu^{2 +}

Pb^{2 +}

- Valency

3

Al^{3 +}

Fe^{3 +}

Acidic radicals are negatively charged and come from acids:

- Valency

1

Cl^{-}

Br^{-}

I^{-}

OH^{-}

(hydroxide),

NO_{3}^{-}

(nitrate),

HCO_{3}^{-}

(bicarbonate),

NO_{2}^{-}

(nitrite)
- Valency

2

O^{2 -}

S^{2 -}

SO_{4}^{2 -}

(sulphate),

CO_{3}^{2 -}

(carbonate),

SO_{3}^{2 -}

(sulphite)
- Valency

3

PO_{4}^{3 -}

(phosphate),

N^{3 -}

(nitride)

Compound radicals are those made of more than one kind of atom, in contrast with simple radicals, which are single atoms.

- Simple:

Na^{+}

Cl^{-}

O^{2 -}

Al^{3 +}

- Compound:

NH_{4}^{+}

OH^{-}

NO_{3}^{-}

SO_{4}^{2 -}

CO_{3}^{2 -}

PO_{4}^{3 -}

Ammonium is the one positive compound radical worth remembering, since all the others in that list are negative — and it behaves chemically like a metal ion even though it contains no metal at all.

Radicals in the atmosphere and their effects. Some acidic radicals are formed in the air itself.

- Burning coal and petroleum releases oxides of sulphur, and lightning and vehicle engines produce oxides of nitrogen. These dissolve in the water vapour of the air to give sulphate and nitrate radicals in rainwater
- The result is acid rain, which damages the leaves of plants, leaches nutrients from the soil, acidifies lakes so that fish cannot survive, corrodes stone buildings and metal structures, and irritates the eyes and lungs of people and animals
- Chlorine-containing compounds released from older refrigerants break down high in the atmosphere to give reactive chlorine species that attack the ozone layer, letting more ultraviolet radiation reach the ground

Nitrate radicals also do good. The nitrogen oxides made by lightning wash down as nitrates and act as a natural fertiliser, which plants use to build protein — the nitrogen cycle depends on it. The same radical is a nutrient in the soil and a pollutant in a lake, and which it is depends entirely on the quantity and where it lands.

A "radical" here is not a "free radical". A radical in this chapter is a charged group that keeps its identity through a reaction. A free radical, met when a covalent bond breaks evenly, is an uncharged species with an unpaired electron and is fiercely reactive. The two words are close and the species are quite different — and the later atomic-structure chapter makes that distinction precisely.

Exam tip

Exam tip: simplify the subscripts and bracket a multiplied group

Always simplify after crossing.

Mg_{2} O_{2}

must become

MgO

, and

Ca_{2} (CO_{3})_{2}

must become

CaCO_{3}

.

Bracket a compound radical only when its subscript exceeds one:

Al_{2} (SO_{4})_{3}

needs them,

Na_{2} SO_{4}

does not.

**A subscript of

1

is never written** —

Na_{2} O_{1}

Na_{2} O

.

Write the positive radical first, then the negative one.

Valency has no sign. Sodium's valency is

1

, not

+ 1

; the sign belongs to the charge on the ion.

**Metal valency

=

valence electrons; non-metal valency

= 8 -

valence electrons. Show the electron arrangement when asked to derive one.

For variable valency, -ous is the LOWER and -ic the HIGHER** — and the value differs by element. Cuprous is

1

, ferrous is

2

, plumbous is

2

.

Use Roman numerals if the question allows — iron(III) chloride removes all doubt.

Capital first letter, small second:

Co

is cobalt and

CO

is carbon monoxide.

Noble gases have valency ZERO, which is a property and not a missing value.

And keep a radical (a charged group) apart from a free radical (uncharged, with an unpaired electron) — the exam sets both words.

Did you know

Why ammonium behaves like a metal without containing one

Every basic radical in the list is a metal ion, with one exception. The ammonium radical,

NH_{4}^{+}

, contains only nitrogen and hydrogen — not a trace of metal — and yet it takes the metal's place in formula after formula.

Compare the sodium and ammonium compounds side by side:

-

NaCl

and

NH_{4} Cl

Na_{2} SO_{4}

and

(NH_{4})_{2} SO_{4}

NaNO_{3}

and

NH_{4} NO_{3}

Na_{2} CO_{3}

and

(NH_{4})_{2} CO_{3}

The pattern is exact. Ammonium is monovalent and positive, just like sodium, so the criss-cross method treats the two identically and produces matching formulae.

The reason is that valency and charge are all the method needs. The criss-cross rule never asks what a radical is made of — it asks only how much charge it carries and what sign. A group of five atoms carrying one positive charge is, for formula-writing purposes, indistinguishable from a single sodium ion.

And the resemblance goes beyond the paper. Ammonium salts really do look and behave like the corresponding sodium and potassium salts — white crystalline solids, soluble in water, and in the case of ammonium nitrate and ammonium sulphate, valuable fertilisers because they supply nitrogen in a form plant roots can take up.

There is one clear difference. Heat ammonium chloride and it does not simply melt like sodium chloride; it breaks up into ammonia and hydrogen chloride gas, which recombine on the cooler part of the tube. A metal ion has nothing inside it to break apart, and a compound radical does — which is the one place where the resemblance stops.

So ammonium is a genuinely useful oddity. It shows that the language of chemistry is built on behaviour rather than on composition, and that a formula records how much charge goes where and nothing more.

Exam relevance

How does valency feed into JEE Main and NEET?

Because formula-writing is assumed silently in every later chemistry chapter, and variable valency becomes an entire block of the periodic table.

This is the foundation for Class 11 Chemistry Classification of Elements and Periodicity, Chemical Bonding and Some Basic Concepts of Chemistry, all examined in JEE Main and NEET. Valency is replaced there by oxidation number, which does carry a sign and can be worked out for any atom in any compound — and the rule that valency comes from the valence electrons becomes a consequence of the electronic configuration and the position in the periodic table.

Variable valency becomes the defining feature of the transition elements. Class 12 The d- and f-Block Elements explains that iron, copper, manganese and chromium show several oxidation states because the

3 d

and

4 s

electrons are close in energy, so different numbers can be removed. The ferrous-and-ferric pair on this page is the accessible case of that whole chapter, and questions on which oxidation states an element shows are recurring.

Formula-writing is the silent requirement of every numerical. A JEE Main or NEET mole-concept, stoichiometry or titration question begins with a balanced equation, and the equation cannot be written without the formulae. So a wrong formula makes the whole question wrong before any chemistry starts, and that is why the criss-cross method must be automatic rather than remembered.

The radicals become ions in solution. Class 11 Equilibrium and Class 12 Electrochemistry treat sulphate, nitrate, carbonate and ammonium as the species actually present in a solution, and qualitative analysis in the practical syllabus is built on identifying exactly the basic and acidic radicals listed here. Match-the-column questions pairing a radical with its test are standard.

The atmospheric material reappears in two places. Class 11 Environmental Chemistry covers acid rain and its effects, and ozone depletion by chlorine species, with the same causes and consequences named here. For NEET Biology, Environmental Issues asks about acid rain and the ozone hole directly, and the nitrogen cycle chapter uses the nitrate radical as the plant-available form of nitrogen.

The free-radical distinction matters in Class 11 Hydrocarbons, where the chlorination of methane proceeds through free radicals with unpaired electrons — a different species entirely from the charged radicals of this page, and a classic point of confusion.

What the questions look like. For board work, expect write symbols and derive valencies from electron arrangements, classify elements by valency, name both compounds of a variable-valency element, write formulae by criss-cross with simplification and brackets, name a compound from its formula, and classify radicals as acidic, basic, simple or compound. For JEE Main and NEET, expect oxidation numbers, transition-metal oxidation states, stoichiometry resting on correct formulae, and environmental-chemistry recall.

How board and competitive emphasis differ. A board paper rewards the shown working — the valencies written above the symbols and the simplification step visible. A competitive paper never asks for a formula on its own; it asks a question whose answer silently depends on getting a dozen formulae right.

The single trap that costs the most marks. Reading -ous as a fixed valency. Cuprous copper is valency

1

and ferrous iron is valency

2

, so the ending means only the lower of this element's two. The defence is to write both of the element's valencies down side by side before choosing — once

Cu

1

2

is on the paper, cuprous oxide comes out as

Cu_{2} O

rather than

CuO

Key takeaways

Symbols, valency, formulae and radicals: quick revision

- Symbols: capital first letter, small second —

Co

is cobalt,

CO

is carbon monoxide.
- Latin-derived symbols: Na natrium, K kalium, Fe ferrum, Cu cuprum, Ag argentum, Au aurum, Pb plumbum, Sn stannum, Hg hydrargyrum.
- **Metal valency

=

number of valence electrons**: Na

(2, 8, 1)

1

; Mg

(2, 8, 2)

2

; Al

(2, 8, 3)

3

.
- **Non-metal valency

= 8 -

valence electrons**: Cl

(2, 8, 7)

1

; O

(2, 6)

2

; N

(2, 5)

3

; C

(2, 4)

4

.
- Valency carries no sign — the sign belongs to the ion's charge.
- Monovalent: H, Na, K, Ag, Cl, Br, I, F. Divalent: Mg, Ca, Zn, Ba, O, S. Trivalent: Al, B, N, P. Tetravalent: C, Si.
- Noble gases have valency ZERO — a complete outer shell, which is a property and not a gap.
- Variable valency: -ous for the lower, -ic for the higher.
- Iron: ferrous

FeCl_{2}

FeSO_{4}

FeO

(

2

); ferric

FeCl_{3}

Fe_{2} (SO_{4})_{3}

Fe_{2} O_{3}

(

3

).
- Copper: cuprous

Cu_{2} O

CuCl

(

1

); cupric

CuO

CuSO_{4}

(

2

).
- Lead: plumbous

PbO

PbCl_{2}

(

2

); plumbic

PbO_{2}

PbCl_{4}

(

4

). Also mercurous/mercuric and stannous/stannic.
- -ous is not a fixed number — cuprous is

1

and ferrous is

2

. Roman numerals, as in iron(III) chloride, remove the ambiguity.
- Criss-cross method: positive radical first, valencies above, cross them down as subscripts, simplify, and bracket any multiplied compound radical.
-

Na_{2} O

CaCl_{2}

Al_{2} O_{3}

Ca_{3} (PO_{4})_{2}

(NH_{4})_{2} SO_{4}

Fe_{2} (SO_{4})_{3}

Zn (NO_{3})_{2}

.
- Simplify:

Mg_{2} O_{2} \to MgO

and

Ca_{2} (CO_{3})_{2} \to CaCO_{3}

. **A subscript of

1

is never written.
- Brackets appear only when the subscript exceeds one** —

Na_{2} SO_{4}

correctly has none.
- A radical is a charged atom or group behaving as one unit in a reaction.
- Basic (positive):

H^{+}

Na^{+}

K^{+}

NH_{4}^{+}

Mg^{2 +}

Ca^{2 +}

Zn^{2 +}

Fe^{2 +}

Fe^{3 +}

Al^{3 +}

.
- Acidic (negative):

Cl^{-}

OH^{-}

NO_{3}^{-}

HCO_{3}^{-}

O^{2 -}

SO_{4}^{2 -}

CO_{3}^{2 -}

PO_{4}^{3 -}

.
- Compound radicals contain more than one kind of atom:

NH_{4}^{+}

OH^{-}

NO_{3}^{-}

SO_{4}^{2 -}

CO_{3}^{2 -}

PO_{4}^{3 -}

. Ammonium is the one positive one.
- In the atmosphere: sulphur and nitrogen oxides from fuels and lightning give sulphate and nitrate radicals in rainwater, causing acid rain — damaged leaves, leached soil, acidified lakes, corroded stone, irritated lungs.
- Chlorine species from older refrigerants attack the ozone layer.
- Nitrates are also a natural fertiliser — the same radical is a nutrient in soil and a pollutant in a lake.
- A radical is not a free radical — the first is charged, the second uncharged with an unpaired electron.

Pick five household chemicals, look up the radicals in each, and write out their formulae by criss-cross — then check whether any needed simplifying.

Why does iron have two different chlorides?

How do valence electrons decide an element's valency?

What does mono-, di-, tri- and tetravalent mean, and why is valency variable?

How does the criss-cross method give you a formula?

What are acidic, basic and compound radicals?

Exam tip: simplify the subscripts and bracket a multiplied group

Why ammonium behaves like a metal without containing one

How does valency feed into JEE Main and NEET?

Symbols, valency, formulae and radicals: quick revision

Related study notes

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