ICSE Class 9 Chemistry • Chapter 1

Language of Chemistry: Radicals & the Formula of a Compound

What Are Radicals?

In chemistry, the word radical stands for “an atom or group of atoms behaving as a unit in a number of compounds.” Radicals are reactive species that combine with other radicals to form compounds. There are three types of radicals:
    • Acidic radical
    • Basic radical
    • Free radical

1. Acidic Radical

The part of a salt that comes from the acid is known as the acidic radical. It is always negatively charged, so it is also called an anion. It forms ionic compounds when joined with a cation. If the cation is the hydronium ion (or hydrogen ion), the two combine to form an acid.

Names and Symbols of Some Anions

1− Valency
AnionFormula
AcetateCH₃COO⁻
AluminateAlO₂⁻
BromideBr⁻
ChlorateClO₃⁻
ChlorideCl⁻
ChloriteClO₂⁻
CyanideCN⁻
Dihydrogen phosphateH₂PO₄⁻
FluorideF⁻
HydrideH⁻
Hydrogen carbonateHCO₃⁻
Hydrogen sulphateHSO₄⁻
Hydrogen sulphiteHSO₃⁻
Hydrogen sulphideHS⁻
HydroxideOH⁻
HypochloriteClO⁻
IodideI⁻
IodateIO₃⁻
NitrateNO₃⁻
NitriteNO₂⁻
PerchlorateClO₄⁻
PermanganateMnO₄⁻
ThiocyanateSCN⁻
2− Valency
AnionFormula
CarbonateCO₃²⁻
ChromateCrO₄²⁻
DichromateCr₂O₄²⁻
OxalateC₂O₄²⁻
OxideO²⁻
PeroxideO₂²⁻
SulphideS²⁻
SulphateSO₄²⁻
SulphiteSO₃²⁻
ZincateZnO₂²⁻
PlumbitePbO₂²⁻
ManganateMnO₄²⁻
ThiosulphateS₂O₃²⁻
3− Valency
AnionFormula
ArsenateAsO₄³⁻
ArseniteAsO₃³⁻
BorateBO₃³⁻
Ferricyanide[Fe(CN)₆]³⁻
PhosphatePO₄³⁻
PhosphideP³⁻
PhosphitePO₃³⁻
NitrideN³⁻
4− Valency
AnionFormula
CarbideC⁴⁻
Ferrocyanide[Fe(CN)₆]⁴⁻

2. Basic Radical

The part of a salt which comes from the base is known as the basic radical. It is positively charged and is also known as a cation. When a cation combines with an anion, they form an ionic compound; if the anion is the hydroxide ion (OH⁻), the compound formed is a base.

Names and Symbols of Some Cations

1+ Valency
CationFormula
AmmoniumNH₄⁺
CaesiumCs⁺
Copper (I)Cu⁺
HydroniumH₃O⁺
LithiumLi⁺
PotassiumK⁺
SilverAg⁺
SodiumNa⁺
Thallium (I)Tl⁺
HydrogenH⁺
Aurous or Gold (I)Au⁺
Mercurous or Mercury (I)Hg⁺
2+ Valency
CationFormula
BariumBa²⁺
CadmiumCd²⁺
CalciumCa²⁺
Chromium (II)Cr²⁺
Cobalt (II)Co²⁺
Copper (II)Cu²⁺
Iron (II)Fe²⁺
Lead (II)Pb²⁺
MagnesiumMg²⁺
Manganese (II)Mn²⁺
Mercury (II)Hg²⁺
StrontiumSr²⁺
NickelNi²⁺
Tin (II)Sn²⁺
ZincZn²⁺
PlatinumPt²⁺
3+ Valency
CationFormula
AluminiumAl³⁺
Chromium (III)Cr³⁺
Cobalt (III)Co³⁺
Iron (III)Fe³⁺
LanthanumLn³⁺
Thallium (III)Tl³⁺
Titanium (III)Ti³⁺
Vanadium (III)V³⁺
BismuthBi³⁺
ArsenicAs³⁺
AntimonySb³⁺
Auric or Gold (III)Au³⁺
4+ Valency
CationFormula
Manganese (IV)Mn⁴⁺
Plumbic or Lead (IV)Pb⁴⁺
Stannic or Tin (IV)Sn⁴⁺
Platinic or Platinum (IV)Pt⁴⁺

3. Free Radical

The part of a covalent compound that forms due to the fission of a bond in a molecular reaction is called a free radical. Free radicals are neutral in nature and are reactive because of the presence of an unpaired electron. Two free radicals join with each other to form a covalent compound.

Some Common Free Radicals

Name of Free RadicalFormula
Hydrogen°H
Methyl°CH₃
Chlorine°Cl
Bromine°Br
Ethyl°C₂H₅

The Formula of a Compound

A formula is a short representation of a compound using the symbols of its constituent elements. For a covalent compound it is called a molecular formula; for an ionic compound it is called a formula unit.

Steps to Write a Formula

    1. Identify the symbol of the cation (first part of the name) and the anion.
    1. Identify the valency (charge) of each and place it in parenthesis just above the symbol.
    1. Balance the total positive and negative charge on the cation and anion — the two must add up to zero.
    1. The numbers needed to balance the charges become the subscripts, placed right after the respective symbol.

Worked Examples

Example 1: Copper (I) Oxide

    1. Symbols: Copper – Cu, Oxide – O
    1. Charges: Copper (I) = 1+, Oxide = 2−
    1. Balance: two Cu⁺ ions balance one O²⁻ ion, since 2(1+) + 1(2−) = 0
    1. Formula: Cu₂O
Note: a subscript of 1 is never written — it is always understood.

Example 2: Calcium Chloride

    1. Symbols: Calcium – Ca, Chloride – Cl
    1. Charges: Calcium = 2+, Chloride = 1−
    1. Balance: one Ca²⁺ ion needs two Cl⁻ ions, since 1(2+) + 2(1−) = 0
    1. Formula: CaCl₂

Example 3: Ethane (Covalent Compound)

    1. The methyl free radical CH₃ contains one unpaired electron.
    1. A second methyl free radical also has one unpaired electron.
    1. The two unpaired electrons pair up to form a covalent bond: CH₃–CH₃

Example 4: Methyl Chloride

    1. The methyl free radical CH₃ has one unpaired electron.
    1. The chloride free radical Cl also has one unpaired electron.
    1. They pair up to form a covalent bond: CH₃–Cl

What Does a Formula Tell Us?

    • The different elements present in the compound.
    • The number of atoms of each element in one molecule of the compound.
    • It represents one molecule of that compound.
    • It represents a definite mass of the compound, equal to its molecular weight.
    • The mass of each element present in one molecule of the compound.

Worked Example: Potassium Permanganate (KMnO₄)

    • Elements present: Potassium, Manganese and Oxygen.
    • Atoms per molecule: 1 atom of Potassium, 1 atom of Manganese, 4 atoms of Oxygen.
    • The formula represents one molecule of Potassium Permanganate.
    • Molecular mass = 39 + 55 + (4 × 16) = 158 g
    • One molecule of KMnO₄ contains 39 g of Potassium, 55 g of Manganese and 64 g of Oxygen.

Names and Formulas of Some Common Compounds

CationAnionFormulaName
NH₄⁺ (Ammonium ion)Cl⁻ (Chloride ion)NH₄ClAmmonium Chloride
Cu²⁺ (Cupric ion)Cl⁻ (Chloride ion)CuCl₂Copper (II) Chloride or Cupric Chloride
K⁺ (Potassium ion)NO₃⁻ (Nitrate ion)KNO₃Potassium Nitrate
Na⁺ (Sodium ion)SO₄²⁻ (Sulphate ion)Na₂SO₄Sodium Sulphate
Fe²⁺ (Ferrous ion)Cl⁻ (Chloride ion)FeCl₂Iron (II) Chloride or Ferrous Chloride
Fe³⁺ (Ferric ion)SO₃²⁻ (Sulphite ion)Fe₂(SO₃)₃Iron (III) Sulphite or Ferric Sulphite
H⁺ (Hydrogen ion)S²⁻ (Sulphide ion)H₂SHydrogen Sulphide
Cu⁺ (Cuprous ion)O²⁻ (Oxide ion)Cu₂OCuprous Oxide or Copper (I) Oxide
Al³⁺ (Aluminium ion)Cl⁻ (Chloride ion)AlCl₃Aluminium Chloride
NH₄⁺ (Ammonium ion)OH⁻ (Hydroxide ion)NH₄OHAmmonium Hydroxide
Ag⁺ (Silver ion)I⁻ (Iodide ion)AgISilver Iodide
H⁺ (Hydrogen ion)F⁻ (Fluoride ion)HFHydrogen Fluoride
Na⁺ (Sodium ion)CO₃²⁻ (Carbonate ion)Na₂CO₃Sodium Carbonate
Mn²⁺ (Manganese ion)SO₄²⁻ (Sulphate ion)MnSO₄Manganese Sulphate
K⁺ (Potassium ion)HCO₃⁻ (Hydrogen Carbonate ion)KHCO₃Potassium Hydrogen Carbonate or Potassium Bicarbonate
C (Covalency 4)Cl⁻ (Chloride ion)CCl₄Carbon Tetrachloride
Na⁺ (Sodium ion)ZnO₂²⁻ (Zincate ion)Pb₂ZnO₂Lead Zincate
Na⁺ (Sodium ion)S₂O₃²⁻ (Thiosulphate ion)Na₂S₂O₃Sodium Thiosulphate
K⁺ (Potassium ion)[Fe(CN)₆]⁴⁻ (Ferrocyanide ion)K₄[Fe(CN)₆]Potassium Ferrocyanide
Na⁺ (Sodium ion)CN⁻ (Cyanide ion)NaCNSodium Cyanide
H⁺ (Hydrogen ion)O₂²⁻ (Peroxide ion)H₂O₂Hydrogen Peroxide
Mg⁺ (Magnesium ion)SO₄²⁻ (Sulphate ion)MgSO₄Magnesium Sulphate
Ca²⁺ (Calcium ion)C₂O₄²⁻ (Oxalate ion)CaC₂O₄Calcium Oxalate
Al³⁺ (Aluminium ion)N³⁻ (Nitride ion)AlNAluminium Nitride