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
Anion
Formula
Acetate
CH₃COO⁻
Aluminate
AlO₂⁻
Bromide
Br⁻
Chlorate
ClO₃⁻
Chloride
Cl⁻
Chlorite
ClO₂⁻
Cyanide
CN⁻
Dihydrogen phosphate
H₂PO₄⁻
Fluoride
F⁻
Hydride
H⁻
Hydrogen carbonate
HCO₃⁻
Hydrogen sulphate
HSO₄⁻
Hydrogen sulphite
HSO₃⁻
Hydrogen sulphide
HS⁻
Hydroxide
OH⁻
Hypochlorite
ClO⁻
Iodide
I⁻
Iodate
IO₃⁻
Nitrate
NO₃⁻
Nitrite
NO₂⁻
Perchlorate
ClO₄⁻
Permanganate
MnO₄⁻
Thiocyanate
SCN⁻
2− Valency
Anion
Formula
Carbonate
CO₃²⁻
Chromate
CrO₄²⁻
Dichromate
Cr₂O₄²⁻
Oxalate
C₂O₄²⁻
Oxide
O²⁻
Peroxide
O₂²⁻
Sulphide
S²⁻
Sulphate
SO₄²⁻
Sulphite
SO₃²⁻
Zincate
ZnO₂²⁻
Plumbite
PbO₂²⁻
Manganate
MnO₄²⁻
Thiosulphate
S₂O₃²⁻
3− Valency
Anion
Formula
Arsenate
AsO₄³⁻
Arsenite
AsO₃³⁻
Borate
BO₃³⁻
Ferricyanide
[Fe(CN)₆]³⁻
Phosphate
PO₄³⁻
Phosphide
P³⁻
Phosphite
PO₃³⁻
Nitride
N³⁻
4− Valency
Anion
Formula
Carbide
C⁴⁻
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
Cation
Formula
Ammonium
NH₄⁺
Caesium
Cs⁺
Copper (I)
Cu⁺
Hydronium
H₃O⁺
Lithium
Li⁺
Potassium
K⁺
Silver
Ag⁺
Sodium
Na⁺
Thallium (I)
Tl⁺
Hydrogen
H⁺
Aurous or Gold (I)
Au⁺
Mercurous or Mercury (I)
Hg⁺
2+ Valency
Cation
Formula
Barium
Ba²⁺
Cadmium
Cd²⁺
Calcium
Ca²⁺
Chromium (II)
Cr²⁺
Cobalt (II)
Co²⁺
Copper (II)
Cu²⁺
Iron (II)
Fe²⁺
Lead (II)
Pb²⁺
Magnesium
Mg²⁺
Manganese (II)
Mn²⁺
Mercury (II)
Hg²⁺
Strontium
Sr²⁺
Nickel
Ni²⁺
Tin (II)
Sn²⁺
Zinc
Zn²⁺
Platinum
Pt²⁺
3+ Valency
Cation
Formula
Aluminium
Al³⁺
Chromium (III)
Cr³⁺
Cobalt (III)
Co³⁺
Iron (III)
Fe³⁺
Lanthanum
Ln³⁺
Thallium (III)
Tl³⁺
Titanium (III)
Ti³⁺
Vanadium (III)
V³⁺
Bismuth
Bi³⁺
Arsenic
As³⁺
Antimony
Sb³⁺
Auric or Gold (III)
Au³⁺
4+ Valency
Cation
Formula
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 Radical
Formula
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
Identify the symbol of the cation (first part of the name) and the anion.
Identify the valency (charge) of each and place it in parenthesis just above the symbol.
Balance the total positive and negative charge on the cation and anion — the two must add up to zero.
The numbers needed to balance the charges become the subscripts, placed right after the respective symbol.
Worked Examples
Example 1: Copper (I) Oxide
Symbols: Copper – Cu, Oxide – O
Charges: Copper (I) = 1+, Oxide = 2−
Balance: two Cu⁺ ions balance one O²⁻ ion, since 2(1+) + 1(2−) = 0
Formula: Cu₂O
Note: a subscript of 1 is never written — it is always understood.
Example 2: Calcium Chloride
Symbols: Calcium – Ca, Chloride – Cl
Charges: Calcium = 2+, Chloride = 1−
Balance: one Ca²⁺ ion needs two Cl⁻ ions, since 1(2+) + 2(1−) = 0
Formula: CaCl₂
Example 3: Ethane (Covalent Compound)
The methyl free radical CH₃ contains one unpaired electron.
A second methyl free radical also has one unpaired electron.
The two unpaired electrons pair up to form a covalent bond: CH₃–CH₃
Example 4: Methyl Chloride
The methyl free radical CH₃ has one unpaired electron.
The chloride free radical Cl also has one unpaired electron.
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
Cation
Anion
Formula
Name
NH₄⁺ (Ammonium ion)
Cl⁻ (Chloride ion)
NH₄Cl
Ammonium Chloride
Cu²⁺ (Cupric ion)
Cl⁻ (Chloride ion)
CuCl₂
Copper (II) Chloride or Cupric Chloride
K⁺ (Potassium ion)
NO₃⁻ (Nitrate ion)
KNO₃
Potassium Nitrate
Na⁺ (Sodium ion)
SO₄²⁻ (Sulphate ion)
Na₂SO₄
Sodium Sulphate
Fe²⁺ (Ferrous ion)
Cl⁻ (Chloride ion)
FeCl₂
Iron (II) Chloride or Ferrous Chloride
Fe³⁺ (Ferric ion)
SO₃²⁻ (Sulphite ion)
Fe₂(SO₃)₃
Iron (III) Sulphite or Ferric Sulphite
H⁺ (Hydrogen ion)
S²⁻ (Sulphide ion)
H₂S
Hydrogen Sulphide
Cu⁺ (Cuprous ion)
O²⁻ (Oxide ion)
Cu₂O
Cuprous Oxide or Copper (I) Oxide
Al³⁺ (Aluminium ion)
Cl⁻ (Chloride ion)
AlCl₃
Aluminium Chloride
NH₄⁺ (Ammonium ion)
OH⁻ (Hydroxide ion)
NH₄OH
Ammonium Hydroxide
Ag⁺ (Silver ion)
I⁻ (Iodide ion)
AgI
Silver Iodide
H⁺ (Hydrogen ion)
F⁻ (Fluoride ion)
HF
Hydrogen Fluoride
Na⁺ (Sodium ion)
CO₃²⁻ (Carbonate ion)
Na₂CO₃
Sodium Carbonate
Mn²⁺ (Manganese ion)
SO₄²⁻ (Sulphate ion)
MnSO₄
Manganese Sulphate
K⁺ (Potassium ion)
HCO₃⁻ (Hydrogen Carbonate ion)
KHCO₃
Potassium Hydrogen Carbonate or Potassium Bicarbonate
