Symbol, Formulae and Naming of Compounds

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Science > Chemistry > Introduction to ChemistryYou are Here
  • In a chemical reaction, the molecular composition changes and it is represented by a chemical equation. In a chemical equation, the various substances involved as reactants or products are written in the form of symbols and formulae. These symbols and formulae are shorthand notations of the molecules involved as reactants or products.

Symbol of an Element:

  • A symbol is a representation of an atom of an element. These are unique 1,2, and 3 letter symbols. The first letter of an Element symbol is always a capital letter and remaining letters are written in lower case only. Many of the element symbols reflect the English name of the element such as Oxygen (O), hydrogen (H), carbon (C), nitrogen.(N), etc. These are single letter symbols. Examples of double letter symbols are aluminium (Al), Magnesium (Mg), calcium (Ca), etc.. Some symbols are derived from Latin names Aurum (gold) (Au), Argentum (silver), Stannum (tin) (Sn), Plumbum (lead) (Pb), etc.(Ag),

The formula of Element or Compound:

  • The formula is a representation of the actual number of atom or atoms of each element present in one molecule of the substance. The formula of element or compound is written by making use of the symbols of the respective elements.
  • The formula for nitrogen is N2, that of oxygen is O2, that of carbon dioxide is CO2, etc.

Radicals or Ions:

  • A radical is a group of atoms of elements carrying a charge, e.g., chlorate [ClO3].
  • Radicals or ions are formed by losing or gaining electrons. When an electron is gained the group of atoms acquire a negative charge and is called negative radical or negative ion.  When an electron is lost the group of atoms acquire a positive charge and is called positive radical or positive ion. Depending upon the number of charges carried by radical they are further classified as monovalent, bivalent (divalent), trivalent and tetravalent radicals or ions and so on.

Positive Radicals:

Valency Radical Symbol Ion
Monovalent Hydrogen H +
Sodium Na Na +
Potassium K +
Mercurous or Mercury (I) Hg Hg +
Cuprous or Copper (I) Cu Cu +
Argenous or silver (I) Ag Ag +
Divalent [2]  Zinc  Zn  Zn2+
Magnesium Mg Mg2+
Calcium Ca Ca2+
Cupric or Copper (II) Cu Cu2+
Mercuric or Mercury (II) Hg Hg2+
Frrrous or Iron (II) Fe Fe2+
Barium Ba Ba2+
Stanous or Tin (II) Sn Sn2+
Plumbous or Lead (II) Pb Pb2+
Trivalent [3] Aluminium Al Al3+
Ferric or Iron (III) Fe Fe3+
Tetravalent [4] Stanic or Tin (IV) Sn Sn4+
Plumbic or Lead (IV) Pb Pb4+
  • Note: In case of the electropositive element showing variable valency. For lower valency, its name is written in form ending with ‘ous’ while with higher valency its name is written ending with ‘ic’. For example, iron shows two valencies 2 and 3. the iron showing lower valency 2 is named as ferrous and by IUPAC system it is named as Iron (II).  the iron showing higher valency r is named as ferric and by IUPAC system it is named as Iron (III).

Negative Radicals

Valency Radical Symbol Ion
 Monovalent Hydroxyl or hydroxide OH OH 
Fluoride F
Chloride Cl Cl 
Bromide Br Br 
Iodide I
Nitrite NO2 NO
Nitrate NO3 NO3
 Hypochlorite ClO ClO 
Chlorite ClO2 ClO
Chlorate ClO3 ClO
Perchlorate ClO4 ClO
Hypobromite BrO BrO 
Bromite BrO2 BrO
Bromate BrO3 BrO
Perbromate BrO4 BrO
Hypoiodite IO IO 
Iodite IO2 IO
Iodate IO3 IO3
Periodate IO4 IO
Acetate CH3COO CH3COO 
Formate HCOO HCOO 
Bisulphide HS HS 
Bisulphite [Hydrogen sulphite] HSO3 HSO
Bisulphate [Hydrogen sulphate] HSO4 HSO
Bicarbonate [Hydrogen carbonate] HCO3 HCO
Cyanide CN CN
Cyanate CNO CNO
Thiocyanate SCN SCN
Permanganate MnO4 MnO4
Oxide O O2
Divalent [2]
Peroxide O2 O22-
Carbonate CO3 CO32-
Sulphide S S2-
Sulphite SO3 SO32-
Sulphate SO4 SO42-
Thiosulphate S2O3 S2O32-
Biphosphate [Hydrogen phosphate] HPO4 HPO42-
Chromate CrO4 CrO42-
Dichromate Cr2O7 Cr2O72-
Oxalate C2O4 C2O42-
Trivalent [3] Phosphite PO3 PO33-
Phosphate PO4 PO43-

Derivation of Formulae of Compound:

Steps Involved:

  1. First, write electropositive element (say A) in its radical form ( Ax+) followed by the electronegative element (say B)  in its radical form ( Ay-). (Use tables for radicals).
  2. Below the element write their numeric charge without considering the nature. ( x and y)
  3. Swap the numbers obtained in step 2 i.e. (y and x)
  4. Now the molecule should be electrically neutral. Hence the formula of the molecule is  AyBx


  • Formula for ferric chloride:

It is compound made up of radicals Iron (Fe3+) and chlorine ( Cl )

Symbol 01

The formula for ferric chloride is FeCl3.

  • Formula for ferric oxide:

It is compound made up of Iron (Fe3+) and oxygen ( O 2-)

Symbol 02

The formula for ferric oxide is Fe2O3.

  • Formula for Aluminium hydroxide:

It is compound made up of Aluminium (Al3+) and hydroxide ( OH )

Symbol 04

The formula for ferric chloride is Al(OH)3.

  • Formula for magnesium phosphite:

It is compound made up of Aluminium (Mg2+) and phosphite ( PO 3-)


The formula for ferric chloride is Mg3(PO3)2.

Naming Binary Compounds:

General Prefixes Used in Naming:

Number Multiplier
1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-

Steps Involved in Naming:

  1. Write the molecular formula. Identify radicals.
  2. Find the valencies of constituent groups. It is important for elements showing variable valency.
  3. Name of electropositive or less electronegative element/ group is written first. (due consideration to be given to elements showing variable valency). followed by name of the electronegative or more electronegative element or group.
Compound Name
NaCl Sodium chloride
MgCl2 Magnesium chloride
CaCl2 Calcium chloride
BaO Barium oxide
H2S Hydrogen sulphide
FeCl3 Ferric chloride or Iron (II) chloride
Fe2Cl3 Ferrous chloride  or Iron (III) chloride
SnCl2 Stannous chloride  or Tin (II) chloride
SnCl4 Stannic chloride  or Tin (IV) chloride
CuCl Cuprous chloride or  Copper (I) chloride
CuCl2 Cupric Chloride  or  Copper (II) chloride
CO Carbon monoxide
CO2 Carbon dioxide
SO2 Sulphur dioxide
SO3 Sulphur trioxide
CCl4 Carbon tetrachloride
P2O5 Phosphorous pentaoxide
UF6 Uranium hexafluoride

Naming of Acids:

Binary Acids:

  • They have general formula HA. The negative radical consists of a single non-metal. Use hydro as prefix and ic suffix.
Binary Acid Name
HF Hydrofluoric acid
HCl Hydrochloric acid
HBr Hydrobromic acid
HI Hydroiodic acid


  • The negative radical consists of non-metal and oxygen. If the percentage of oxygen is less, then suffix ous is used. If the percentage of oxygen is more, then the suffix ic is used.
Binary Acid Name
H2SO3 Sulphurous acid
H2SO4 Sulphuric acid
HNO2 Nitrous acid
HNO3 Nitric acid
H3PO3 Phosphorous acid
H3PO4 Phosphoric acid
  • If acid contains less number of oxygen than its corresponding ‘ous’ acid a prefix hypo is given to the negative radical. If acid contains more number of oxygen than its corresponding ‘ic’ acid a prefix per is given to the negative radical.
Binary Acid Name
HClO Hypochlorous acid
HClO2 Chlorous acid
HClO3 Chloric acid
HClO4 Perchloric acid

Naming of Bases:

  • Base have general formula BOH. They contain hydroxyl (OH-) group. Metal is the electropositive radical and hydroxyl group is negative radical. The name of metal or group is written first followed by the word hydroxide.


Base Name
NaOH Sodium hydroxide
KOH Potassium hydroxide
Al(OH)3 Aluminium hydroxide
Mg(OH)2 Magnesium hydroxide
NH4OH Ammonium hydroxide

Naming of Salts:

  • Salt is assumed to be a product of neutralization reaction between acid and base. The positive radical of salt comes from the base while the negative radical comes from the acid.
  • The name of salt is written with positive radical followed by negative radical.
  • In case of oxyacids, if the negative radical is derived from ic acid the suffix ate is used for naming the salt. if the negative radical is derived from ous acid the suffix ite is used for naming the salt.

Salts involving Binary acids:

Salt Name
NaCl Sodium chloride
NH4Cl Ammonium chloride
CH3COONa Sodium acetate
MgCl2 Magnesium chloride
KBr Potassium bromide
KI Potassium iodide

Salts involving oxyacids:

Salt Corresponding  Acid Suffix Name
ZnSO4 Sulphuric acid ( H2SO4) ate Zinc sulphate
NaNO3 Nitric acid (HNO3) ate Sodium nitrate
AlPO4 Phosphoric acid (H3PO4) ate Aluminium phosphate
CaSO3 Sulphurous acid ( H2SO3) ite Calcium sulphite
ZnNO2 Nitrous acid ( HNO2) ite Zinc nitrite
Mg(PO)2 Magnesium phosphate ite  Magnesium phosphite

Science > Chemistry > Introduction to ChemistryYou are Here
Physics Chemistry  Biology  Mathematics


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