Valency

HCl –Hydrogen chloride (Hydrochloric acid)

H₂O-Hydrogen oxide (water)

 NH₃-Hydrogen nitride (ammonia)

 Looking at these molecules, we find that while one chlorine atom combines with only on hydrogen atom, one oxygen atom combines with two; one nitrogen atom combines with three, that is, the atoms of chlorine, oxygen and nitrogen show different combining capacities.

            This combining capacity of an atom or a radical is called its valency. It is measures in term of hydrogen atom or oxygen atoms. Valency of an atom or radical is the number of hydrogen atoms or double the number of oxygen atoms which will combine with it.

          From the above formulae, the valencies of chlorine, oxygen and nitrogen are 1, 2 and 3 respectively. Similarly from the formulae CO₂ and SO₃, valencies of carbon and sulphur are 4 and 6 respectively.

Elements with variable Valency

          Elements like iron, mercury, copper and tin are those which have more than one valency number. In the cases, the radical with lower valency is designated by –ous, while one with higher valency by –ie. For example:

FeCl₂ is ferrous chloride       ;    FeCl₃ is ferric chloride

Hg₂Cl₂ is mercurous              ;   HgCl₂ is mercuric

CuCl is cuprous                    ;   CuCl₂ is cupric.

Modern chemists make use of Roman numerals in place of these trivial names. FeCl₂ is iron (II) chloride, FeCl₃ is iron (III) and so on.

Radical

          

The molecule of a compound is usually made up to two parts which are separately known as radicals. For example, the radicals present in sodium chloride molecule are sodium and chloride while those in potassium nitrate are potassium and nitrate.

Radicals are group of atom that reacts as single atoms and keep their identity in many reactions.

A radical is called a simple radical when it is an atom only, e.g., sodium, potassium etc. It is known as a compound radical when it is made up of a group of two or more different atoms, nitrate –made up of one nitrogen atom and three oxygen atoms and sulphate- made up of one sulphur atom and four oxygen atoms, a compound radical can be introduced or expelled from combination without change.

When an acid reacts with a base, a salt is produced as a result of neutralization.

Sodium hydroxide (Base) + Hydrochloric acid (acid)  =    Sodium chloride (salt) + water.

 In the molecule of sodium chloride the sodium radical has been contributed by the base sodium hydroxide and is therefore, called Basic Radical. Similarly its chloride radical has been contributed by hydrochloric acid and is, therefore, termed as Acid Radical.

            In the case of inorganic salts, the metallic radicals of the type of sodium are called basic radicals while others like nitrate, sulphate and carbonate are called acid radicals. When a salt is dissolved in water, it splits up into the constituents radicals. The basic radicals then carry a positive charge (positive ions) and are because of this reason, termed electro-positive radicals. The acid radicals under these circumstances carry negative charge (negative ions) and are called electronegative radicals.

 Basic or electropositive radicals:

Monovalent	Bivalent	Trivalent	Tetravalent
Ammonium       NH4 Gold(Aurous)      Au Copper               Cu Mercury(MercurousHg	Barium           Ba              Cadmium       Cd Manganous    Mn Plumbous      Pb Stannous       Sn Ferrous         Fe Cupric          Cu	Aluminium  Al Auric         Au Chromic    Cr Ferric        Fe	Stannic           Sn Platinic            Pt Plumbic          Pb

Acid or electronegative radicals:

Monovalent	Bivalent	Trivalent	Tetravalent
Bicarbonate         HCO3  Bisulphite            HSO3 Bishuphate          HSO4 Bromide               Br Chloride               Cl Chlorate               ClO3 Cyanide               CN Fluoride               F Hydroxide           OH Nitrate                 NO3 Permanganate     MnO4 Nitrite                  NO2	Carbonate          CO3 Oxide                   O Sulphide             S Sulphite              SO3 Sulphate             SO4 Silicate               SiO3 Stannate            SnO3 Zincate              ZnO3	Nitride                N Phosphide          P Phosphite           PO3 Phosphate          PO4	Carbide   C

  











Aim to memory

           To remember the symbols and valencies easily and thus be able to write formulae correctly we should know a few correct formulae. If we know these well, writing of formulae will be nothing more than a simple game. Read the following correct formulae to memory:

HCl             Hydrogen chloride                                                    (Hydrochloric acid) H2SO4         Hydrogen sulphate                     (Sulphuric acid) H2SO3         Hydrogen sulphite                     (sulphrous acid) HNO3          Hydrogen nitrate                   (nitric acid) HNO2        Hydrogen nitride                    (nitrous acid) HBr            Hydrogen bromide                   (Hydrobromic acid) HI               Hydrogen iodide                    (Hydrogen acid) HCLO3        Hydrogen Chlorate                    (chloric acid) H3PO4        Hydrogen phosphate                    (phosphoric acid) H3BO3        Hydrogen borate                    (Boric acid) H2CO3        Hydrogen carbonate                      (carbonic acid) H2O           Hydrogen oxide                      (water) HOH          Hydrogen hydroxide                     (water) H2S           Hydrogen sulphide                   (Hydrosulphuric acid)

NaCL            Sodium chloride MgSO4          Magnesium sulphate BaSO3           Barium sulphite AgNO3          Silver nitrate LiNO2            Lithium nitrate AgBr             Silver bromide CuI                Copper iodide KCLO3         Potassium chlorate FePO4           Iron phosphate AlBO3           Aluminium borate CaCO3          Calcium carbonate CoO              Cobalt oxide NH4OH         Ammonium hydroxide CuS               Copper sulphide

Compound 1-14 are hydrogen compounds; thus valencies of radical combined with hydrogen are directly given by the number of hydrogen atoms combined in each case. All compound radicals have been underlined to facilitate their selection.

In compound 15-28, hydrogen atoms of compounds 1-14 have been replaced by other basic radicals. The positive valency of the basic radicals is equal to the negative valency of the acid radicals with which these are combined. For example:

Valency of NO₃ from (4) =1 and positive valency of Ag from (18) is the same as that of NO₃ i.e., =1.

Valency of SO₄ from (2) =2 and positive valency of Mg from (16) is the same as that of SO₄; i.e., =2, and so on.