valency

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)
H2SO       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)
H3PO      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.
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