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How can i know the valency of a compond to react it with another elemet to form a chemical reaction? For example - Suppose I have to make CaCO subscript 3. Then i have to first react Carbon with Oxygen then react that compond with calcium. So how will i know the valency of compound formed from reaction of carbon with oxygen. We can know the valency of elements from their atomic number and electronic configuration, but how can i know the valency of compound?  Please help,waiting for your reply, Thank you. 
Asked by hk.sachdev | 12 Oct, 2018, 22:38: PM
answered-by-expert Expert Answer
Important points to remember:
 
1. Valency is always the whole number.
2. Valency of metal is always positive. (e.g. Na+)
3. Valency of non-metal is always negative. (e.g. Cl−)
4. Monovalent elements have valency one(e.g. K1+)
5. Divalent elements have valency two.(e.g. Zn2+)
6. Trivalent elements have valency three(e.g. Al3+)
7. Certain elements exhibit variable valency i.e. more than one valency. e.g., Cu1+, Cu2+ or Fe2+, Fe3+
 
Valency chart of elements and radicals

 

1)    Valencies of common elements for reference

Monovalent

Divalent

Trivalent

Tetravalent

Potassium  K1+

Sodium Na1+

 

Copper Cu+

 

Hydrogen    H1+

Calcium Ca2+

Magnesium Mg2+

Zinc Zn2+

Copper Cu2+

Iron Fe2+

Lead             Pb2+

Aluminium Al3+

 

 

 

Iron              Fe3+

 

 

 

 

 

 

 

Lead       Pb4+

 

Monovalent

Divalent

Trivalent

Tetravalent

Chlorine      Cl1−

Bromine      Br1−

Iodine          I1−

Oxygen         O2−

Sulphur         S2−

Nitrogen       N3−

 

Carbon     C4−
 

2)    Valencies of radicals

 

Monovalent

Divalent

Trivalent

Ammonium NH4+

 

Carbonate         CO32−

Sulphite             SO32−

Sulphate            SO42−

 

Phosphate            PO43−

Bicarbonate      HCO31−

Bisulphite          HSO31−

Bisulphate         HSO41−

Nitrate               NO31−

Hydroxide          OH−
 

3)    Variable valencies of common elements


Elements

Monovalent

Divalent

Trivalent

Tetravalent

Copper

Iron

Lead

Cu1+

 

Cu2+

Fe2+

Pb2+

 

Fe3+

 

 

Pb4+
 
 
Now, it seems you have a problem with understanding the valency of a radical like Carbonate CO32−. 
Let's understand this,
 
From the above structure, it is clear that valency of carbon if 4 which is fulfilled but out of three oxygens valency of two oxygen atoms are not fulfilled.
And the valency of non-metal is negative hence this radical has valency -2.
Answered by Ramandeep | 15 Oct, 2018, 12:10: PM

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