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 . 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. 

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.