Chemical formulae

To write the correct chemical formula of a compound

1. Identify the elements that makes this compound

e.g: Calcium chloride is made with Calcium and Chlorine

2. Write the correct symbols of the elements in the order they appear in the chemical name

eg: Ca    Cl

3. Write the valency of the elements

Valency means the number of electrons given, taken or shared by an atom of the element when making a compound
Valency is equal to the group number for elements in group numbers I to IV
Valency is equal to ( 8 - group number) for elements in group numbers V to VII
Valency is equal to zero for group number 8 elements as they usually does not make compounds

Ca is in group number II . Its valency is 2
Cl is in group number VII .Its valency is (8-7)= 1

Ca  2   Cl  1

4. Exchange the valencies

Ca1 Cl2

5. Simplify and write the formula

further simplification is not possible in this example. If we take the formula of Magnesium oxide
and exchange their valencies it is

Mg 2    O2
2 and 2 can be simplified into 1 to 1

6. Write the correct formula
(ratio between atoms should be simplified in to whole numbers and number 1 is not written

CaCl2


This method is used only for compounds made between two elements. In addition to the simple elements there are complex cations and anions that makes compounds. (Cation: a positive ion
anion: a negative ion)
If the compound is made with simple elements they will end as :' ide' added to the end of the last element
e.g: sodium chloride : NaCl

mostly the compounds made with complex anions ends as : 'ate'
e.g: Sodium Sulfate Na2SO4

However the rules are same for writing the the formula but we have to keep in mind the name , formula and the charge of these complex anions and also cations

Complex anion name       formula         valency

                    

Nitrate                            NO₃^{-                           -1}

Hydroxide                      OH^{-                             -1}

Bicarbonate                    HCO₃^{-                     -1}

Carbonate                      CO₃^{2-                       -2}

Sulphate                         SO₄^{2-                       -2}

ThioSulphate                  S₂O₃^{2-                  -2}

Phosphate                       PO₄^{3-                   -3}

Formula of the  complex cation  Ammonium ion is      NH₄⁺

Now lets consider an example

1.  Aluminium Phosphate

Elements/ complex cation/anion

Al       and PO₄^3- 

^{Valancies Al =3  Phospahate= 3}

^{Exchange and simplify we get}

^AlPO4

2. Calcium hydroxide

Ca =2  OH^-   
exchange and simplify you get  Ca(OH)2



Questions:

Write the correct formula of

1. Sodium Carbonate
2. Aluminium Hydroxide
3. Magnesium Phosphate

Next on Mole concept

Arunigd 10:00 PM   chemical formula   complex anions   complex cations   GCSE Chemistry   valency   writing chemical formula

Author : Arunigd

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