The combination of elements to formed compound is governed by the below five laws

Law of Conservation of Mass

It states that mass remains conserved.
For radioactive and nuclear reactions, 1st law is modified – Law of Conservation of mass & energy.
Law of Conservation of Mass & Energy:
It status that during chemical reaction sum of mass & energy is conserved.
C + O2      ->      CO2
12 + 32                        12 + (16)  
44 gm                           44 gm
Mass of Reactants = Mass of Products

Law of constant or definite proportion

It states that elements combine in their definite proportion of mass to give compounds.    
CO2 -> C : O
      12 : 32
         3 : 8
H2O                         H : O
                                2: 16
                                 1: 8

Law of Multiple Proportion:

It states that when two different elements combined to form more than one compound and if mass of one element is fixed then other elements mass bears simple whole no. ratio.
NO                  14:16         28: 32
NO2                 14:32         28: 64
N2 O4              28 :64        28: 64
N2 O5               28:80         28: 80
         Now, ratio of oxygen:
         =32: 64: 64: 80
         = 2: 4: 4: 5
CO                  12: 16
CO2                 12: 32
Now ratio of Oxygen
=16: 32= 1: 2
PO2                 16: 32
PO5                 16: 80
Now Ratio of O
=32: 80=2: 5

Gay Lassac Law of Gaseous

 When gases combine, they bear simple whole no. ratio of their volumes provided all the gases are at the same temperature and pressure
1 N2 + 3H2      ->         2N H3  (Haber’s Process)
Ratio 1:3. So 100 ml of Nitrogen combines with 300 ml of oxygen to form 200ml of Ammonia
H2 + I2   ->      2H I (Hydrogen Iodide)
Ratio 1:1. So 100 ml of hydrogen combines with 100 ml of Iodine to form 200ml of Hydrogen Iodide

It states that equal volume of gases at the same temperature and pressure should contain equal no. of molecules.
Extra reading

Law of Reciprocal Proportion:

It states that when two different elements combine with fixed mass with 3rd element & when their ratio are compound when they combine with each other, bears simple whole no.


  1. Atoms can neither be created nor be destroyed
  2. Atoms of same elements are similar.
  3. Atoms of different elements are different.
  4. Atoms are indivisible.  

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