definition of molecular weight in chemistry ? what is atomic weight ? We have studied about atoms and molecules. Do you know what is atomic weight or what is definition of molecular weight ? not ?

Okay no worry , today we will discuss about atomic weight and molecular weight , after today’s session , you will easily define both the terms atomic weight and molecular weight.

Let us start with atomic weight.

**Atomic weight **: as we have studied that the atoms are very small particles in size , so atoms can not be weighted in grams .

In other words , the gram unit is very large to weight the atoms .

We understand this thing with an example : if we weight the carbon in grams unit we will get the carbon weight as 1.95 x 10 ^{-23} grams.

So we can see that in gram unit , it is hard to express the weight of atoms.

So we use 1/12th of the weight of a carbon as a unit to find out or calculate or represent the weight of any atom.

This is called one atomic mass unit (1 amu) , sometimes atom of same elements found with different masses and these elements are called isotopes.

Here is an example of isotopes : the chlorine has two isotopes , one of them isotope contains 35 amu weight and other have 37 amu weight.

We mixed the both isotopes in such proportion that we get the cl atomic weight 35.5 amu.

So the atomic weight of chlorine will be

Cl = 35.5 amu

So now can you tell me what is exact definition of atomic weight ?

**Definition of atomic weight** : this is the average mass of the atom of any element called atomic mass unit.

The carbon-12 (C-12) is used as standard unit and carbon-12 atomic mass is taken 12 amu.

How to use C-12 as standard unit ?

Let us suppose , we have magnesium and it’s atomic weight is 24 , so we can say that the magnesium has twice atomic weight as compared to carbon atom.

**Molecular weight** : as we know that molecule is the smallest particle of any element which have it’s individual existence. And the molecular weight defined as , it is relative weight as compared to carbon atom that is 12 amu.

For example : magnesium carbonate has 84 molecular weight , it means magnesium carbonate has 7 times heavy as compared to the carbon-12 (C-12) molecular weight .