^{12}isotopes as 12 amu.

For Example:

Naturally occurring Chlorine contains 75% Cl

^{35}and 25% Cl

^{37}, so atomic of Chlorine

In other words, atomic weight is the number which show how many times an atom of an element is heavier that 1/12^{th} part of an atom of ^{12}Cl isotopes.

**Fractional Atomic Weight:**

Atomic Weight is the average weight per atom of an element. So it can be in fraction due to the presence of isotopes.

For Example:

Naturally occurring Chlorine contains 75% Cl^{35} and 25% Cl^{37}, so atomic of Chlorine

**Gram Atomic Weight:**

When atomic weight of an element is expressed in gram. Then it is called gram atomic weight. One gram atomic weight of each element contains 6.023×10^{23} atoms.

For eg.

Atomic Weight of Sodium = 23

23 gram sodium = 1 gram atom of sodium

= 6.023×10^{23} atoms of sodium

**Q. Calculate the number of gram atoms and atoms of sodium present in 60 gram of sodium.**

Solution:

Given,

Weight of Sodium = 60 gram

Atomic Weight of Sodium = 23

No. of gram atoms of sodium = ?

No. of atoms of sodium = ?

1 gram atom = 6.023×10^{23} atoms

2.608 gram atoms = 6.023×10^{23}×2.608 atoms

= 1.57×10^{24} atoms

**Q.What is the weight of one atom of sodium?**

Solution:

Atomic Weight of sodium = 23

23 gram of sodium = 1 gram atom of sodium

= 6.023×10^{23} atoms of sodium

6.023×10^{23} atoms of sodium = 23 gram

= 3.81×10^{-23} gram.

**Q.What is the weight of 1 amu carbon?**

Atomic weight of carbon = 12

12 gram of carbon = 1 gram atom of carbon

= 6.023×10^{23}atoms of carbon

= 6.023×10^{23}×12 amu