# 05. Atomic Weight

Atomic Weight is the average relative weight per atom of an element according to their abundance considering an atom of C

For Example:

Naturally occurring Chlorine contains 75% Cl

In other words, atomic weight is the number which show how many times an atom of an element is heavier that 1/12

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

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

For eg.

Atomic Weight of Sodium = 23

23 gram sodium = 1 gram atom of sodium

= 6.023×10

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

2.608 gram atoms = 6.023×10

= 1.57×10

Solution:

Atomic Weight of sodium = 23

23 gram of sodium = 1 gram atom of sodium

= 6.023×10

6.023×10

= 3.81×10

Atomic weight of carbon = 12

12 gram of carbon = 1 gram atom of carbon

= 6.023×10

= 6.023×10

6.023×10

^{12}isotopes as 12 amu.For Example:

Naturally occurring Chlorine contains 75% Cl

^{35}and 25% Cl^{37}, so atomic of ChlorineIn 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}atoms2.608 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 sodium6.023×10

^{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 amu6.023×10

^{23}×12 amu = 12 gram