Atomic Mass, Relative Atomic Mass, and the differences

Atomic Mass (A)
          Is the total mass of all of the subatomic particles in the nucleic.


Relative Atomic Mass (Ar)
          Is comparison of average mass of an atom to 1/12 mass of 12 C atom.


Difference about both:

• Mass Number : use integer form
•  Relative Atomic Mass : able to use fraction number
•  

extra :::

Why average mass of an H atom is 1.008 amu , therefore mass number of Hidrogen isotopes are 1 , 2 , and 3
So, we get (1+2+3)/3 = 2 ?

because depend of abundance of each isotope in the nature environment