Difference between relative ISOTOPIC mass and relative ATOMIC mass?

Can anyone tell me is there any difference between relative ISOTOPIC mass and relative ATOMIC mass???

In my class they are introduced separately, but it seems like they are same when I read some other textbooks/reference.

Besides difference, how are they similar to each other??

Please don't copy the paragraphs in Wikipedia "Atomic Mass".........

Thanks.

Additional Details

so, do they refer to the same thing or just different??

and... what is RIM?

joeyeehu...

Best Answer - Chosen by Voters

Most natural occurring elements can exist in a few isotopic form. Some has two, some has three and some has six ....ect.

For example, chlorine can exist as two isotopic forms, namely chlorine-35 and chlorine-37.

We can say that the relative isotopic mass of chlorine-35 is 35 and the relative isotopic mass of chlorine-37 is 37.

To determine the relative atomic mass of chlorine, then you must know the relative abundance of isotope Cl-35 and that of isotope Cl-37.

It is found that the relative abundances of Cl-35 and Cl-37 are 75% and 25%.

The relative atomic mass of Cl is the weighted average of the relative isotopic mass of Cl-35 (75%) and the relative isotopic mass of Cl-37 (25%).

So, relative atomic mass of chlorine ={ (75 x 35) + (25 x 37) } / 100
......................................… 35.5

So, RIM of element X refers to the relative mass of a particular isotope only.
RAM of element X refers to the average relative mass of all isotopes of X.