Chemistry help!

[bravegirl123]

Plz answer this question:

?The relative atomic mass of an element is the weighted average atomic mass of the
element relative to 1/12th the mass of an atom of the 12C isotope. What doe the word 'weighted' mean in the definition?

 

[hredoymohammad]

A "weighted average" allows for the fact that there won't be equal amounts of the various isotopes. This example should make that clear:
In chlorine, there are 3 atoms of chlorine-35 for every 1 of chlorine-37.
Suppose you had 4 typical atoms of chlorine.
The total mass of these would be (3 x 35) + (1 x 37) = 142
The average mass of these 4 atoms would be 142 / 4 = 35.5.
35.5 is the relative atomic mass of chlorine.
Notice the effect of the "weighted" average. A simple average of 35 and 37 is, of course, 36. Our answer of 35.5 allows for the fact that there are more of the lighter isotope of chlorine - and so the "weighted" average ought to be closer to that.
(copied from another site)

 

[bravegirl123]

thanks!!