relative atomic mass of an element ????

[faisal]

A sample of neon was found to cotain 90.9% of 20Ne, 0.26% of 21Ne, & 8.8% of 22Ne, Calculate the relative atomic mass of neon.
this is what i'v done so far (90.9x 20)+ (0.26x21)+(8.8x22)= 2017/?
i dont know what to devide it by :confused: ??? I'm not given a mass spectra?
P.s.
why r relative atomic masses for some elements not whole numbers, is it because they r isotopes????

[Azrioch]

Divide by 100.

That will get you 20.17

Yes, they are not whole numbers because of isotopes. That's what you're taking account for by finding the relative atomic mass.

[faisal]

Thanx, i did the question again and i got 20.16 this is what i did (90.9/100x20)+(0.26/100x21)+(8.8/100x22)= 20.16

[Azrioch]

Either way works. :)