[tabby124]

Ladies and gentlemen, by subject is my question.
An answer would be greatly appreciated.

[macman104]

Because each element has various isotopes that are present in nature, and the atomic mass is an average of all of these isotope weights.

[nj_bartel]

*by their relative abundance

[Mitch]

The better question is why an isotope's mass isn't a whole number. Think about that one for a little bit.

[nj_bartel]

Because the masses of protons/neutrons/electrons aren't whole numbers? 

I can't tell if that's a complex nuclear chem question or not

[Yggdrasil]

There's also the nuclear binding energy which will slightly affect the mass.  As it turns out the mass of a nucleus is less than the sum of its parts.

[happyanimesh]

This is because the atomic mass of an element is the weighed average of the atomic masses of all the isotopes of an element.

For example, oxygen occurs in nature as a mixture of isotopes¹⁶O,¹⁷O and ¹⁸O having atomic masses of  15.995 u, 16.999 u and 17.999 u respectively.The relative abundance of the three isotopes is 99.763%,0.037% and 0.2%.
Thus the atomic mass of oxygen is expressed as the weighed average of the atomic masses of these three isotopes.

 Atomic mass of oxygen= [(99.763*15.995)+(0.037*16.999)+(0.2*17.999)]/100                                   

                                     = 15 u