[jayjay112]

What is the difference between atomic weight and mass number??

[ARGOS++]

Dear jayjay112;

Please compare:  http://en.wikipedia.org/wiki/Mass_number 
and:                    http://en.wikipedia.org/wiki/Atomic_weight#Determination_of_atomic_weight 

Good Luck!
                    ARGOS⁺⁺

[jayjay112]

thats the part i get confused with, are they not both the sum of the protons and nuetrons in an atom?

[sjb]

At a simple level, yes, however at a more advanced level no.

Consider ³⁵Cl, an atom of this has mass 34.96885268(4) amu, which is close, but not the same as 35 amu. Or ¹⁰B (mass 10.0129370(4) amu / 10 amu).

Are you aware of the terms binding energy and mass defect? (http://en.wikipedia.org/wiki/Binding_energy#Mass_defect)

[ARGOS++]

Dear jayjay112; Dear sjb;

The binding energy can nearly be neglected if we compare these both properties.

Much more important is that the atomic weight reflects/is influenced also (by) the natural Abundance of the isotopes.
That's why for example the atomic weight of Cl is close to 35.453 because the mayor Isotopes involved are ³⁵Cl and ³⁷Cl with their relative atom masses.

See also the Silicon example given in the second link.

Good Luck!
                    ARGOS⁺⁺

[sjb]

Ahh, my bad, I read atomic weight as the mass of a single atom, in this instance. It could be that you're after the average atomic weight, in which case ARGOS++ is correct.