Hi,

 

How do you find the mass number of element? Internet search told me that if I round the atomic weight, I get the mass number.

 

I never heard of anything like this before so can anyone confirm this? Thanks.

Mass number is the number of protons + the number of neutrons. usually an average of how common the different isotopes are.

 

usually you just go look it up on a periodic table.

Well, I know THAT... Mass number = # of protons + # of neutrons, and I also know that it's the average of isotopes.

 

I know ALL that. But you haven't helped my question yet.. I searched for Google , and only 100% sure thing I found was that atomic mass= mass number if the atomic mass is an integer. However, what about all the other cases where the atomic mass is not an integer?

 

Round up to find mass number?

the mass number doesn't need to be an integer.

 

and averages don't always produce integers. also, due to E=mc^2 there is a bit of mass from the binding energy and protons and neutrons have slightly different weights and all that it throws it off.

 

atomic mass is the actual mass of the atom and mass number is the number of nucleons. they don't always correspond.

Chlorine alone should tell you that, how would you round 35.45 (or 35.5 in some tables)?

 

as IA said, it`s worked out with respect to Isotope abundance.

 

 

Edit: to elucidate a little more, and using the Cl example, it Still has 17 Protons (and obviously electrons), so if we take that away from the 35.5 we get 18.5 neutrons, and that`s a perfectly valid answer.