Atomic Mass

[sensfan776]

The eqation for atomic mass, is it as simple as adding the number of neutrons to the number of protons? Does the number of electrons come into play or are they not relevent?

[JackBean]

e-'s mass is ~9 . 10^-31 kg http://en.wikipedia.org/wiki/Electron
p+'s mass is ~1.67 . 10^-27 kg http://en.wikipedia.org/wiki/Proton
n's mass is ~1.67 . 10^-27 kg http://en.wikipedia.org/wiki/Neutron
(their masses seem similar, but differ little)

as you can see, the e-'s mass is ~1000x smaller, than that of nucleons, so you had to have ~ 1000 e- to have some impact (but such atom would have ~3000 nucleons, so it would be again just a little...), so yes, you can neglect the e-'s mass.

But no, you can't just add the masses of nucleons together, because when you add them together a little energy is released and this equal (by the famous E = mc^2) to loss of weight and that makes atoms stable

[sensfan776]

In a question when asked to predict the atomic mass is it feasable to simply add the number of protons with the number of neutrons? Before I posted this question I read in my text that that is the way to do it, I consulted Wiki for conformation (as the text I am using is old) and was thrown a curve ball. So is my text just giving me a way to quickly estimate. I am obviousley not on the same level as the people who frequent this forum and I appreciate you people coming down to a highschool level for me. Thanks

[JackBean]

well, basically, you can, e.g. you know, that oxygen has 8 both protons and electrons, what makes the atom mass ~ 16 units, that's probably fairly accurate for your calculations

[JackBean]

the problem comes, when the element has more isotopes (especially, if they are relevantly occuring like with bromine) or if the released energy is big with both small and large atoms

[sensfan776]

Thanks for your help.

[jwalin]

well, basically, you can, e.g. you know, that oxygen has 8 both protons and electrons, what makes the atom mass ~ 16 units, that's probably fairly accurate for your calculations

jackbean....!!!

i think you had worked a lot on other things and were tired or something before posting that...
for how could you make such a mistake....
oxygen had 8 protons and 8 nuetrons therefore the atomic mass is 16gm/mol of O and not oxygen molecule
when calculating the atomic mass its the addition of numbers of NUETRONS and PROTONS... to give the atomic mass... or the mass of 6.023 * 10^23 species or atoms...
electrons play no significant role. avagadro's number into mass of electron gives really low value. so it does affect but the effect is negligible

[JackBean]

the first person talking about molecules is you, so chill out.

When talking about individual electrons or atoms, you're not supposed to use Avogadro's number...

[mith]

the relative abundance of isotopes will affect the "atomic mass" reported on the periodic table since it's an average of all isotopes for that element

[JackBean]

of course, but I meant, that if you had isotope e.g. 30 in 99% and isotope 31 in 1%, than you can quite easily calculate only with isotope 30

[sensfan776]

Oh God help me I am so confused!!!

[mith]

Just add the neutrons and protons, but you should be aware that this does not produce the exact numbers in the periodic table.