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Atomic Mass

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Atomic Mass

Postby sensfan776 » Thu Mar 18, 2010 12:59 pm

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? :?:
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Postby JackBean » Thu Mar 18, 2010 1:28 pm

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 ;)
http://www.biolib.cz/en/main/

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Re: Atomic Mass

Postby sensfan776 » Thu Mar 18, 2010 1:41 pm

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
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Postby JackBean » Thu Mar 18, 2010 2:26 pm

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
http://www.biolib.cz/en/main/

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Postby JackBean » Thu Mar 18, 2010 2:28 pm

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
http://www.biolib.cz/en/main/

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Re: Atomic Mass

Postby sensfan776 » Thu Mar 18, 2010 2:34 pm

Thanks for your help.
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Re:

Postby jwalin » Thu Mar 18, 2010 3:33 pm

JackBean wrote: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
it isn't what you do that matters but it is how you do it
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Postby JackBean » Thu Mar 18, 2010 3:56 pm

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...
http://www.biolib.cz/en/main/

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Postby mith » Fri Mar 19, 2010 3:04 am

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
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Postby JackBean » Fri Mar 19, 2010 7:13 am

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 ;)
http://www.biolib.cz/en/main/

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Postby sensfan776 » Sun Mar 21, 2010 3:38 pm

Oh God help me I am so confused!!!
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Postby mith » Mon Mar 22, 2010 2:45 am

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