aga
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Atomic mass question
Hydrogen has 1 Proton + 1 Neutron + 1 Electron, i.e. 3 particles.
This implies that set of 1(P+N+E) has an atomic mass of 1.008 g mol<sup>-1</sup>.
Helium has 2(P+N+E) implying that the mass should be 2.016 g mol<sup>-1</sup> yet it is almost double: 4.003
Ignoring that and basing the next step on the atomic mass of He, 1(P+N+E) = 2.0015, so Lithium should be 3x 2.0015 = 6.0045, yet it is 6.94 g
mol<sup>-1</sup>, representing a deviation of almost 1g per mol.
Perhaps somebody could offer an explanation, or point out where the logic/maths is flawed.
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j_sum1
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Let me introduce you to isotopes.
http://www.ptable.com/#Isotope
Some 92.4% of Li atoms have four neutrons and hence an atomic mass of roughly 7. (Most of the rest of Li atoms have three neutrons.) Hence, by
weighted average, Lithium has an atomic mass of 6.94.
The same can be done for any element on the table. Most have more than a few stable isotopes.
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aga
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Holy crap ! Even H has isotopes !
I thought i knew something about isotopes. Obviously not.
Thanks for the explanation j_sum1.
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j_sum1
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No probs.
Concerning hydrogen, it is possible to buy gas discharge tubes of deuterium. You can also get little glowing vials of tritium (plus some fluorescing
compounds.) And for some fun, I will recommend Cody'sLab on YT. He made himself some heavy water (D2O -- that is the hydrogen isotope with mass of
2.) It took some time but it was all using low tech methods that anyone could do.
Amazing how easy it is to form a gap in your knowledge and yet not know about it.
Most of my journey over the last 18 months has been spent plugging gaps Undoubtedly there are more to fill. (I know there is more to learn on the
Nernst equation, but I have not had a need to go there.) Soon I will be ready to progress onto new ground...
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aga
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I just realised that Hydrogen has no neutron either, so that's one major error right there at the start.
DOH !
Yes, it is easy to imagine some 'advancement' is happening only to find a vast gaping hole in one's knowledge around every corner.
I'll go have a look at Cody's lab. Cheers.
[Edited on 28-2-2016 by aga]
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j_sum1
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Yeah, Cody. Interesting guy. I have known about him for a while but only recently given his channel a decent viewing.
Trained as a geologist. Lives on old family land with some old mines and he has obtained rights to dig. A lot of practical know-how and a penchant
for doing stuff in a DIY fashion. Chemistry topics include some refining with a bit of EM. Working with gold, silver zinc, lead and mercury -- most
of which is found on his land. Almost a (good) mythbusters kind of experimentation for some things. I recently watched his experiment inspired by
the man-hole cover that was possibly launched into space. He also has done a lot of work with bees and has interests in every area of science. I
think you will find him a bit of a tonic, aga.
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blogfast25
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Quote: Originally posted by aga  | Hydrogen has 1 Proton + 1 Neutron + 1 Electron, i.e. 3 particles.
This implies that set of 1(P+N+E) has an atomic mass of 1.008 g mol<sup>-1</sup>.
Helium has 2(P+N+E) implying that the mass should be 2.016 g mol<sup>-1</sup> yet it is almost double: 4.003
Ignoring that and basing the next step on the atomic mass of He, 1(P+N+E) = 2.0015, so Lithium should be 3x 2.0015 = 6.0045, yet it is 6.94 g
mol<sup>-1</sup>, representing a deviation of almost 1g per mol.
Perhaps somebody could offer an explanation, or point out where the logic/maths is flawed. |
With sole exception of protonium (<sup>1</sup>H) calculating atomic masses of isotopes from particle masses also requires taking into
account nuclear binding energy, i.e. the so-called mass deficit:
https://en.wikipedia.org/wiki/Nuclear_binding_energy
Amazing factoid: did you know that after an exothermic chemical reaction the reaction products also weigh a teensie, teensie, teensie, teensie,
teensie, teensie, wee bit lighter than the reactants? See:
http://physics.stackexchange.com/questions/11449/conversion-...
Antoine Lavoisier was a teensie, teensie, teensie, teensie, teensie, teensie, wee bit wrong! 
[Edited on 28-2-2016 by blogfast25]
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j_sum1
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If only we were all as wrong as Lavoisier!
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annaandherdad
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| Quote: Originally posted by blogfast25 |
Amazing factoid: did you know that after an exothermic chemical reaction the reaction products also weigh a teensie, teensie, teensie, teensie,
teensie, teensie, wee bit lighter than the reactants? See
[Edited on 28-2-2016 by blogfast25] |
It depends on whether you let the heat escape. If you don't, it weighs exactly the same as before the reaction.
Any other SF Bay chemists?
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aga
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Surely is all about Conservation of Stuff, rather than aribrary views of the 'stuff' being, mass, matter, energy etc.
What goes in is what comes out, so Lavoisier was pretty much right.
If the heat can't escape, then there's a whole new universe right there, inside which it is all warm and cosy.
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blogfast25
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Yes. I implied heat left of course. Ta.
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blogfast25
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| Quote: Originally posted by aga | Surely is all about Conservation of Stuff, rather than aribrary views of the 'stuff' being, mass, matter, energy etc.
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'arbitrary views'? You don't think it's important what form 'stuff' takes?
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blogfast25
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Its alleged that when they executed him someone said: 'it takes a 100 years for a mind like that to some along and 5 seconds to chop his head
off!'
[Edited on 29-2-2016 by blogfast25]
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j_sum1
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Yup the French Revolution was not pretty and its anti-intellectualism was one of the more disturbing and destructive features.
(Mostly my fault, but this thread on atomic mass has taken a rather circuitous route.)
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