Sciencemadness Discussion Board

Atomic mass question

aga - 28-2-2016 at 01:44

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.

j_sum1 - 28-2-2016 at 01:50

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.

aga - 28-2-2016 at 01:55

Quote: Originally posted by j_sum1  
Let me introduce you to isotopes.
http://www.ptable.com/#Isotope

Holy crap ! Even H has isotopes !

I thought i knew something about isotopes. Obviously not.

Thanks for the explanation j_sum1.

j_sum1 - 28-2-2016 at 02:02

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...

aga - 28-2-2016 at 02:11

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]

j_sum1 - 28-2-2016 at 02:29

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.

blogfast25 - 28-2-2016 at 06:58

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! :D

[Edited on 28-2-2016 by blogfast25]

j_sum1 - 28-2-2016 at 14:46

If only we were all as wrong as Lavoisier! :D

annaandherdad - 28-2-2016 at 15:02

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.

aga - 28-2-2016 at 15:06

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.

blogfast25 - 28-2-2016 at 16:51

Quote: Originally posted by annaandherdad  

It depends on whether you let the heat escape. If you don't, it weighs exactly the same as before the reaction.


Yes. I implied heat left of course. Ta.

blogfast25 - 28-2-2016 at 16:53

Quote: Originally posted by aga  
Surely is all about Conservation of Stuff, rather than aribrary views of the 'stuff' being, mass, matter, energy etc.



'arbitrary views'? You don't think it's important what form 'stuff' takes? :D

blogfast25 - 28-2-2016 at 17:07

Quote: Originally posted by j_sum1  
If only we were all as wrong as Lavoisier! :D


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]

j_sum1 - 28-2-2016 at 17:35

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.)