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size difference of N2 vs O2

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IamSmooth

Fri Jun 14 2013, 03:05PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

It is often mistakenly assumed that "molecular size" correlates directly with "molecular weight". O2 does have a greater
molecular weight (32) than N2 (28), but O2 is actually smaller in size. Thus, O2 fits through the relatively tight passage
ways between polymer chains in the rubber more easily than does N2. The difference is size between O2 and N2 is
very small, only about 0.3 times 10 to the -10th meters (0.00000000003 meters).

The above text is something I recently read. I am preparing to make a PSA. Why is the O2 molecule smaller than the N2 molecule if its atomic weight is bigger? Is it that the electron shell is still not filled, so we don't have a larger shell surrounding the atom? Does having two more protons cause a slightly tighter attraction with the electrons?

Fraggle

Fri Jun 14 2013, 06:02PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Chemistry not my area but remember there`s not really a `shell` there, that`s just a way to visualise the eigenstates of the electrons. The `size` of an atom is the extent of its interactions with other atoms and can`t really be derived by using weight alone as it is down to electromagnetic interactions between instantaneously fluctuating dipoles and stuff.

Reading: Van der Waal`s interaction, Van der Waal volume, Van der Waal equation of state and covalent bond length (if you don`t like spherical cows).

IamSmooth

Fri Jun 14 2013, 08:26PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Fraggle wrote ...

it is down to electromagnetic interactions between instantaneously fluctuating dipoles and stuff.

I understand that part. I was hoping there was a more concrete explaination as to what is phyically happening, unless this is not truly known.

Fraggle

Sat Jun 15 2013, 02:06AM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I`m afraid I can`t answer that properly, I don`t understand it fully. Sorry.

Dr. Slack

Sat Jun 15 2013, 10:07AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

'Size' is a concept that's only really meaningful at large scales, just like trying to figure out what the 'pressure' or 'temperature' of a gas is when you focus down on individual molecules. When you try to nail down what size means, for instance when the coupling between adjacent electron orbitals switches from attractive to repulsive, then you are deep into quantum mechanics. You must start asking questions like 'if I put the particles into this configuration, how do they behave?' If you choose to model that behaviour as a pore size and a molecule size, then the parameters you use in that model must be informed by what the particles actually are observed to do. That a substrate could appear to have smaller pores for a molecule with one set of orbitals than another is perhaps no great surprise.

Mari

Sun Jul 21 2013, 04:39PM

Registered Member #19991 Joined: Sat May 25 2013, 11:21PM
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It's because there is more protons in oxygen(8) than in nitrogen(7). This larger positive charge of the oxygen attracts the electron cloud and makes it smaller than the nitrogen. Sodium has even more protons but it's larger because of the shielding effect of a complete sub shell + 1 electron.

radiotech

Mon Jul 22 2013, 07:50AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Not chemistry, but I have worked in a facility that separated nitrogen from air to make
an oxygen pure enough for a bio reactor cleaning water. Te size of the molecule through
the membrane (blown with large fans) separates the gases

Essentially two huge motors, controls, instrumentation, computer.

The process is called Praxair :

Ash Small

Mon Jul 22 2013, 10:30AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

radiotech wrote ...

Not chemistry, but I have worked in a facility that separated nitrogen from air to make
an oxygen pure enough for a bio reactor cleaning water. Te size of the molecule through
the membrane (blown with large fans) separates the gases

Essentially two huge motors, controls, instrumentation, computer.

The process is called Praxair :

Zeolites can also be used for this purpose:

IamSmooth

Tue Jul 23 2013, 10:04PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I'm building a large-scale PSA nitrogen generator. I'll post pictures and text when it is done.

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