查看完整版本 : Can molecular size be determined by its relative atomic mass??

cyw1226 2013-8-6 03:48 AM

Can molecular size be determined by its relative atomic mass??

Can molecular size be determined by its relative atomic mass??  

"That the larger the molecular size, the greater will be the van der waals' forces between  molecules."
However, how can I know which molecule has a greater molecular size??

jmlo 2013-8-6 10:50 AM

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[quote] Can molecular size be determined by its relative atomic mass??  [/quote]
No. Think about CO and C2H4 which have the same molecular mass. Which one is bigger?

[quote] "That the larger the molecular size, the greater will be the van der waals' forces between  molecules."
However, how can I know which molecule has a greater molecular size?? [/quote]
You need two additional pieces of information: (1) The 3D shape of the molecule; and (2) The van der Waals radii of atoms that make up the molecule. With these, the molecular size can be worked out by simple geometrical calculations.

Comparison will be much easier if it is done within a homologous series (e.g. alkane molecules) because the molecules share a similar molecular shape. The change of molecular size due to additional structural unit (e.g. CH2 unit) can be easily justified.

dustbus 2013-8-6 11:02 PM

[quote]原帖由 [i]jmlo[/i] 於 2013-8-6 10:50 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=368762252&ptid=22273153][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

No. Think about CO and C2H4 which have the same molecular mass. Which one is bigger?


You need two additional pieces of information: (1) The 3D shape of the molecule; and (2) The van der Waals r ... [/quote]


jmlo ching,Have you memorized CO and C2H4 have the same molecular mass:smile_41::smile_41:

jmlo 2013-8-6 11:29 PM

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Not really. These two molecules just popped out in my mind when answering your question.

The molecular masses (or molar masses more precisely) of simple molecules can be soon deduced if you remember the atomic masses of several popular atoms such as H, C, N, O, F, Si, P, S and Cl. Of course, there will be small deviations from the actual masses but are acceptable for comparison purpose only.

Zzlaz 2013-8-7 04:05 PM

大約比較到



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dustbus 2013-8-7 08:31 PM

THX:lDD_001: :lDD_001:
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查看完整版本: Can molecular size be determined by its relative atomic mass??