# Avodadro's number **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [June 2, 2002, 6:48pm UTC](https://boards.straightdope.com/t/avodadros-number/112030 "2002-06-02T18:48:23Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![PosterChild](https://avatars.discourse-cdn.com/v4/letter/p/e274bd/32.png) [@PosterChild](https://boards.straightdope.com/u/PosterChild)\ **Post date:** [June 2, 2002, 6:48pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/1 "2002-06-02T18:48:23Z")
I probably should be able to remember this from college/high school, but I can’t. In [this column](http://www.straightdope.com/mailbag/mavogadro.html) Son of Dex explains Avogadro’s number pretty well, but I got confused when he said: > [@](#): > > Avogadro’s number (also called “a mole”) is roughly 6.022x1023. **The number is precisely calculated so that when you have one mole of atoms of a given molecular weight M, it weighs M grams.** > The chemists are still with me, but they aren’t the ones asking the question. So let’s take an example. Hydrogen gas, H2, has a molecular weight of 2 g/mol. Thus, if you had one mole, (6.022x1023 molecules) of hydrogen gas, it would weigh two grams. Similarly, one mole of carbon dioxide, with a molecular weight of 44 g/mol, would weigh 44 grams. > {my bolding} It sounds circular to me. If a mole were defined as half of what it is now, then H[sub]2[/sub] would weigh 1 g/mol and 1 mole would weigh 1 gram and everything would still work out, right? (I assume not, but don’t know why.) ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [June 3, 2002, 1:50am UTC](https://boards.straightdope.com/t/avodadros-number/112030/2 "2002-06-03T01:50:10Z")
Uh, what do you mean by “working out”? What possible use would it be to have a number that gives you **half** of the gram-molecular weight? Avogadro’s number is a measurement, the number of particles in a mole. The definition of a mole is one of convenience, so that you can convert from molecular weights to mass without arithmetic. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 3, 2002, 6:53pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/3 "2002-06-03T18:53:37Z")
> [@](#): > > Hydrogen gas, H2, has a molecular weight of 2 g/mol. There’s the problem, right there. Hydrogen has a _molar_ weight of 2 g/mol. It has a _molecular_ weight of 2 amu/molecule. An amu is (approximately) the mass of a proton or neutron. In other words, there are two nucleons in a molecule of hydrogen, so a mole of hydrogen is two grams. There are 32 nucleons in a molecule of normal oxygen, so a mole of oxygen is 32 grams, etc. ---
**Author:** ![Jerevan\_Somerville](https://avatars.discourse-cdn.com/v4/letter/j/c37758/32.png) [@Jerevan\_Somerville](https://boards.straightdope.com/u/Jerevan_Somerville)\ **Post date:** [June 4, 2002, 10:09pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/4 "2002-06-04T22:09:57Z")
**Chronos** has gotten to the gist of it: one mole of protons and/or neutrons (nucleons) weighs one gram – or close enough for government work. Think of Avogadro’s number as a wonderful convenience: it allows mass and the number of nucleons in an atom/molecule to be numerically interchangeable. ---
**Author:** ![PosterChild](https://avatars.discourse-cdn.com/v4/letter/p/e274bd/32.png) [@PosterChild](https://boards.straightdope.com/u/PosterChild)\ **Post date:** [June 5, 2002, 8:21am UTC](https://boards.straightdope.com/t/avodadros-number/112030/5 "2002-06-05T08:21:20Z")
Thanks. I understand now. 🙂 PC ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [June 9, 2002, 3:52am UTC](https://boards.straightdope.com/t/avodadros-number/112030/6 "2002-06-09T03:52:56Z")
Follow-up question: why the word “mole”? where does it come from? 😕 (and, how much would a mole of moles weigh? :eek: ) ---
**Author:** ![mok](https://avatars.discourse-cdn.com/v4/letter/m/edb3f5/32.png) [@mok](https://boards.straightdope.com/u/mok)\ **Post date:** [June 9, 2002, 8:15pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/7 "2002-06-09T20:15:08Z")
> [@](#): > > \*Originally posted by Northern Piper \* > \*\*Follow-up question: why the word “mole”? where does it come from? \*\* From the word “ **mole** cule”. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [June 10, 2002, 4:46pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/8 "2002-06-10T16:46:24Z")
> [@](#): > > \*Originally posted by Northern Piper \* > \*\*Follow-up question: why the word “mole”? where does it come from? 😕 (and, how much would a mole of moles weigh? :eek: ) \*\* To put it most simply, it comes from Latin: _moles_, “mass” (_molecule_ is the diminutive of _moles_). However, the word “mole” is actually short for _molekulargewicht_, German for “molecular weight.” Properly defined, it’s “the amount of pure substance containing the same number of elementary entities as there atoms in 12 grams of carbon-12.” Since there is no “elementary entity” called a mole, there can be no such thing as a mole of moles. If what you mean is “how much would Avogadro’s number, squared, of something mass?”, all I can say is, it masses 6 x 10[sup]23[/sup] times the molar mass of whatever it is. Hydrogen, f’rinstance, would be 6 x 10[sup]20[/sup] kilograms; everything else would mass even more. ---
**Author:** ![The\_Ryan](https://avatars.discourse-cdn.com/v4/letter/t/7feea3/32.png) [@The\_Ryan](https://boards.straightdope.com/u/The_Ryan)\ **Post date:** [June 10, 2002, 10:14pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/9 "2002-06-10T22:14:10Z")
> [@](#): > > \*Originally posted by Nametag \* > Since there is no “elementary entity” called a mole, there can be no such thing as a mole of moles. I understood NP’s question to be “how much would one mole (chemist definition) of moles (the little animals) be?” In that case, the answer would be that a mole of moles would fill up a box with each side being 84 million moles long, which works out to be about the size of the Earth. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [June 10, 2002, 10:36pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/10 "2002-06-10T22:36:29Z")
I assumed that if NP had meant those kind of moles, he’d’ve specified the eastern mole, hairy-tailed mole, star-nosed mole, broad-footed mole, Townsend’s mole, or one of the others. Y’know, “African or European” and all that. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [June 17, 2002, 11:09am UTC](https://boards.straightdope.com/t/avodadros-number/112030/11 "2002-06-17T11:09:40Z")
I can’t believe nobody has fixed the thread title yet. Shouldn’t that be _Ava **c** ado’s Number_? 😉 ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [June 17, 2002, 11:57am UTC](https://boards.straightdope.com/t/avodadros-number/112030/12 "2002-06-17T11:57:25Z")
We only bother to fix thread titles if they’re misleading or enormously unclear (like a threat title “My Question”, say). :: Singing (to tune of “Oklahoma”):: A—vacado, It’s the finest fruit that’s known to man! It’s so bright and clean, So nice and green; And it’s fresh, not taken from a can! ---
**Author:** ![tracer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tracer/32/20578_2.png) [@tracer](https://boards.straightdope.com/u/tracer)\ **Post date:** [June 17, 2002, 7:15pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/13 "2002-06-17T19:15:23Z")
Chronos wrote: > [@](#): > > There’s the problem, right there. Hydrogen [gas] has a _molar_ weight of 2 g/mol. It has a _molecular_ weight of 2 amu/molecule. Actually, it’s worse than that. Hydrogen gas, H[sub]2[/sub], has a molar weight of 2.0158 g/mol, and a molecular weight of 2.0158 amu/molecule. And this little difference, this little deviation from exactly 2.00000000 g/mol, isn’t due to the 2 electrons. They only contribute about 0.001 g/mol. The difference is because, despite the fact that a [sup]12[/sup]carbon nucleus is made up of six protons and six neutrons, it **weighs slightly less** than six protons and six neutrons taken separately. And our mole/amu system is based around [sup]12[/sup]carbon. Why is a [sup]12[/sup]carbon nculeus lighter than the sum of its parts, I hear you cry? Why, because of binding energy! Lots and lots of binding energy! If you take six protons and six neutrons and slam them together to make a [sup]12[/sup]carbon nucleus\*, you release a **tremendous** amount of energy in the process. It’s called nuclear fusion. You release so much energy, in fact, that it lowers the mass of what’s left over. E = mc[sup]2[/sup] and all that. Thus, the [sup]12[/sup]carbon nucleus, which is essentially the “ashes” of a 6-proton-6-neutron fusion reaction, weighs a little bit less than 6 protons and 6 neutrons. [sub]\*) Well, okay, you can’t form a [sup]12[/sup]carbon nucleus **directly** by slamming 6 protons and 6 neutrons into each other. For one, bare neutrons only have a half-life of 15 minutes. You can, however, bash 2 protons into each other to form a deuteron (a heavy hydrogen nucleus), then smash another proton into the deuteron to form a [sup]3[/sup]helium nucleus, then smash this into another [sup]3[/sup]helium nucleus to form an alpha particle (a [sup]4[/sup]helium nucleus) and release a couple of protons as by-products, **then** you can slam three of those alpha particles together to form your [sup]12[/sup]carbon nucleus.[/sub] ---
**Author:** ![K2Rage101](https://avatars.discourse-cdn.com/v4/letter/k/c57346/32.png) [@K2Rage101](https://boards.straightdope.com/u/K2Rage101)\ **Post date:** [June 21, 2002, 1:14pm UTC](https://boards.straightdope.com/t/avodadros-number/112030/14 "2002-06-21T13:14:52Z")
not quite =) 2 protons won’t smash into eachother. There is simply too much of what relativity has come to call “the strong force” or “nuclear force” I think this is just semantics - you probably meant to say: “bash a proton into a NEUTRON to form a deuteron (a heavy hydrogen nucleus)” From here on out, your chemistry is a little off. Atomic number, and thus the element, is defined by the proton count. So smashing protons together DOESN’T make the “heavy” version of that element; the heavy version has additional NEUTRONS. i.e., H has 1 proton, 1 electron Deuterium has 1 proton, 1 neutron, 1 electron Tritium has 1 proton, 2 neutrons, 1 electron Helium has 2 protons, 2 neutrons, 2 electrons Alpha Particles are 2 protons, 2 neutrons, no electrons. you can do the theoretical atom smashing from here, ad nauseum. But ask a rocket scientist how often smashing two particles actually yields what they are looking for =)