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When I was in school, I was taught that we need oxygen to breathe, but it actually constitutes only a small fraction of the atmospheric composition, and that nitrogen constituted the largest fraction of the atmosphere's composition.

This quick Google search mentions oxygen as the most abundant element.

How can this be reconciled?

Gimelist
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Matthew Layton
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  • Where does Google say this? https://en.wikipedia.org/wiki/Atmosphere_of_Earth – Torbjørn T. Mar 07 '16 at 14:53
  • https://www.google.co.uk/?gfe_rd=cr&ei=3pXdVtz-NYvS8Aelgb_oDg&gws_rd=ssl#q=most+abundant+element+on+earth – Matthew Layton Mar 07 '16 at 14:53
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    It isn't strictly speaking Google that says it. Anyways, that page describes Earth's mass, not its atmosphere. – Torbjørn T. Mar 07 '16 at 14:56
  • Further, judging by https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust compared to https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Abundance_of_elements_in_the_Earth it looks like the site Google highlights actually refers to the crust, not the entire earth. – Torbjørn T. Mar 07 '16 at 14:59
  • By mass? Please write better questions (the title is not even a question). [Edit] it and put in the actual quote. – Jan Doggen Mar 07 '16 at 16:17
  • That's because rocks are mostly made out of oxygen (by volume, atomic proportions, and mostly by mass as well). It's a bit weird considering that oxygen is a gas, but when you combine it with most other elements it kind of "glues" them together as solids. – Gimelist Mar 08 '16 at 04:19
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    Earth's atmosphere is just a tiny part of the Earth as a whole. Your teacher talked about the atmosphere only, where oxygen isn't the most abundant. In the planet as a whole, oxygen dominates by far (or, taken by mass, iron - oxygen is still second, though). Look around you - all of that soil, rock etc. is most likely formed of oxygen. Also, look at the oceans - all that water contains huge amounts of oxygen (1:2 by part, 8:1 by mass). – Luaan Mar 09 '16 at 15:25
  • No, it's (surmised to be) iron. Please edit the title to include the word atmosphere or crust. – Mazura Mar 10 '16 at 16:32
  • Perhaps a better question would be: What happened to the carbon, neon and nitrogen when you compare "surmised" Milky Way abundances versus "surmised" Earth abundances? Just orbiting the sun in interplanetary space? – Jack R. Woods Apr 09 '18 at 16:43
  • I am presuming that Jupiter is expected to mirror solar system abundances so my comment above would pertain to the zone where terrestrial bodies were formed. – Jack R. Woods Apr 09 '18 at 16:48
  • Proposed answer to my own question: It was blown away by solar event that cleared most of the solar system after the formation of the terrestrial planets. (It's somewhere "out there".) – Jack R. Woods Apr 09 '18 at 17:16
  • Look at the dang stars in the Universe. Stars are composed of mainly hydrogen. The vast clouds of star stuff out there is hydrogen. No one quoted hydrogen? – stormy Oct 18 '18 at 21:33

2 Answers2

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Both of them.

The composition of the atmosphere, crust, mantle, core and bulk earth are all notably different.

The atmosphere is composed of ~78% nitrogen and ~21% oxygen, with small amounts of other gases.

enter image description here

The bulk composition of the earth by weight is mostly, iron, oxygen, silicon and magnesium, in that order, with all the other elements making up only about 5% of the earth's weight. Most of the earth's iron is in the core, which is about 85% iron. The rest of the earth is dominated by oxygen and silicon, primarily in the form of silicate minerals, which consist of $\ce{SiO4^{4-}}$ tetrahedra linked in different ways and with different cations filling in the gaps.

enter image description here

bon
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"How can this be reconciled?" In two words: silicon dioxide :-)

Yes, that's simplistic, but reflects the fact that virtually all the oxygen occurs in chemical combinations with other elements, not as free oxygen. The same is true for other elements in the crust & mantle.

jamesqf
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  • Actually, silicon oxide (as quartz or other silica minerals) is only that common in the uppermost part of the crust. There is hardly any silica, if at all, more than a few tens of km deep – Gimelist Mar 08 '16 at 04:15
  • Unless you are talking about normative oxide analysis, Michael, instead of quarts and other mineral forms. There is a lot of silicon bonded to oxygen that is counted as silicon dioxide. – haresfur Mar 08 '16 at 05:07
  • And also MgO, CaO, Al2O3, FeO and whatever. Singling out SiO2 out of all oxides misses the point. – Gimelist Mar 08 '16 at 07:52
  • @Michael: No, it makes a point (though maybe you missed it :-)), which is that virtually all the oxygen is bound up in compounds. Indeed, the only reason there's a significant amount of free oxygen in the atmosphere is photosynthesis. – jamesqf Mar 08 '16 at 22:03
  • @jamesqf in compounds that are not necessarily silicon oxide. Therefore, there is no reason to single it out. Read my previous comment. – Gimelist Mar 09 '16 at 01:51
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    This is interesting and I'm going to spin off as a new question. – haresfur Mar 10 '16 at 23:13