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?

  • $\begingroup$ Where does Google say this? en.wikipedia.org/wiki/Atmosphere_of_Earth $\endgroup$ – Torbjørn T. Mar 7 '16 at 14:53
  • $\begingroup$ google.co.uk/… $\endgroup$ – Matthew Layton Mar 7 '16 at 14:53
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    $\begingroup$ It isn't strictly speaking Google that says it. Anyways, that page describes Earth's mass, not its atmosphere. $\endgroup$ – Torbjørn T. Mar 7 '16 at 14:56
  • $\begingroup$ Further, judging by en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust compared to en.wikipedia.org/wiki/… it looks like the site Google highlights actually refers to the crust, not the entire earth. $\endgroup$ – Torbjørn T. Mar 7 '16 at 14:59
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    $\begingroup$ 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). $\endgroup$ – Luaan Mar 9 '16 at 15:25

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

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    $\begingroup$ Note that the pie charts are by mass. Different pie charts would result if the elements were expressed in terms of molar fraction. In that sense, oxygen is the most abundant element on the Earth (including the atmosphere), while iron by more than a factor of three. $\endgroup$ – David Hammen Mar 7 '16 at 20:03
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    $\begingroup$ What does "bulk silicate earth" mean? $\endgroup$ – naught101 Mar 8 '16 at 0:27
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    $\begingroup$ @naught101 crust + mantle $\endgroup$ – Gimelist Mar 8 '16 at 4:16
  • $\begingroup$ This seems like a good answer, but could you add a source please? That would also make clear if we speak about numbers or mass fractions. $\endgroup$ – AtmosphericPrisonEscape Oct 18 '18 at 21:42

"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.

  • $\begingroup$ 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 $\endgroup$ – Gimelist Mar 8 '16 at 4:15
  • $\begingroup$ 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. $\endgroup$ – haresfur Mar 8 '16 at 5:07
  • $\begingroup$ And also MgO, CaO, Al2O3, FeO and whatever. Singling out SiO2 out of all oxides misses the point. $\endgroup$ – Gimelist Mar 8 '16 at 7:52
  • $\begingroup$ @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. $\endgroup$ – jamesqf Mar 8 '16 at 22:03
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    $\begingroup$ This is interesting and I'm going to spin off as a new question. $\endgroup$ – haresfur Mar 10 '16 at 23:13

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