Oxygen in water: How is it single if it's always double?

This is simple question, I think. At college (I'm an Intro Chem. student), the professor explained that oxygen is always found in molecules of 2 oxygen atoms combined together - $\ce{O2}$. On the other hand, molecule of the water is $\ce{H2O}$. So I don't get, how is it that O is single in this molecule, if it's always found coupled? 




I think you are confusing something here: **Molecular** oxygen is most commonly found as the gas we know, $\ce{O2}$ and ozone, $\ce{O3}$. However, this doesn't limit it to only that. Oxygen can appear in a wide variety of (bio-) chemicals and there, it only needs two bonds to other atoms (or a double bond to one atom).


Good examples for this are:


* The aforementioned water $\ce{H2O}$ where the oxygen is bonded to two hydrogen atoms.
* One of my most loved molecules: Ethanol. Here, the oxygen is bonded to a hydrogen and a carbon atom.
* In an ester, which usually are quite fragrant compounds, there actually are two oxygen atoms. But they are not connected to each other. Instead, one has a double bond and the other two bonds, each of them to carbon respectively.


I hope this has made it clear that oxygen doesn't have to bond to itself (but it can, see [peroxides](http://en.wikipedia.org/wiki/Peroxide)), and that oxygen doesn't have to come in pair of twos for the molecule to not fall apart.


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Free single oxygen atoms are unstable and will bind to another free single oxygen atom that is present in the air because this is more stable.(So you get $\ce{O2}$) When you look at electron configurations then you see that both oxygen atoms share their electrons with each other and so have the same electron configuration as Neon, which is very stable: $\ce{1s^2 2s^2 2p^6}$
This is also the same for other non metals such as nitrogen gas : $\ce{N2}$ and also chlorine gas ($\ce{Cl2}$) and fluorine gas ($\ce{F2}$)


Now, the oxygen atom in a water molecule is bonded to not another oxygen molecule, but to two hydrogen atoms.($\ce{H2O}$) The oxygen atom has a **partial** negative charge of -2 and both hydrogen atoms have a positive **partial** charge of +1. This has as a result that all atoms once again have stable electron configurations : oxygen once again has a stable electron configuration, equal to Neon. 


$\ce{O2}$ is the pure element, water is a molecule. But they both achieve stable electron configurations. Just with different atoms.


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