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As I recall a Lewis acid is a substance that can accept an electron pair from a donor and oxygen seems to be able to do that. For example:

$\ce{H2 + O -> H2O}$

So, is oxygen a Lewis acid?

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Usually the terms Lewis acid and Lewis base are defined within the context of a specific chemical reaction. So in a reaction, if there is lewis acid there must be a lewis base.

Lone-pair acceptors are lewis acids where as lone-pair donors are lewis bases.

Some compounds, such as $\ce{H2O}$, are both Lewis acids and Lewis bases, because they can either accept a pair of electrons or donate a pair of electrons, depending upon the reaction.

In reaction $\ce{H2 + O -> H2O}$, you can say that oxygen is lewis acid but that would be contradictory since hydrogen doesn't have a lone pair to donate.

Also, Oxygen acts as a lewis base by donating a lone pair to hydrogen ion when Acids are dissolved in water.

To be more clear on Lewis Acid/Base concept, see this.

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    $\begingroup$ The reaction $\ce{2H2 + O2 -> 2H2O}$ is a redox reaction (also if you go to atomic recombination), there is no Lewis acid or base, but electron transfer and this results in the contradiction you observed. $\endgroup$ – Martin - マーチン Jul 19 '14 at 14:06
  • $\begingroup$ @Martin : I kinda felt like writing it but I doubted it since it's been a while since I left field of chemistry. :) $\endgroup$ – ashu Jul 20 '14 at 5:04
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Oxygen is a Lewis base (that too a weak one), not a Lewis acid.

REASON: It has lone pair of electrons, which can be donated to electron-deficient species (Lewis acids). The reason which makes it a weak lewis base is that the electronegativity of Oxygen is 3.0 on the Pauling Scale, which increases its tendency to hold on to its lone pair of electrons. Due to this difficulty in parting lp of electrons from Oxygen, it is regarded as a weak Lewis base.

Furthermore, due to its high electronegativity, its repulsion to incoming electrons is very high, and hence it doesnt not accept electrons as easily as a Lewis acid would, which leads to the conclusion that Oxygen cannot be regarded as a Lewis acid in day-to-day terms.

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