I know that this might sound wrong but, in the reaction

$$\ce{2 Mg(s) + O2(g) → 2 MgO(s) + energy}$$

isn't oxygen supposed to be oxidised as it has excess electrons $(\ce{O^2-})$ and can donate a few to magnesium as it has less electrons $(\ce{Mg^2+})?$

I have searched for it in almost all books and it is written that magnesium gets oxidised. Please clear my misconception. It would be helpful if I can get a reaction mechanism for this, because it includes transfers of electrons.

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    $\begingroup$ In layman's terms both magnesium and oxygen within MgO are "happy" since both $\ce{O^2-}$ and $\ce{Mg^2+}$ have configuration of neon, a noble gas. For a more in-depth explanation probably have a look at the elements' electronegativities or dive into physical chemistry (review free Gibbs energy for the burning process). $\endgroup$ – andselisk Jun 17 '19 at 16:21
  • $\begingroup$ But MgO is formed after the reaction. I am talking about their reaction to achieve the stable state of MgO. $\endgroup$ – Shishir Maharana Jun 17 '19 at 16:28
  • $\begingroup$ But, oxidation means loss of electrons. So, if magnesium is oxidized it has to lose electrons, but it clearly has less electrons, then why will it lose them? $\endgroup$ – Shishir Maharana Jun 17 '19 at 16:37
  • $\begingroup$ Well, then why did magnesium lose 2 electrons and oxygen gain 2 electrons? I mean why did Oxygen take 2 electrons from magnesium? $\endgroup$ – Shishir Maharana Jun 17 '19 at 16:51
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    $\begingroup$ The reactants are neutral Mg and neutral O2. The magnesium doesn't turn into Mg++ until after reaction, and it gets that way by being oxidized. The O2 doesn't get extra electrons and go to 2 O-- until after reaction. The MgO is the product and doesn't do any oxidizing or reducing, so we imagine it to be happy as a stable lattice of Mg++ and O-- ions. $\endgroup$ – James Gaidis Jun 18 '19 at 13:32

I'm unsure the level which this question is being asked, but hopefully this answers your question.

As you pointed out "why did Oxygen take 2 electrons from magnesium" well if you would allow me to be rather 'hand-wavy' about the whole thing the reason is due to a activation energy vs ionization energy and bond-dissociation energy.

If you stick some fresh magnesium in an oxygen rich environment it won't all spontaneously react. This is because there is an amount of energy required for the reaction to happen (I will refer to this as activation energy). Now to make the reaction between magnesium and oxygen gas happen two things need to happen, the magnesium needs to lose two of its electrons (ionization energy) and the oxygen needs to break into two individual oxygen atoms(bond-dissociation energy). The misconception here is that oxygen gas will split into two -2 ions. If you count the number of electrons in one molecule of oxygen gas you get 12 electrons. Two -2 oxygen ions come out to total 16 electrons.

In reality, O2 would split into two neutral oxygen atoms as the oxygen gas is a neutral species not an ion. This is obviously incredibly unstable which is why it will look for the nearest source of electrons to pull from and magnesium's electrons are relatively easy to grab vs the oxygen. Reply if you want any more detail on anything I've said so far (as a lot of it is rather hand-wavy on my end), but if this is for a intro chemistry class I don't want to overload you with more information than you need.

  • $\begingroup$ No, no, don't think so. Actually, I got this question wrong on a test and wanted to find out the detailed reason about it. So, if you think that there is a detailed reason, then feel free to answer. $\endgroup$ – Shishir Maharana Jun 18 '19 at 4:34
  • $\begingroup$ What part of my explanation do you want more detail about? If it is on how the Oxygen is a neutral it's simply because there is no charge on either species. You can determine this by finding the oxidation state of each element on the reactants side vs the product side. Here are some general rules for you to follow to determine this: socratic.org/questions/… $\endgroup$ – Sabata Jun 18 '19 at 18:20

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