Why metals react with non-metals
 

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The question

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Explain in terms of the electronic structure of magnesium and oxygen atoms why magnesium reacts with oxygen to give magnesium oxide:

2Mg (s)  + O2 (g)  →   2MgO (s)


GCSE contents
Reactions catalogue
Extraction of metals

The usual response

 

When asked this question students sometimes give an answer something along the lines of the following:

  • The magnesium atom has electronic structure 2,8,2 and wants to lose the outer two electrons to become stable;

  • The oxygen atom has electronic structure 2,6 and wants to gain two electrons to become stable;

  • Both ions now have noble gas electronic structures and are stable.

The reason that students give this answer is because it is what they have been taught and because it is what many books imply.

Every statement in the list above is wrong.

What is particularly irritating about this nonsense is that the correct model is not at all difficult to understand. Furthermore, atoms cannot 'want' anything; they can't think and certainly have no desires.

The word 'stable' should be reserved for use only when you want to know where to keep a horse. Otherwise it is a very dangerous word in Chemistry. Is sodium stable? Probably half of the students who are asked this will say that it is not. But sodium, kept away from air and water and anything else that reacts with it will remain as sodium until the end of time. It is perfectly stable. To ask if something is stable is not the same as asking if it is reactive. So if you do want to use the s-word, it must always be qualified. Is sodium stable if kept in a sealed tube in a vacuum? Yes. Is it stable in the presence of water? Most certainly not.


 

How magnesium reacts with oxygen

 

If a metal reacts with a non-metal, the reaction occurs because the ionic compound produced is at a lower energy level than the mixture of the metal and non-metal at the start. Magnesium oxide is at a lower energy level than a mixture of magnesium and oxygen. You can tell this because when magnesium burns it gives out a great deal of energy both as heat and in the intense white light of the flame. Energy is lost, so the magnesium oxide must have a lower energy level.

Consider the following - all true.

  • Magnesium, Mg, forms the magnesium ion Mg2+, by losing two electrons. The electronic structure changes from 2,8,2 in Mg to 2,8 in Mg2+ . This process requires energy because the negative electrons have to be pulled away from the attractive positive charge of the nucleus. So the ion is at higher energy than the atom - it is energetically less stable.

  • Oxygen forms the oxide ion by gaining two electrons. The electronic structure changes from 2, 6 in the O atom to 2,8 in the O2- ion. This process requires energy. Addition of one electron to form O- gives out energy since the electron is attracted to the nucleus. Adding a second to give O2- requires more energy because the incoming electron is repelled by the one already there. The oxide ion is at a higher energy than the atom - it is energetically less stable.

Most accounts of bonding stop here. However:

The driving force for bonding is what happens next.

Once the magnesium and the oxide ions are formed, they attract one another because they are of opposite charge. The force with which they attract depends on:

  • The distance between the ions. The shorter this distance the stronger the force - halving the distance multiplies the force by four.

  • The charges on the ions. The force is proportional to the charges multiplied together, so a 2+ and a 2- ion will attract with four times the force of a 1+ and 1- ion at the same distance.

  • When ions attract they give out energy. If this is hard to see, you only have to imagine pulling them apart again - you would certainly have to put in energy, and it is the opposite process.

  • The energy given out when the ions attract and bond is much greater than the energy that was taken in to form the ions in the first place.


 

Maximising the attraction

 

Why does the formation of ions usually result in the formation of an electron octet?

It is not because the atoms 'want to be stable'. I hope you have by now abandoned this idea. It is that formation of the octet leads to the maximum possible attraction between the ions.

It takes three times as much energy to make Mg2+ than it does to make Mg+. But all magnesium compounds contain Mg2+ which has an octet of electrons in its outer shell; none contain Mg+. The reason is that the Mg2+ ion is only half the size of the Mg+ ion since the atom loses a complete electron shell in order to form it. As a result Mg2+ can get much closer to any anion than Mg+ could. The resulting increase in attraction more than makes up for the extra energy needed to form the 2+ ion.

In the case of O2-, the ion is somewhat larger than the oxygen atom from which it came and is also larger than the O- ion. However, the increase in size (which weakens any attractions) is more than compensated by the higher charge which strengthens the attractions.

Formation of the octet therefore leads to the maximum possible attraction between the ions for the minimum expenditure of energy in forming them. It has nothing to do with ions being 'more stable'.

 

JRG Beavon 2007