Energy changes and bonding

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What makes a full shell most stable?

Orion replies ...
You could rephrase this question to ask: 'Why does the octet rule work?' Of course it often does not work but in some cases it is possible to give a very detailed answer in terms of the energy changes involved.
 
The short answer is that in many cases the energy needed to break bonds or form ions is more than compensated by the energy given out when new bonds are formed if the compound produced has an electron configuration with a full shell (or full s and p orbitals in the third shell).
 
Stability
Any question about stability has to be answered using ideas from thermochemistry. Also in chemistry, questions about stability are always relative. To take a specific example related to this question, why does magnesium react with chlorine to form magnesium chloride with the magnesium in the 2+ state rather than the 1+ state or the 3+ state.
 
Electron configurations:

Mg³⁺: 2.7
 
In the case of ionic compounds it is possible to answer questions of stability with the help of Born-Haber cycles. It is possible to draw up a Born-Haber cycle for MgCl₂ and compare it with estimates of what the cycles for MgCl and MgCl₃ would be like.
 
Roughly speaking the stability of an ionic compounds is a balance between the energy needed to ionise the metal ion and the energy given out when the ions come together to form a crystal lattice (the lattice energy).
 
Here is an outline of the analysis for magnesium chloride showing why the ion with eight electrons in the outer shell is the one that forms.
 
Why is MgCl₂ more stable than MgCl?
MgCl is more stable than its elements but MgCl₂ is even more stable. Why? Forming 2+ ion from Mg atoms needs more energy than forming 1+ ions. However losing both outer electrons to give an ion with a full outer shell removes the outer shell and makes the 2+ ion considerably smaller than the 1+ ion. So the Mg²⁺ ions have a larger charge and can pack much closer to chloride ions. This means that the lattice energy for MgCl₂ is much greater than for MgCl. The increase in lattice energy more than compensates for the greater energy needed to form a 2+ ion.
 
Why is MgCl₂ more stable than MgCl₃?
The outer two electrons in a magnesium atom are screened from the full attraction of the 12+ nucleus by 10 electrons in inner full shells. So the outer two electrons are relatively easy to remove. However electrons in the second shell are only shielded by two electrons so they are held much more strongly. In other words the third ionisation energy of magnesium is very, very much higher than the first and second ionisation energyies.
 
Mg has a larger charge than Mg but it is not much smaller. So, if it existed, MgCl₃ would have a larger lattice energy than MgCl₂ but by no means large enough to make this compound stable.

Risk assessment
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updated: 21 November 2003