Why are full electron shells considered stable?

[keigoh]

I've always heard that when atoms make ions or create chemical blonde, thru try to make their electron configurations the same as that of a noble thereby filling their electron shells and making them stable. Why then, are noble gas election configurations considered stable?

 

[DrDu]

The point is that electrons in the same shell are not very effective in screening each other from the nuclear charge, i.e. in filling up a shell, the electrons see a higher and higher effective nuclear charge, thus increasing the binding energy. This effect is maximal for a filled shell and minimal for a shell with just one or two electrons.