Closed shells and negative ions

Under suitable conditions (more details will be given in section 2.24), it is possible to attach an extra electron to a neutral atom, and the resulting, very loosely bound object is called a negative ion. It is then an obvious step to search for stability of negative ions with closed shells, whose outermost configuration is that of a rare gas. One might expect that they would mirror the behaviour of the periodic table for neutral atoms, but this turns out not to be the case. 

The H- ion possesses a single bound state and has an ionisation poten- 

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