The ground-state configuration of a negative ion of a halogen

The ground state configuration for the negative ion of a halogen. ... A possible excited state electronic configuration. 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^1 6s^1. Its symbol is the double solid lin, // or ||, in a cell diagram.

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