what is penultimate shell.

Dear Firsenbat

The word penultimate means that which comes next to last. Many use the term incorrectly. The penultimate shell would be the shell below the one presently being filled by electrons. If you have an electron shell configuration of 2, 8, 1, the shell configuration for Na, the penultimate shell would be the second shell. You could say that the 3rd shell, in this case, is the ultimate shell.

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