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Given: $1s^2 2s^2 2p^6 3s^1$

can generally be used to find the electron configuration of an element. There are a few special cases:

1st column, hydrogen + alkali metals: $1s^1$

2nd column, alkaline earth metals: $1s^2$

3rd-12th columns, transition metals: $d^1 -> d^6$

13th-18th columns, Boron - neon: $p^1 -> p^6$

Inner transition metals: $f^1 -> f^14$

Look at the last set of electrons (valence) in your example: $3s^1$

This means the element falls within the column 1: alkali metals. The $3$ represents the number of energy levels (periodic table row).

This means $1s^2 2s^2 2p^6 3s^1$ = the electron configuration for sodium (Na).

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