An atom of element X has one more shell of electrons than an atom of beryllium, but it has one less valence electron than beryllium. Which element is element X?

Beryllium is in period 2, so its atoms have two electron shells, and it has two valence electrons. Element X must have three shells and one valence electron. Sodium is in period 3 and has one valence electron, so the correct choice is sodium (Na).

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