An atom of an element X has one more shell of electrons than an atom of berylium but it has one less valence electron thatn berylium which element is X

Sodium

Explanation:

Sodium in period 3 has three electron shells. Recall that the period to which an element belongs shows the number of electron shells present in each atom of the element.

Beryllium is in period two, hence it possesses only two electron shells. However, beryllium is a member of group two hence it has two valence electrons in its outermost shell.

Sodium on the other hand has only one electron in its outermost shell since it is a member of group one. Hence the element must be sodium.