Question

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?

sodium (Na)

boron (B)

magnesium (Mg)

lithium (Li)

Answer 1

The correct answer is Sodium

Answer 2

sodium (Na)

Step-by-step explanation:

The valence electron configuration of the given elements are as follows:

Sodium (Na) = 3s¹

Boron (B) = 2s²2p¹

Magnesium (Mg) = 3s²

Lithium (Li) = 2s¹

The valence electron configuration of beryllium (Be) = 2s²

The outer shell in Na is 3s i.e. n = 3 relative to that of Be where n = 2

Also the valence shell in Na has 1 s electron i.e. one electron lower than Be.