Answer:
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.