The reason why beryllium loses electrons while sulfur gains electrons when forming ionic bonds lies in their electron configurations and their desire to achieve a stable octet (8 valence electrons).

Beryllium (Be):

* Electron configuration: 1s² 2s²

* Valence electrons: 2

* Tendency: Beryllium has only 2 valence electrons. To achieve a stable octet, it's easier for it to lose these 2 electrons, leaving it with a full 1s² shell. This results in a +2 charge, forming a Be²⁺ ion.

Sulfur (S):

* Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁴

* Valence electrons: 6

* Tendency: Sulfur has 6 valence electrons. To achieve a stable octet, it's easier for it to gain 2 electrons, completing its 3p orbital. This results in a -2 charge, forming a S²⁻ ion.

In summary:

* Beryllium has a lower electronegativity (tendency to attract electrons) than sulfur.

* It's energetically favorable for beryllium to lose 2 electrons and sulfur to gain 2 electrons, leading to a stable ionic compound (like BeS) where the oppositely charged ions are attracted to each other.