I thought in the third shell there are 18 electrons, but why are they gaining only 2 electrons?? Pls tell me... I have exam tomorrow.

Answerδh > 0explanationc₆h₆  (s)  → c₆h₆ (l)if you are changing it from a solid to a liquid, you are pretty much melting the solid. this requires heat energy. therefore, the reaction is endothermic (takes in energy); the system enthalpy increases as the reaction proceeds: δh > 0.

In any atom of any element, you will find protons and neutrons in the nucleus of any atom 

"electrical energy" transforms to "thermal energy".hope my answer has you : )