What is the chemical equation when Na2SO4 decahydrate reacts in the atmosphere and becomes efflorescent?

Answer 1

Think about what is happening. Under hot, dry conditions, the decahydrate can lose water to make anhydrous sodium sulphate:

Na2SO4.10H2O(s) = Na2SO4(s) + 10 H2O(vapour)

But perhaps you really mean "deliquescent", which can happen to sodium sulphate in moist air. This is simply the sodium sulphate dissolving in water from the atmosphere.