Noble gases' outer shells are already filled with 8 electrons (other than He, which has 2, but is still filled and stable). Atoms bond with other elements to fill their outer shell, but they don't need to do that and are already stable.
A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention, because the enthalpy of the products is lower than the enthalpy of the reactants of the system. The enthalpies of these reactions are less than zero, and are therefore exothermic reactions
Photosynthesizing plants and algae convert light energy into chemical energy, which then gets passed through the food web to plant eaters, flesh eaters, and ultimately to scavengers and decomposers.
So the empirical formula is Mg3N2
D has a total of four significant figures.
