What structure do all of these elements have in common?
a) number of protons
b) numbers of electrons
c) number of energy levels
d) number of valence electrons

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.