Can isotopes have different atomic numbers

No

Explanation:

Atomic number represents the identity of atoms

using number of protons which is equal in isotopes.

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.