Given that the reaction consumes 462 kJ of energy. It means that the reaction is a endothermic reaction.
In the energy profile of the endothermic diagrams, The reactants are at a very low level as compared to the products and hence, energy is supplied to overcome this difference.
Hence, The energy levels of products are <u>higher</u> than the energy level of reactant.
Hence, already stated, heat is required by the reaction and thus heat is written to the reactant side which is to the <u>left</u> side of the equation.
The number of electrons in a neutral atom is equal to the number of protons. The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus.
