Answer:
Exothermic reaction: In exothermic reaction, energy is transferred to the surroundings, and the surrounding temperature increases, this is known as exothermic reaction. In other words energy exits in exothermic reaction. Some example of exothermic reactions are:
1) Neutralisation reaction.
2) Combustion reaction.
3) Some oxidation reaction.
Endothermic reaction: In endothermic reaction, energy is taken in from the surrounding, and the surrounding temperature decreases, this is known as endothermic reaction. In other words energy enters in endothermic reaction. Some example of exothermic reactions are:
1) Thermal decomposition.
2) Reaction between citric acid and sodium hydrogen carbonate.
The correct answer is D. Using the law of conservation of mass the number of atoms on each side of the equation should be equal. Through introspection, we find that there are 2
atoms on reactant side as opposed 3Cl atoms on product side. If we add a coefficient of 3 on 
we get
.
Now there are 6Cl atoms on reactant side and 2 on product side, hence we add a coefficient of 2 on both 
and 
. The balanced chemical equation is,
You are correct, the atomic number is always equal to the number of protons in the element. It is the relative atomic mass that is equal to the number of protons+neutrons. Electrons have no mass
