Why are some ions known as poly atomic ions?

NH4+ammonium ionhydroxide ion

SO32-sulfite ionperchlorate ion

NO3-nitrate ionchlorate ion

NO2-nitrite ionchlorite ion

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.