Which part of the atom is most important when it comes to forming bonds with other
atoms?

Valence shell
The number of electrons in the outermost shell of a particular atom determines its reactivity, or tendency to form chemical bonds with other atoms. This outermost shell is known as the valence shell, and the electrons found in it are called valence electrons.

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A  reaction  in which  energy  is  released  to the surroundings is called an exothermic  reaction. in this type of  reaction  the enthalpy, or stored  chemical energy, is lower for the products than the reactants. when ammonium nitrate is dissolved in water,  energyis  absorbed  and the water cools.