If an exothermic chemical reaction occurs, the surroundings become warmer. where does this energy come from?

It comes to from the chemical reaction itself. Exothermic reaction release heat because the energy level of the products is lower than that of the reactants.  Therefore, the net enthalpy change of the reaction is negative (-), meaning heat or light (or any other form of energy) is released into the environment. An example of an exothermic reaction is the combustion of fuel.  

the ozone layer acts as a filter for the shorter wavelength and highly hazardous ultraviolet radiation (uvr) from the sun, protecting life on earth from its potentially harmful effects. when the sky is clear, there is an inverse relationship between stratospheric ozone and solar uvr measured at the earth's surface.

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements