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.  

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."