Amongst exothermic and endothermic reactions, which one is spontaneous?

spontaneous reaction may also be non spontaneous

Explanation:

        as energy is released to the surroundings as heat there is a decrease in the energy of the system The combination of energy decrease and entropy increase dictates that combustion reactions are spontaneous reactions.            

        If there's an endothermic reaction that is spontaneous, then the reverse reaction is an exothermic reaction that's non-spontaneous. Melting ice is spontaneous at room temperature, but absorbs heat so it's endothermic. Therefore freezing of water at room temperature is an example, if you consider that a "reaction"            

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.