When little Al Pratt first appeared in 1940, the Atom was the tiniest known particle in nature that had near limitless potential. Al was merely a college student trying to protect himself and his girlfriend from bullies. After training, he became a human dynamo at the pinnacle of physical protection. When as the hero named the Atom he encountered the villain Cyclotron, Pratt's body became a sponge for the tholium energy his foe emitted. As a result, he was modeled after the atom bombs on 1945, gaining tremendous superstrength and an "atomic punch".
Before he would return for a new generation, another atom appeared on the scene in the form of Captain Atom, a soldier turned superhero thanks to an atomic accident. He too had atomic powered strength as well as flight and energy blasts that Cyclotron never passed on to Pratt. Were these two to have met in battle, the Captain may've won by shear power alone. But for heart and true grit, the original Atom would've taken him hands down.