Final answer:
Nuclear power is the energy source that involves splitting heavy atoms into lighter ones to release energy, a process known as nuclear fission.
Step-by-step explanation:
The alternative energy source based on splitting heavy atoms into lighter ones to release energy is nuclear power. This process is known as nuclear fission. In a nuclear reactor, heavy elements such as uranium or plutonium are split into lighter elements, releasing a significant amount of energy. This energy can be harnessed to generate electricity.