This process is called nuclear fission. Nuclear power plants split uranium atoms to make heat. The heat boils water and generates steam. The steam turns a turbine to generate electricity. Another nuclear process could also yield energy.
Nuclear fusion takes place when two hydrogen atoms join together to form a larger helium atom.
The helium atom is not quite as heavy as the two hydrogen atoms were. The lost mass is converted into energy. Nuclear fusion is the process that makes the sun burn. People have not been able to recreate fusion on Earth yet, except in very brief tests. Researchers are still studying ways to do this.