The correct answer is Sir Ernest Rutherford.
One of the first findings on atomic energy theory was reached in 1911 by Rutherford, who became known as the father of nuclear physics.
He discovered that the mass of the atom was concentrated in its nucleus, and proposed that the nucleus has a positive charge and is surrounded by negatively charged electrons, which had been discovered in 1897 by J. J. Thomson.
His theory was complemented in 1913 by Niels Bohr, who placed the electrons in definite shells or quantum levels.