The liberation of energy in any form, chemical, electrical, or nuclear, involves the loss of an equivalent amount of mass, in accordance with the Einstein formula. When 3,000 metric tons of coal are burned to ashes, the residual ashes and the gaseous products weigh one gram less than 3,000 tons; that is, one three-billionth part of the original mass will have been converted into energy. The same is true with the liberation of nuclear energy by the splitting or fusing (as will be explained later) of the nuclei of certain elements. The difference is merely that of magnitude. In the liberation of chemical energy by the burning of coal, the energy comes from a very small loss of mass resulting from the rearrangement of electrons on the surface of the atoms. The nucleus of the coal atoms is not involved in any way, remaining exactly the same as before. The amount of mass lost by the surface electrons is one thirtieth of one millionth of one per cent. On the other hand, nuclear energy involves vital changes in the atomic nucleus itself, with a consequent loss of as high as one tenth to nearly eight tenths of one per cent in the original mass of the nuclei. This means that from one to nearly eight grams per thousand grams are liberated in the form of energy, as compared with only one gram in three billion grams liberated in the burning of coal. In other words, the amount of nuclear energy liberated in the transmutation of atomic nuclei is from 3,000,000 to 24,000,000 times as great as the chemical energy released by the burning of an equal amount of coal. In terms of TNT the figure is seven times greater than for coal, as the energy from TNT, while liberated at an explosive rate, is about one seventh the total energy content for an equivalent amount of coal. This means that the nuclear energy from one kilogram of uranium 235, or plutonium, when released at an explosive rate, is equal to the explosion of twenty thousand tons of TNT. Nuclear energy can be utilized by two diametrically opposed methods. One is fission—the splitting of the nuclei of the heaviest chemical elements into two uneven fragments consisting of nuclei of two lighter elements. The other is fusion—combining, or fusing, two nuclei of the lightest elements into one nucleus of a heavier element. In both methods the resulting elements are lighter than the original nuclei. The loss of mass in each case manifests itself in the release of enormous amounts of nuclear energy.