Since each loss of mass manifests itself by the release of energy, it can be seen that to obtain energy from the atom’s nucleus requires either the fusion of two elements in the first half of the periodic table or the fission of an element in the second half. From a practical point of view, however, fusion is possible only with two isotopes (twins) of hydrogen, at the beginning of the periodic table, while fission is possible only with twins of uranium, U-233 and U-235, and with plutonium, at the lower end of the table. The diameter of the atom is 100,000 times greater than the diameter of the nucleus. This means that the atom is mostly empty space, the volume of the atom being 500,000 billion times the volume of the nucleus. It can thus be seen that most of the matter in the universe is concentrated in the nuclei of the atoms. The density of matter in the nucleus is such that a dime would weigh 600 million tons if its atoms were as tightly packed as are the protons and neutrons in the nucleus. The atoms of the elements (of which there are ninety-two in nature, and six more man-made elements) have twins, triplets, quadruplets, etc., known as isotopes. The nuclei of these twins all contain the same number of protons and hence all have the same chemical properties. They differ, however, in the number of neutrons in their nuclei and hence have different atomic weights. For example, an ordinary hydrogen atom has a nucleus of one proton. The isotope of hydrogen, deuterium, has one proton plus one neutron in its nucleus. It is thus twice as heavy as ordinary hydrogen. The second hydrogen isotope, tritium, has one proton and two neutrons in its nucleus and hence an atomic mass of three. On the other hand, a nucleus containing two protons and one neutron is no longer hydrogen but helium, also of atomic mass three. There are hundreds of isotopes, some occurring in nature, others produced artificially by shooting atomic bullets, such as neutrons, into the nuclei of the atoms of various elements. A natural isotope of uranium, the ninety-second and last of the natural elements, contains 92 protons and 143 neutrons in its nucleus, hence its name U-235, one of the two atomic-bomb elements. The most common isotope of uranium has 92 protons and 146 neutrons in its nucleus and hence is known as U-238. It is 140 times more plentiful than U-235, but cannot be used for the release of atomic energy.