Robert Mageau, Jr., age 10, of Exeter, Rhode Island, for his question:
How come uranium is radioactive?
Atoms, as we know, come in assorted weights and sizes and it seems that certain hefty types tend to be o€f balance. They shed particles and spurts of energy until finally they are reduced to smaller, stable atoms. This dramatic reducing process is radioactivity. Uranium is one of the heavy unstable atoms. However, certain smaller atoms also are radioactive because the forces that hold them together are unbalanced.
The secret of radioactivity is in the nucleus, that tight little fist of assorted particles amid the atom's swarming electrons. Its mayor particles are the protons and neutrons that give the atom its weight or mass. The positively charged proton equals the mass of 1836 negative electrons. The neutral neutron slightly outweighs the proton and these two particles make up 99.9 per cent of an atom's total mass. It seems that the radioactivity of substances is determined by neutrons.
The nucleus of the ordinary uranium atom has 92 protons and 146 neutrons giving it a total mass number of 238. Apparently the nucleus cannot bind and hold such a large number of particles together. A few atoms in a uranium ore sample have 143 and a very few have 142 neutrons. All of these uranium isotopes are unstable. Step by precise step, the sample breaks down through a series of smaller atoms, emitting nuclear fragments and energy in the form of nuclear radiations. The smaller atoms also are unstable until the final stage. .
After 4 1/Z billion years, half the atoms in our uranium sample become a form of lead. This atom, with 82 protons and 124 neutrons, is stable and no longer involved in the radioactive process. During the next half life of 4 1/2 billion years, SO per cent of the remaining uranium is reduced to this stable lead. And 50 per cent of the remainder is reduced during the next half life. Obviously the many particles is the massive uranium nucleus have something to do with its radioactivity, though this is not the whole story.
Some radioactive substances have only three or so neutrons. However, they are small atoms with only one or two protons. For example, the smallest atom is hydrogen. Its atomic number is one, for the one proton in its nucleus. An ordinary hydrogen atom has no neutrons. But heavy hydrogen weighs twice as much because this isotope has one neutron. Apparently the forces that bind the one proton nucleus together can cope with an extra neutron because heavy hydrogen, alias deuterium, is stable. Tritium is a hydrogen isotope with two neutrons and it is radioactive. In an atomic explosion, these two isotopes of the smallest atom release the fury of the H bomb.
Uranium and all the heavy elements known have large numbers of neutrons and all of them are radioactive. However, there are many smaller radioactive atoms in nature. Some are formed as uranium decays through its series of smaller, unstable atoms. Others are isotopes of common elements. It seems that even the smallest atoms become unstable when extra neutrons upset the forces that bind its nucleus together.