Naomi Walker, age i4, of Medicine Lodge, Kan., for her question:
What is an unstable atom?
An unstable person is somewhat off balance and likely to explode in temper tantrums. An unstable atom also is somewhat unbalanced and likely to explode in a tantrum of searing energy. Unstable atoms, however, are more reliable than unstable people. Scientists can predict when and exactly how they will explode.
Unstable atoms are radioactive atoms. Most of them are big fellows stuffed with particles of all kinds. In fact, one reason for their radioactivity may be because they have more particles than they can hold together. They may burst apart in radioactive decay in order to reach a more balanced arrangement of their particles. These atoms make up the radioactive elements to be found on the Periodic Table.
Some of them exist in nature. The rest have been discovered in this Atomic Age or are man made in the laboratory. In the Periodic Table of the Chemical elements, most of them are listed together as the actinide metals. This list begins with actinium, atomic number 89, and ends with the latest element so far discovered, Lawrencium, which has atomic number 103.
The atomic number of an element Telis the number of protons in the nucleus of its atom. The protons are particles charged with positive electricity. Also in the nucleus there are electrically neutral neutrons and an assortment of more than 20 other particles, some positive, some negative and some neutral. A big atom seems unable to Cope with all the particles in its bulky nucleus. At definite intervals, certain of these particles leave home with a burst of radioactivity.
Every element has its own number, and the list runs from one to 103. When Uranium, atomic number 92, loses a proton, it is no longer uranium. It is Protactinium, atomic number 91. When it loses another proton, it becomes thorium, Atomic number 90.
As the big, unstable atoms decay into smaller atoms, they shoot off high speed particles of various kinds, plus radioactive energy. The process of decay proceeds step by step at a definite rate. After 4.5 billion years, half a supply of radioactive uranium has decayed into a type of lead, whose atomic number is 82. This idea atom is stable and the radioactive decay is at an end.
The rate of radioactive decay is measured by the half life. In 1620 years, exactly half of every supply of radioactive radium will decay. In the next 1620 years, half of the remaining supply will decay, and so on and on. The half life of radium, then, is 1620 years. The half life of a radioactive substance may be days or hours or no more than a brief fraction of a second. In any case, after each half life period, half the supply of each radioactive substanee has decayed and become some other substance.