32 | The History and Use of Our Earth’s Chemical Elements interaction (reaction). The previously noted example is a type of transmutation in which the

32 | The History and Use of Our Earth’s Chemical Elements interaction (reaction). The previously noted example is a type of transmutation in which the

nuclei of larger atoms disintegrate (transmutate) into nuclei of smaller atoms. Another type of transmutation occurs when nuclei of smaller elements combine to form nuclei of larger ele- ments (i.e., fusion). An example of this form of transmutation is the nitrogen nuclei capturing

helium nuclei (alpha) to form oxygen nuclei plus hydrogen ( 7 N-14 + 2 He-4 >nuclei combine>

8 O-16 + 1 H-1). The time it takes for radioactive decay to occur is the half-life for that isotope. The half-life is the time required for the activity to decrease to one-half of the radioactivity of the original isotope, followed by a decrease of one-half of that remaining half, and half of that remaining half of the nuclei decaying, and so on. This means that the nuclei of radioisotopes do not decay all at once, but rather undergo decay to more stable forms of atoms—giving off radia- tion in each of the sequences of decay until stable nuclei of an element result. Interestingly, scientists cannot measure or predict the half-life of a particular single nucleus. They do not have the computing power for this analysis, so they work with averages and mathematical probabilities.

The half-lives of some radioisotopes are measured in billions of years; for others, the half- life is measured in fractions of seconds. Following are some examples of the half-lives of a few isotopes: uranium-238 = 4.6 billion years; carbon-14 = 5730 years; strontium-90 = 38 years; phosphorus-32 = 14.3 days; radon-222 = 3.8 days; uranium-239 = 23.5 minutes.

Radiation is one area of science not well understood by the lay public, and often the media information relating to radioactivity is misleading and misunderstood. To some extent, the topics of radioactivity and radiation have become a political issue. The public is somewhat scientifically illiterate about radiation, and many people do not have a very clear understand- ing of the physical nature, sources, uses, benefits, and dangers of radiation and radioactivity. We can all learn more about radioactivity so that it can be used for the benefit of mankind without undue fear. After all, it is very natural and universal. Radioactivity takes place both inside and on the surface of our Earth. Not only does it exist in space, but it also is penetrat- ing our bodies at all times from natural sources, and small amounts of radiation exist in our tissues and organs. It is part of all life.

Some of the sources of radiation that affect us follow: 1. Potassium is essential to our diets and is found in many foods. Our bodies cannot distin-

guish between potassium-39 ( nonradioactive) and the smaller quantities of radioactive potassium-40 found in our foods. Radioactive K-40 makes up almost one-fourth of all the atomic radiation we normally receive.

2. Radon is a radioactive gas that seeps into our homes, schools, and offices. It is produced by the natural decay of radium in the ground. Radon gas is thought to be a cause of some cancers, particularly lung cancer, as it seeps into the ground levels of buildings. Kits are available for testing the levels of radon that may exist in your home—particularly the basement or ground-level areas.

3. There are other sources of radiation from the decay of radioactive elements in the Earth’s crust. 4. We receive radiation from outer space as cosmic rays, solar radiation, and upper-atmo- sphere radiation. The higher the altitude at which you live, the greater will be your expo- sure to cosmic radiation from space. Since nuclear radiation accumulates in our bodies

The Periodic Table of Chemical Elements | 33

over time, people living in Denver, Colorado, receive more radiation in their lifetimes than do people living in areas at sea level.

5. Humans are exposed to radiation from the testing and explosion of nuclear weapons and the wastes of nuclear reactors and power plants. Strontium-90 is a fission product from nuclear reactors. It is of particular concern because it has a long half-life of 38 years and becomes concentrated in the food chain, particularly plants-to-milk. The ban on atmospheric testing of nuclear weapons has reduced this hazard. Strontium-90 does have some industrial uses.

6. Most people in developed countries receive minor exposure to radiation through medical procedures such as X-ray and various treatments for some diseases.

The greatest hazard from strong radiation is the ionization of atoms in cells and tissues of humans that may result in changes in somatic (body) cells. In addition, reproductive sperm and egg cells may also be affected by strong radiation as well as by cosmic rays. Radioactive potassium is thought to be a source of genetic mutations (genetic changes) in the DNA of plant and animal cells. This natural process of radiation and mutation has a relationship to the evolution of plant and animal species because all plants and animals require potassium to survive and live with constant cosmic radiation. As mentioned earlier, living organisms cannot tell stable potassium from the radioactive form in their diets.

Our exposure to man-made radioactive sources, such as from nuclear power plants, is negligible when compared to the total radiation we receive. Man-made radiation accounts for less than 3% of the total radiation we receive in the United States, but in some countries, this figure is higher. The vast majority of the 3% of man-made doses of radiation we receive in our lifetime results from medical uses, and the vast majority of the 97% of the total exposure to all radiation we receive comes from natural sources.

In summary, the structure and recurring characteristics of elements are represented in the catalog-like periodic table of chemical elements. A review of the material covered thus far follows:

1. The negatively charged electrons surround the positively charged nucleus. Electrons can exist in shells or energy levels orbiting around the nucleus, or they can easily be stripped from the atom and exist as free electrons as electricity or beta particles. An electron is 1/1837 the mass of a proton. Even so, an electron’s electrical charge balances the positive charge of a proton. In addition, the distance between the electrons and the nucleus of an atom is about one million times greater than the diameter of the nucleus. In a neutral atom, the number of electrons equals the number of protons.

2. The positively charged protons are compacted in a tiny, dense center of the atom called the nucleus. The number of protons in the nucleus determines the atomic number for each element. The periodic table lists the number of protons in progression from the first

number, hydrogen ( 1 H, with one proton), to the most recently discovered superactinide elements and yet-to-be-discovered elements with the highest atomic numbers.

3. The neutron was initially more difficult to identify because it has no electrical charge. Neutrons are found in the nucleus with the protons. They have approximately the same mass as the protons, and together they make up the atomic mass of the atoms for each of the elements. To determine the number of neutrons in the atoms of an element, one can subtract the atomic number (protons) from the total atomic weight.

34 | The History and Use of Our Earth’s Chemical Elements

4. Isotopes are atoms with more than the usual number of neutrons in their nuclei. Their atomic number does not change, but their atomic mass does. Many isotopes of atoms with high atomic masses are radioactive.

5. Ions are atoms that have gained or lost one or more electrons in a chemical reaction to form a molecule of a new compound. Thus, one might think of ions as atoms with electri- cal charges.

The background presented here will aid you in learning how to use the periodic table of the chemical elements. Once you study and understand the organization of this remarkable chart, you can glean a great deal of information about the Earth’s chemical elements. The structure and characteristics of elements may be identified by their position in the table. How

a given element will react with other elements can be predicted by the element’s placement in relation to other elements in the table. The periodic table contains an abundance of intriguing and useful information. The periodic table of the chemical elements is the Rosetta Stone for decoding the nature of chemistry.

As mentioned, a number of different forms of tables, charts, and diagrams for the arrange- ments of the chemical elements have been proposed and designed. One of the newest and unique designs is the Chemical Galaxy version of the periodic table devised by Philip Stewart. Copies of this unique periodic table are available at the Chemical Galaxy Web site (http:// www.chemicalgalaxy.co.uk/).

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Guide to the Elements