Introduction
The Atomic Century
Before the Atomic Age: "Shadow Pictures," Radioisotopes, and the Beginnings of Human Radiation Experimentation
The Manhattan Project: A New and Secret World of Human Experimentation
The Atomic Energy Commission and Postwar Biomedical Radiation Research
The Transformation in Government - Sponsored Research
The Aftermath of Hiroshima and Nagasaki: The Emergence of the Cold War Radiation Research Bureaucracy
New Ethical Questions for Medical Researchers
Conclusion
The Basics of Radiation Science
What Is Ionizing Radiation?
What Is Radioactivity?
What Are Atomic Number and Atomic Weight?
Radioisotopes: What Are They and How Are They Made?
How Does Radiation Affect Humans?
How Do We Measure the Biological Effects of External Radiation?
How Do We Measure the Biological Effects of Internal Emitters?
How Do Scientists Determine the Long-Term Risks from Radiation?
What Are Atomic Number and Atomic Weight?
What is an element?
Chemical behavior is what originally led scientists to classify matter intovarious elements. Chemical behavior is the ability of an atom tocombine with other atoms. In more technical terms, chemical behavior dependsupon the type and number of the chemical bonds an atom canform with other atoms. In classroom kits for building models of molecules,atoms are usually represented by colored spheres with small holes forpegs and the bonds are represented by the small pegs that can connectthe spheres. The number of peg holes signifies the maximum number of bonds anatom can form; different types of bonds may be represented by different typesof pegs. Atoms that have the same number of peg holes may have similarchemical behavior. Thus, atoms that have identical chemical behavior areregarded as atoms of the same element. For example, an atom is labeled a"carbon atom" if it can form the same number, types, and configurations ofbonds as other carbon atoms. Although the basics are simple to explain, howatoms bind to each other becomes very complex when studied in detail; newdiscoveries are still being made as new types of materials are formed.What is atomic number?
An atom may be visualized as a miniature solar system, with a large centralnucleus orbited by small electrons. The bonding capacity of an atom isdetermined by the electrons. For example, atoms that in their normalstate have one electron are hydrogen atoms and will readily (and sometimesviolently) bond with oxygen. This bonding capacity of hydrogen was the causeof the explosion of the airship Hindenburg in 1937. Atoms that in their normalstate have two electrons are helium atoms, which will not bond with oxygen andwould have been a better choice for filling the Hindenburg.We can pursue the question back one step further: What determines the numberof electrons? The number of protons in the nucleus of the atom. Here,the analogy between an atom and the solar system breaks down. The force thatholds the planets in their orbits is the gravitational attraction between theplanets and the sun. However, in an atom what holds the electrons in theirorbit is the electrical attraction between the electrons and the protonsin the nucleus. The basic rule is that like charges repel and oppositecharges attract. Although a proton has more mass than an electron, theyboth have the same amount of electrical charge, but opposite in kind.Scientists have designated electrons as having a negative chargeand protons as having a positive charge. One positive proton canhold one negative electron in orbit. Thus, an atom with one proton in itsnucleus normally will have one electron in orbit (and be labeled a hydrogenatom); an atom with ninety-four protons in its nucleus will normally haveninety-four electrons orbiting it (and be labeled a plutonium atom).
The number of protons in a nucleus is called the atomic number andalways equals the number of electrons in orbit about that nucleus (in anonionized atom). Thus, all atoms that have the same number of protons--theatomic number--are atoms of the same element.
What is atomic weight?
The nuclei of atoms also contain neutrons, which help hold the nucleustogether. A neutron has no electrical charge and is slightly moremassive than a proton. Because a neutron can decay into a proton plus anelectron (the essence of beta decay), it is sometimes helpful to think of aneutron as an electron and a proton blended together, although this is at bestan oversimplification. Because a neutron has no charge, a neutron has noeffect on the number of electrons orbiting the nucleus. However, becauseit is even more massive than a proton, a neutron can add significantly to theweight of an atom. The total weight of an atom is called the atomicweight. It is approximately equal to the number of protons and neutrons,with a little extra added by the electrons. The stability of the nucleus, andhence the atom's radioactivity, is heavily dependent upon the number ofneutrons it contains.What notations are used to represent atomic number and weight?
Each atom, therefore, can be assigned both an atomic number (the number ofprotons equals the number of electrons) and an atomic weight (approximatelyequaling the number of protons plus the number of neutrons). A normal heliumatom, for example, has two protons and two neutrons in its nucleus, with twoelectrons in orbit. Its chemical behavior is determined by the atomicnumber 2 (the number of protons), which equals the normal number ofelectrons; the stability of its nucleus (that is, its radioactivity) varieswith its atomic weight (approximately equal to the number of protons andneutrons). The most well-known form of plutonium, for example, has an atomicnumber of 94, since it has 94 protons, and with the 145 neutrons in itsnucleus, an atomic weight of 239 (94 protons plus 145 neutrons). In World WarII, its very existence was highly classified. A code number was developed: thelast digit of the atomic number (94) and the last digit of the atomicweight (239). Thus, in some of the early documents examined by theAdvisory Committee, the term 49 refers to plutonium.Styles of notation vary, but usually isotopes are written as:
atomic number Chemical abbreviation atomic weight
or as
atomic weight Chemical abbreviation
Thus, the isotope of plutonium just discussed would be written as:
94Pu239
or as
239Pu
Since the atomic weight is what is often the only item of interest, it mightalso be written simply as Pu-239, plutonium 239, or Pu239.
