What notations are used to represent atomic number and weight?
atomic number and weight Each atom, therefore, can be assigned both an atomic number (the number of protons equals the number of electrons) and an atomic weight (approximately equaling the number of protons plus the number of neutrons).
A normal helium atom, for example, has two protons and two neutrons in its nucleus, with two electrons in orbit. Its chemical behavior is determined by the atomic number 2 (the number of protons), which equals the normal number of electrons; the stability of its nucleus (that is, its radioactivity) varies with its atomic weight (approximately equal to the number of protons and neutrons).
The most well-known form of plutonium, for example, has an atomic number of 94, since it has 94 protons, and with the 145 neutrons in its nucleus, an atomic weight of 239 (94 protons plus 145 neutrons). In World War II, its very existence was highly classified. A code number was developed: the last digit of the atomic number (94) and the last digit of the atomic weight (239). Thus, in some of the early documents examined by the Advisory Committee, the term 49 refers to plutonium.
Styles of notation vary, but usually isotopes are written as:
atomic number Chemical abbreviation atomic weight
atomic weight Chemical abbreviation
Thus, the isotope of plutonium just discussed would be written as:
Since the atomic weight is what is often the only item of interest, it might also be written simply as Pu-239, plutonium 239, or Pu239.