This site should be read in connection with the Nuclear Table

The nucleus is made up of protons and neutrons. The symbol for the number of protons is "Z" (also called the atomic number); the symbol for the number of neutrons is "N". The total number of protons and neutrons is called "A". A = N + Z, and A is called the mass number.

Isotopes
It is possible to have two or more nuclei with the same number of protons but different numbers of neutrons. Such sets of nuclei are called isotopes. For example, there is a nucleus with 2 protons and 1 neutron, and another nucleus with 2 protons and 2 neutrons. Since they both have the atomic number Z = 2, they are both called "helium". Suppose you form atoms by putting two electrons around these nuclei. Matter made of these two types of atoms will have the same chemical and physical properties. (They are gases at room temperature; chemically they are inert - it is almost impossible to get them to form molecules.) This is true of isotopes in general; they have the same chemical and physical properties.


Notation
A given nucleus is represented as follows: We use the chemical symbol, e.g. "He" for helium, "C" for carbon, "Fe" for iron. Then the mass number, A, is written as a superscript, and the atomic number, Z, is written as a subscript -- both on the left. For example, the two helium isotopes mentioned above are 32He and 42He. The three isotopes of carbon are 126C, 136C, and 146C. The number of neutrons must be determined by subtracting: N = A - Z. For example, 136C has 7 neutrons, 146C has 8 neutrons.

Note that the subscript, Z, is redundant. If the nucleus is "C", carbon, it must be Z = 6; Helium must be Z = 2. So the subscript is sometimes omitted, and we write just 14C, or 4He. Sometimes also we write nuclei as "carbon-14", "helium-4", etc.

The Nuclear Table represents a sample of nuclei from Z = 1 up to 118. There are about 115 known chemical elements (a few are missing between 100 and 118), and more than 3000 known nuclei.


Nuclear properties
The following general points should be noted from the table:
  • For the lighter nuclei, if we look at the most common isotope, N is approximately equal to Z.
  • As we get to heavier nuclei, past Z = 20, we begin to see N considerably greater than Z. This is more and more true as nuclei get heavier.
  • Bismuth is the heaviest stable nucleus. Heavier nuclei exist but they are all unstable -- they undergo certain spontaneous changes which we observe as radioactivity. Nuclei from Z = 84 (polonium) to 92 (uranium) are found in nature (on earth) and all their isotopes are radioactive.
  • Nuclei heavier than uranium exist but they are all artificial -- they have been created by scientists in laboratories. The heaviest known nucleus has Z = 118. It was produced just recently.

KEY CONCEPTS

  • Atomic number, mass number
  • Isotopes:
    • Same Z, different N
    • Same chemical properties
  • Symbol for a nucleus
  • N larger than Z for heavier nuclei
  • Nuclei unstable beyond Z = 83
  • Found in nature and radioactive from Z = 84 to 92
  • Artificial nuclei beyond 92