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The Periodic Table and Chart of the Nuclides

The isotopes of hydrogen.
The isotopes of hydrogen.  

The number of protons in the atom defines the element. The number of electrons in a neutrally-charged atom is equal to the number of protons. The number of electrons determines the chemical properties of that atom.

Atoms with the same number of protons but different numbers of neutrons are different isotopes of the same element. Isotopes have the same chemical properties but different nuclear properties. For example, the element U consists of two principle isotopes U-235 and U-238. U-235 has 235 nucleons, 92 protons, and 143 neutrons. We say that U-235 has an atomic number (Z) of 92. It has an atomic mass (A) of 235. It has a neutron number (N) of 143. U-238 has the same Z, but a different N (N for U-238 is equal to 146). Thus, U-238 has an A of 238.

A particular combination of neutrons and protons is called a nuclide. Thus, H-3 has a Z of 1 and an N of 2. H-3 is a nuclide and is an isotope of the element H. He-4 has a Z of 2 and an N of 2. He-4 is a different nuclide than H-3. He-4 is an isotope of the element He.

nuclides2 
The four forces at work in the universe. Click to view larger.

The very small nucleus is composed of positively-charged protons that are repelling due to the electromagnetic force. The strong nuclear force counteracts this force and holds the nucleus together. The strong nuclear force is a very powerful attractive force but operates over a very limited range. By adding neutrons to the nucleus this adds strong nuclear force for attraction without adding repulsive electromagnetic force. However, the neutrons also add weak nuclear force which forces decay of neutrons into electrons and protons. Thus, too many neutrons makes a nucleus unstable as does too few neutrons for any particular number of protons. A balance of neutrons and protons is necessary for stability.

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