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Most of the heat and radioactivity from spent nuclear fuel dissipates within 50 years. For more information, see: For a lesson about this topic, see: |
What makes each element different is the number of protons, neutrons and electrons the atoms making up that element contain.
The number of protons in an atom is used to identify each element and is called the atomic number, which sometimes is referred to by Z. Each atom of the element helium has 2 protons. Each atom of carbon has 6 protons. The number of electrons in a neutral atom equals the number of protons. So since carbon has 6 protons, it has six electrons.
Each element also has its own mass. We use atomic mass to describe how much matter there is in that element, and the units are atomic mass units (amu). Based on the properties of protons & neutrons, we know they have an approximate mass of 1 amu each. Generally speaking, the atomic mass is the number of protons and neutrons added together (since the electrons’ contribution to atomic mass is negligible in comparison). Each element can be represented using an atomic symbol as in the example of gold below.
Based on the information above, we can calculate the number of neutrons each atom has by using the following formula:
[Number of Neutrons] = [Rounded Atomic Mass] - [Atomic Number]using our example of gold (above)...
[Number of Neutrons] = 197 - 79
[Number of Neutrons] = 118
Gold has 118 neutrons in its nucleus (in general, mass numbers in the periodic table are not integers, since they represent weighted averages of all the mass numbers of all the isotopes of a given element).
Here are some more examples of atomic symbols:
(helium), and 
( iron)