Isotopes
The atomic number Z characterizes an atom.
It is possible that 2 or more atoms have the same Z, so that they are the same element, but different mass number A. Such atoms are called isotopes.
Example: hydrogen is the most simple atom, as it has only 1 electron. It may have 3 different masses:
1) hydrogen with 1 proton
2) hydrogen with 1 proton and 1 neutron (called deuterium)
3) hydrogen with 1 proton and 2 neutrons (called tritium)
In all 3 cases above the number Z=1, so that we are dealing with the element hydrogen. However their masses are 1, 2 and 3 respectively. So, what is the mass number that is shown in the periodic table? (check yourself here)
It is 1.008 ! Strange?
That is because the mass number A is an average of the mass of all the isotopes of an element. The reason the number in this case is so close to 1 is because this is an weighted average, i.e., the mass number of each isotope enters the average in proportion to its abundance. Because the simple hydrogen atom (Z=1, A=1) is by far the most abundant, the average is very close to 1.
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