Isotopes and Atomic Mass
Every atom of a given element has the same number of protons. However, like the electrons, the amount of neutrons in any atom of that element can be different. Elements that are of the same kind (i.e., 2 carbon atoms) that have different masses, are called isotopes. The chemical properties of each isotope are identical because they are the same element. The only difference is the number of neutrons within the nucleus. Therefore, the only difference between the atoms are their masses. See figure to the right
Hydrogen has 3 naturally occurring isotopes. Hydrogen-1 has a mass of 1 (1 proton and 0 neutrons). Hydrogen 2 has a mass of 2 (1 proton and 1 neutron). Hydrogen-3 has a mass of 3 (1 proton and 2 neutrons). Each atom is hydrogen, and is identical in chemical properties. The only difference is the number of neutrons, which affects the mass of the atom. This fact that you can have atoms of the same element with different masses is what we call isotopes. To identify the different isotopes, scientist add a mass number after the element’s name. The mass number is the sum of the number of protons and neutrons for that isotope. For example, there are two naturally occurring isotopes of chlorine.
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Chlorine 37 has two more neutrons that chlorine 35.
Determining the Atomic Mass from Isotope Data
You will notice that the average atomic weight for chlorine on the periodic table is not 36 (the average of the numbers 35 & 37). The average atomic mass for chlorine is 35.453. This discrepancy is due to the abundance of chlorine on this planet. According to the percent abundance data, over 75% of all the chlorine that exists on this planet is of isotope chlorine-35. A little less than 25% of the chlorine is chlorine-37. Because these percentages are not 50/50, the percents must be figured into the individual mass of each isotope to determine the average mass of the element.
The easiest way to do this is to pretend that I had 100 atoms of chlorine. If I wanted to determine the mass of 100 atoms of chlorine, I would need to know how many of each isotope of chlorine are in 100 atoms. I know that in 100 atoms, 75.53 of those atoms would be chlorine-35 and 24.47 of those atoms would be chlorine-37. I could then determine the mass of each isotope by multiplying the number of atoms times the mass in amu:
You will notice that the average atomic weight for chlorine on the periodic table is not 36 (the average of the numbers 35 & 37). The average atomic mass for chlorine is 35.453. This discrepancy is due to the abundance of chlorine on this planet. According to the percent abundance data, over 75% of all the chlorine that exists on this planet is of isotope chlorine-35. A little less than 25% of the chlorine is chlorine-37. Because these percentages are not 50/50, the percents must be figured into the individual mass of each isotope to determine the average mass of the element.
The easiest way to do this is to pretend that I had 100 atoms of chlorine. If I wanted to determine the mass of 100 atoms of chlorine, I would need to know how many of each isotope of chlorine are in 100 atoms. I know that in 100 atoms, 75.53 of those atoms would be chlorine-35 and 24.47 of those atoms would be chlorine-37. I could then determine the mass of each isotope by multiplying the number of atoms times the mass in amu:
Then, I can determine the mass of 100 atoms by adding these products together:
From this mass of 100 atoms of chlorine, I can determine the average mass by dividing by 100:
For each of the atomic masses on the periodic table, this process was completed for all the isotopes of that particular element. Thus, the numbers on the periodic table that we call the atomic mass is actually an average of all the isotopes and their abundance for each element.