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Atomic masses of some elements:
Atomic number | Name | Symbol | Atomic mass |
\(1\) | Hydrogen | \(H\) | \(1.008\) |
\(2\) | Helium | \(He\) | \(4.003\) |
\(3\) | Lithium | \(Li\) | \(6.941\) |
\(4\) | Beryllium | \(Be\) | \(9.012\) |
\(5\) | Boron | \(B\) | \(10.811\) |
How to calculate the average atomic mass?
Let us understand this with the help of the following examples.
Example: 1
Oxygen is the most abundant element in both the Earth’s crust and the human body. In nature, it exists as a combination of three stable isotopes, as shown in the table:
Isotopes of oxygen:
Isotope | Mass | % abundance |
\(_{8}\textrm{O}^{16}\) | \(15.9949\) | \(99.757\) |
\(_{8}\textrm{O}^{17}\) | \(16.9991\) | \(0.038\) |
\(_{8}\textrm{O}^{18}\) | \(17.9992\) | \(0.205\) |
Hence, the atomic mass of oxygen is \(15.999\) \(amu\).
Example: 2
Boron naturally found as a mixture of boron-\(10\) (\(5\) protons + \(5\) neutrons) and boron-\(11\) (\(5\) protons + \(6\) neutrons) isotopes. The percentage abundance of boron-\(10\) is \(20\), and that of boron-\(11\) is \(80\). The atomic mass of boron is then determined as follows:
Hence, the atomic mass of boron is \(10.8\) \(amu\).
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