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why lithium atomic mass is 7

The atomic mass of lithium is approximately 7 atomic mass units (amu), and it is usually rounded to 7 for simplicity in the periodic table. This value represents the weighted average of the masses of all naturally occurring isotopes of lithium.

Lithium has two stable isotopes:

  1. Lithium-6 (Li-6): This isotope has a mass of approximately 6.0151214 amu.
  2. Lithium-7 (Li-7): This isotope has a mass of approximately 7.0160034 amu.

The abundance of these isotopes in naturally occurring lithium is as follows:

  • Lithium-6 is less abundant and makes up about 7.5% of natural lithium.
  • Lithium-7 is more abundant and makes up the remaining 92.5% of natural lithium.

To calculate the weighted average atomic mass of lithium, you can use the following formula:

Average Atomic Mass = (Fractional Abundance of Isotope 1 × Mass of Isotope 1) + (Fractional Abundance of Isotope 2 × Mass of Isotope 2)

In the case of lithium:

Average Atomic Mass = (0.075 × 6.0151214 amu) + (0.925 × 7.0160034 amu)

Calculating this expression gives an average atomic mass of approximately 6.94 amu, which is rounded to 7 amu in the periodic table.

So, the atomic mass of lithium is approximately 7 amu, but this value is based on the weighted average of its isotopes' masses and their respective abundances.

Published on: Sep 15, 2023, 02:17 AM  


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