Atomic Weight

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Atomic Weight

Atomic weight is the average of the atomic masses of all the isotopes in an element. Isotopes of an element have the same number of protons, but differ in their number of neutrons, and consequently, their atomic mass. The difference in the number of neutrons does not change chemical reactivity significantly, but there is an important difference in boiling points.

For example, Carbon-12 and Carbon-13 are naturally occurring isotopes of Carbon. C-12 has 6 protons and 6 neutrons, whereas C-13 has 6 protons and 7 neutrons.

The percent of each isotope in an element is called the natural abundance.

Sample Calculations Using Atomic Weight and Natural Abundance

Use the following information to calculate the atomic weight of Oxygen

Isotope- Isotope Mass- Natural Abundance
O-16 15.995 99.757
O-17 16.999 0.038
O-18 17.999 0.205


1: Convert---> 99.757/100=0.99757 --- 0.038/100 =0.00038 --- 0.205/100=0.00205

2: (15.995 x 0.99757) + (16.999 x 0.00038) + (17.999 x 0.00205) = ....

3: Make sure you have the units and that the answer is close to what you would expect

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