Understanding Isotopes and Atomic Mass

Ever wonder why chlorine's atomic mass on the periodic table is 35.5, not a whole number? The answer lies in isotopes. Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons.

For example, chlorine has two common isotopes: chlorine-35 and chlorine-37. Both have 17 protons (making them chlorine), but Cl-35 has 18 neutrons while Cl-37 has 20 neutrons. This is written as $^{35}{17}Cl$and$^{37}{17}Cl$ - where the subscript shows the number of protons and the superscript shows the mass number $protons + neutrons$.

The average atomic mass of an element accounts for all its naturally occurring isotopes and their relative abundances. For chlorine, Cl-35 makes up 75.8% of natural chlorine while Cl-37 accounts for 24.2%. We calculate the average mass by multiplying each isotope's mass by its percentage abundance and adding them:

$\frac{(35 \times 75.8)+(37 \times 24.2)}{100} = 35.484$

Did you know? Most elements in nature exist as mixtures of isotopes, which is why atomic masses on the periodic table are usually decimal numbers, not whole numbers!