How do isotopes impact molecular weight calculations?

Isotopes affect molecular weight calculations by contributing to the average atomic mass of an element.

Isotopes are variants of a particular chemical element which, while having the same number of protons and electrons, have different numbers of neutrons. This difference in neutron number results in different atomic masses for the isotopes of the same element. For example, carbon-12 and carbon-14 are both isotopes of carbon, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons, making carbon-14 heavier.

When calculating the molecular weight of a compound, we need to consider the average atomic mass of the elements involved. The average atomic mass is not simply the mass of the most common isotope, but rather a weighted average that takes into account the relative abundance of all the isotopes of that element. This is where isotopes come into play.

For instance, let's consider chlorine, which has two main isotopes: chlorine-35 and chlorine-37. About 75% of naturally occurring chlorine is chlorine-35, and about 25% is chlorine-37. To calculate the average atomic mass of chlorine, we would do a weighted average of the two isotopes, which gives us an average atomic mass of approximately 35.5 atomic mass units (amu).

Therefore, when calculating the molecular weight of a compound that contains chlorine, we would use 35.5 amu as the atomic mass of chlorine, not 35 or 37. This is how isotopes impact molecular weight calculations. They contribute to the average atomic mass of an element, which is then used in the calculation of the molecular weight of compounds containing that element.