How do isotopes affect atomic mass?

Isotopes affect atomic mass by contributing to the average mass of an element's atoms.

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 is what gives isotopes their different atomic masses. For example, carbon-12 and carbon-14 are both isotopes of carbon, but carbon-12 has 6 neutrons while carbon-14 has 8. This difference in neutron number results in carbon-14 having a greater atomic mass than carbon-12.

The atomic mass of an element is not simply the mass of one atom of that element, but rather the weighted average of the masses of all the naturally occurring isotopes of that element. This means that the more abundant an isotope is, the more it contributes to the atomic mass of its element. For instance, carbon-12 is much more abundant than carbon-14, so the atomic mass of carbon is closer to 12 than to 14.

To calculate the atomic mass of an element, you multiply the mass of each isotope by its natural abundance (expressed as a decimal), then add these values together. For example, if an element has two isotopes, one with a mass of 20 and an abundance of 90%, and another with a mass of 22 and an abundance of 10%, the atomic mass would be (20 x 0.9) + (22 x 0.1) = 18 + 2.2 = 20.2.

In summary, isotopes affect atomic mass by having different masses and natural abundances. The atomic mass of an element is a weighted average of the masses of its isotopes, with more abundant isotopes contributing more to the atomic mass.