Atomic Weights and Isotopes

To report the mass of something, one generally gives a number together with a unit like pounds,
kilograms (kg), grams (g), et cetera. Because the mass of an atom is so small, however, these units
are not very convenient, and new ways have been devised to describe how much an atom weighs. A
unit that can be used to report the mass of an atom is the atomic mass unit (amu). One amu is
approximately the same as 1.66 × 10–24 g. How is this particular value chosen? Why not, for example,
have 1 amu be equal to a nice round number like 1.00 × 10–24 g instead? The answer is that it is
chosen so that a carbon-12 atom, with 6 protons and 6 neutrons, will have a mass of 12 amu. In
other words, the amu is defined as one-twelfth the mass of the carbon-12 atom. It does not convert
nicely to grams because the mass of a carbon-12 atom in grams is not a nice round number. In
addition, since the mass of an electron is negligible, all the mass of the carbon-12 atom is
considered to come from protons and neutrons.

Since the mass of a proton is about the same as that of a neutron, and there are 6 of each in the

carbon-12 atom, protons and neutrons are considered to have a mass of

each.

While it is necessary to have a way of describing the weight of an individual atom, in real life one
generally works with a huge number of them at a time. The atomic weight is the mass in grams of
one mole (mol) of atoms. Just like a pair corresponds to two, and a dozen corresponds to twelve, a
mole corresponds to about 6.022 × 10^23 . The atomic weight of an element, expressed in terms of
g/mol, therefore, is the mass in grams of 6.022 × 10^23 atoms of that element. This number, roughly
6.022 × 10^23 , to which a mole corresponds, is known as Avogadro’s number. Why this particular value
and not something like 1.0 × 10^24 , for example? Once again, the answer lies in the carbon-12 atom: a
mole of carbon-12 atoms weighs exactly 12 g. In other words, a mole is defined as the number of
atoms in 12 g of carbon-12. A mole of atoms of an element heavier than carbon-12 (such as oxygen)
would have an atomic weight higher than 12 g/mol, while a mole of atoms of an element lighter
than carbon-12 (such as helium) would have an atomic weight less than 12 g/mol. Six g of carbon-12
would mean 3.011 × 10^23 carbon-12 atoms, et cetera.