Atomic mass

How much does an atom weigh? If we could weigh one single hydrogen atom on super sensitive scales, they would read 1.7 septillionths of one gram. Whereas the heaviest atoms would give a reading of half a sextillionth of one gram. Talking about the mass of an atom in grams is not at all practical. We need to use another unit, one that denotes a really tiny mass.

One such unit is the unified atomic mass unit. Its symbol is a lowercase u. Defined in atomic mass units, atoms weigh between about one u and three hundred u. These numbers are a bit easier to work with. There is a quick way of finding approximately how many ‘u’ an atom weighs.

This atom has three protons, four neutrons and three electrons. It’s a Lithium-7 atom. The number seven shows how many particles there are in the nucleus. It is the atom’s mass number. The mass of each nuclear particle, expressed in atomic mass units, is almost exactly one u.

This means that a nucleus with three protons and four neutrons, weighs about 7 u. What about the three electrons? Actually, they are so light, they don’t affect the atomic mass that much. Each electron weighs only half a thousandth of one u. So the atomic mass of the entire lithium-7 atom is still very close to 7 u.

To determine the approximate mass of an atom in ‘u’, it’s enough to know the number of nuclear particles in the atom. The two expressions – mass number and atomic mass – sound similar, but they mean different things. The mass number is the number of particles in the nucleus. This is always a whole number, since there are no half protons or quarters of neutrons. The atomic mass is how much the atom weighs.

This is almost never a whole number, unless it’s rounded to the nearest integer. The mass number for a lithium atom with seven nuclear particles is exactly seven, while its atomic mass is 7.016 u. Here is another atom of the same element, lithium. This also has three protons, but only three neutrons. This means its mass number is six, and the atomic mass approximately 6 u.

Lithium-7 and lithium-6 are the only isotopes of lithium that occur in nature. Almost all lithium atoms, 92 percent of them, are the heavier lithium-7 isotope. The rest are lithium-6. This makes the average mass of lithium atoms close to 7 u. We can look up the average atomic mass of all the elements in the periodic table.

When we look at the square for lithium, we see that its atomic mass is 6.94 u. None of the lithium atoms weigh 6.94 u: the mass is either 7.016 or 6.015 u. It’s the average mass of lithium’s isotopes that is 6.94 atomic mass units. So how much does an atom weigh? The answer is: about as many atomic mass units as the mass number of the atom.

When an element has more than one isotope, we use the average mass of its isotopes.