The atomic mass of an atom is the mass of that atom compared to some standard, such as the mass of a particular type of carbon atom. The terms atomic mass and atomic weight are often used interchangeably, although, strictly speaking, they do not mean the same thing. Mass is a measure of the total amount of matter in an object. Weight is a measure of the heaviness of an object. In general, the term atomic mass is preferred over atomic weight.
Scientists usually do not refer to the actual mass of an atom in units with which we are familiar (units such as grams and milligrams). The reason is that the numbers needed are so small. The mass of a single atom of oxygen-16, for example, is 2.657 × 10 −23 grams, or 0.000 000 000 000 000 000 000 026 57 grams. Working with numbers of this magnitude would be very tedious.
Early chemists knew that atoms were very small but had no way of actually finding their mass. They realized, however, that it was possible to express the relative mass of any two atoms. The logic was as follows: suppose we know that one atom of hydrogen combines with one atom of oxygen in a chemical reaction. It is easy enough to find the actual masses of hydrogen and oxygen that combine in such a reaction. Research shows that 8 grams of oxygen combine with 1 gram of hydrogen. It follows, then, that each atom of oxygen has a mass eight times that of a hydrogen atom.
This reasoning led to the first table of atomic masses, published by John Dalton (1766–1844) in 1808. Dalton chose hydrogen to be the standard for his table of atomic masses and gave the hydrogen atom a mass of 1. Of course, he could have chosen any other element and any other value for its atomic mass. But hydrogen was the lightest of the elements and 1 is the easiest number for making comparisons.
One problem with which Dalton had to deal was that he had no way of knowing the ratio in which atoms combine with each other. Since there was no way to solve this problem during Dalton's time, he made the simplest possible assumption: that atoms combine with each other in one-to-one ratios (unless he had evidence for some other ratio).
The table Dalton produced, then, was incorrect for two major reasons. First, he did not know the correct combining ratio of atoms in a chemical reaction. Second, the equipment used at the time to determine mass ratios was not very accurate. Still, his table was an important first step in determining atomic masses. Some of the values that he reported in that first table were: nitrogen: 4.2; carbon: 4.3; oxygen: 5.5; phosphorus: 7.2; and sulfur: 14.4.
Atomic mass unit (amu): A unit used to express the mass of an atom equal to exactly one-twelfth the mass of a carbon-12 atom.
Isotopes: Two or more forms of an element with the same atomic number (same number of protons in their nuclei), but different atomic masses (different numbers of neutrons in their nuclei).
Mass: Measure of the total amount of matter in an object.
Standard: A basis for comparison; with regard to atomic mass, the atom against which the mass of all other atoms is compared.
Weight: The measure of the heaviness of an object.
Within two decades, great progress had been made in resolving both of the problems that troubled Dalton in his first table of atomic masses. By 1828, Swedish chemist Jöns Jakob Berzelius (1779–1848) had published a list of atomic masses that was remarkably similar to values accepted today. Some of the values published by Berzelius (in comparison to today's values) are: nitrogen: 14.16 (14.01); carbon: 12.25 (12.01); oxygen: 16.00 (16.00); phosphorus: 31.38 (30.97); and sulfur: 32.19 (32.07).
One of the major changes in determining atomic masses has been the standard used for comparison. The choice of hydrogen made sense to Dalton, but it soon became clear that hydrogen was not the best element to use. After all, atomic masses are calculated by finding out the mass ratio of two elements when they combine with each other. And the one element that combines with more elements than any other is oxygen. So Berzelius and others trying to find the atomic mass of elements switched to oxygen as the standard for their atomic mass tables. Although they agreed on the element, they assigned it different values, ranging from 1 to 100. Before long, however, a value of 16.0000 for oxygen was chosen as the international standard.
By the mid-twentieth century, another problem had become apparent. Scientists had found that the atoms of an element are not all identical with each other. Instead, various isotopes of an element differ slightly in their masses. If O = 16.0000 was the standard, scientists asked, did the 16.0000 stand for all isotopes of oxygen together, or only for one of them?
In order to resolve this question, researchers agreed in 1961 to choose a new standard for atomic masses, the isotope of carbon known as carbon-12. Today, all tables of atomic masses are constructed on this basis, with the mass of any element, isotope, or subatomic particle being compared to the mass of one atom of carbon-12.
The atomic mass of an element is seldom a whole number. The reason for this is that most elements consist of two or more isotopes, each of which has its own atomic mass. Copper, for example, has two naturally occurring isotopes: copper-63 and copper-65. These isotopes exist in different abundances. About 69.17 percent of copper is copper-63 and 30.83 percent is copper-65. The atomic mass of the element copper, then, is an average of these two isotopes that takes into account the relative abundance of each: 63.546.
Students sometimes wonder what unit should be attached to the atomic mass of an element. For copper, should the atomic mass be represented as 63.546 g, 63.546 mg, or what? The answer is that atomic mass has no units at all. It is a relative number, showing how many times more massive the atoms of one element are compared to the atoms of the standard (carbon-12).
Still, occasions arise when it would be useful to assign a unit to atomic masses. That procedure is acceptable provided that the same unit is always used for all atomic masses. Scientists have now adopted a unit known as the atomic mass unit for atomic masses. The abbreviation for this unit are the letters amu. One may represent the atomic mass of copper, therefore, either as 63.546 or as 63.546 amu.