Relative atomic mass

Relative atomic mass is a ratio of the average mass of the atoms of a particular element to one-twelfth of the mass of a carbon-12 atom. It is sometimes known as “atomic weight,” a term some object to because mass is technically not a weight. Weight refers to the amount of force being exerted on an object within a field of gravity; mass refers to the actual, physical quantity of a material. Mass remains the same regardless of what gravity field the object is in, while the weight of that mass will change according to the strength of the gravity field. This relationship is seen most clearly in the difference in weight between an object on Earth and that same object on the moon. A bowling ball that weighs 10 pounds on Earth would weigh only 1.66 pounds on the moon, but at both locations the mass of the ball would remain the same. Thus, scientists tend to prefer to measure mass rather than weight, because it is a value that will not fluctuate according to gravitational pull.

Any measurement, even one that strives to represent an absolute rather than a relative value, must always be in relation to some standard unit value. Thus, a unit of relative atomic mass also requires a reference point, and therefore one-twelfth of the mass of a carbon-12 atom is used. This quantity is equal to one unified atomic mass unit (u).

Background

Relative atomic mass is an average value because a degree of variation naturally exists in the atomic structure, and therefore in the atomic mass, of every sample of an element. For example, most carbon atoms contain six protons, six electrons, and six neutrons. However, in any sample of natural carbon, there will be a small percentage of atoms with seven neutrons, and another small percentage with eight. Such atypical atoms of an element are called isotopes.

Therefore, relative atomic mass is given as an average, because the only way to reach an exact value would be to measure every single atom within each sample of carbon—an impractical task. Instead, through repeated measurements, scientists have been able to pinpoint approximately what percentage of each common isotope a sample of an element will contain. With chlorine, for example, about 75 percent of the atoms in a sample will have thirty-five neutrons, and about 24 percent of the atoms will have thirty-seven neutrons. Knowing the masses of chlorine-35 and chlorine-37 and the percentage of each that will occur in a given sample allows scientists to calculate the average mass for the sample and for the element in general.

Overview

Measuring the mass of atoms is an extremely complex process. The atoms are first vaporized and then given a positive charge by removing electrons. These positively charged atoms, known generally as ions (atoms with an electrical charge) and more specifically as cations (ions with a positive electrical charge), are then exposed to a magnetic field, which causes them to be deflected by amounts proportional to their mass and to the size of the positive charge they have been given. The amount of the deflection is then measured and compared with the amount of charge the cations were given to determine their mass.

Relative atomic mass has several practical applications. Carbon dating, for example, is the method used to determine roughly how old a quantity of organic material is. For example, when the remains of a mastodon have been unearthed by paleontologists who estimate that the creature died more than eleven thousand years ago, carbon dating is the method they use to arrive at that estimate.

Carbon dating adheres to the following procedure. Most carbon atoms have six protons and six neutrons, for an atomic weight of twelve. In every sample of carbon, though, there will be a known percentage of isotopes of different weights, one of which is carbon-14. Carbon-14 is interesting to scientists for two reasons. First, it decays, or breaks apart, into ordinary carbon-12 at a known rate. Second, carbon-14 is acquired by living organisms only while they are still alive. Organisms acquire the isotope by breathing in air and by eating food that contains carbon and other elements. Once an organism dies, it ceases to absorb any carbon-14.

As a result, a scientist can look at the carcass of a ten-thousand-pound mastodon and calculate that when it was alive it would have contained 98 percent carbon-12 atoms and 2 percent carbon-14 atoms. The scientist can then measure the carcass and discover that it now contains 99.5 percent carbon-12 and only 0.5 percent carbon-14, due to the decay of the carbon-14 over time. Because experiments have shown that carbon-14 has a half-life of approximately 5,730 years, meaning that it takes that long for half of any given sample of carbon-14 to decay, and 75 percent of the mastodon’s carbon-14 atoms have decayed, the scientist can conclude that the mastodon died approximately 11,460 years earlier. Note that in the second half-life cycle, only half of the remaining sample of carbon-14—that is, 0.5 percent of the overall carbon content—decayed, compared to the first half-life cycle, when 1 percent of the overall carbon content decayed from carbon-14 to carbon-12. This is because radioactive material decays exponentially. If another 5,730 years were to pass, only half of the remaining carbon-14 atoms, or 0.25 percent of the overall carbon content, would decay during that time.

Relative atomic mass has many other applications. It underlies virtually all chemical reactions; without it, society would be unable to manufacture many of the materials on which it has come to rely. Preparing a chemical reaction is similar to assembling the ingredients needed for a recipe; one must know the exact quantity required for each ingredient, and relative atomic mass is the unit used to measure these quantities.

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