Promethium (revised)

PROMETHIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Promethium is one of the most fascinating of all chemical elements. It has never been found on the Earth's surface. Scientists know of it only because it can be prepared artificially in particle accelerators ("atom smashers") and in other unusual reactions. Its existence was predicted as early as 1902, but its discovery was not confirmed until 1945.

All of the known isotopes of promethium are radioactive. That is, they break down and give off radiation spontaneously.

At one time, promethium was strictly a laboratory curiosity. Today, however, it has a number of practical industrial applications.

SYMBOL
Pm

ATOMIC NUMBER
61

ATOMIC MASS
144.9128

FAMILY
Lanthanide
(rare earth metal)

PRONUNCIATION
pruh-MEE-thee-um

Discovery and naming

In the late 1860s, Russian chemist Dmitri Mendeleev (1834-1907) discovered the periodic law. The periodic law provides a way of organizing the chemical elements to show how they are related to each other. It is usually represented by a table with 18 columns and 7 rows. Each chemical element belongs in one of the boxes of the periodic table.

By about 1900, most of the chemical elements had been discovered, but a few empty boxes remained on the periodic table. Chemists wondered why those boxes were still empty. In 1902, Czech chemist Bohuslav Brauner predicted that there should be an element between neodymium (number 60) and samarium (number 62). Chemists began searching for the element based on the characteristics of the elements around it.

In 1924, Italian chemists Luigi Rolla and Rita Brunetti claimed to have found element 61. They suggested the name florentium for their home town of Florence. At about the same time, scientists at the University of Illinois also announced the discovery of element 61. They proposed the name illinium for Illinois.

Gradually, scientists began to believe that element 61 was radioactive. A radioactive element is one that breaks apart and gives off some form of radiation. One way to make radioactive elements is to fire very small particles at atoms. The particles stick in the atoms and make them radioactive. In the late 1930s, scientists at Ohio State University thought they had found element 61. They suggested the name cyclonium, after the kind of particle accelerator they used to make the element, a cyclotron.

None of the "discoveries"—from Italy, Illinois, or Ohio—could be confirmed by other scientists. A great debate went on for many years as to whether element 61 had really been found or not. Finally, the problem was solved. During World War II (1939-45), scientists at the Oak Ridge Laboratory in Oak Ridge, Tennessee, were studying the materials formed during atomic fission. Atomic fission is the process in which large atoms break apart, releasing large amounts of energy and smaller atoms. The smaller atoms are called fission products.

The Oak Ridge scientists proved that element 61 was present in fission products of uranium. They named it promethium, after the Greek god Prometheus. According to legend, Prometheus stole fire from the gods and brought it to Earth for human use.

Physical properties

Promethium is a silver-white metal with a melting point of 1,160°C (2,120°F) and no measured boiling point. Its density is 7.2 grams per cubic centimeter. The physical properties of promethium are of less interest to scientists than its radioactive properties.

Chemical properties

Promethium behaves like other rare earth elements. The chemical properties of promethium are of less interest to scientists than its radioactive properties.

Occurrence in nature

Promethium has never been found in the Earth's crust. It has been observed, however, in the spectra of some stars in the galaxy of Andromeda. The spectrum (plural: spectra) of a star is the light given off by the star.

Isotopes

Fifteen isotopes of promethium are known. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

The only isotope generally available is promethium-147, with a half life of 2.64 years. The half life of a radioactive element is the time it takes for half of a sample of the element to break down. That means for promethium-147 that after 2.64 years, only half of a 100-gram sample, for example, or 50 grams, will be left. Another isotope, promethium-145, has a longer half life of 18 years.

Extraction

Promethium is not found in the Earth's surface.

Uses and compounds

Promethium has limited uses. It can be used as a source of power. The radiation it gives off provides energy, similar to that from a battery. A promethium battery can be used in places where other kinds of batteries would be too heavy or large to use, as on satellites or space probes. Such batteries are far too expensive for common use, however.

Promethium is also used to measure the thickness of materials. For example, suppose thin sheets of metal are being produced on a conveyor belt. A sample of promethium metal is placed above the metal and a detector is placed below. The detector counts the amount of radiation passing through the metal. If the metal sheet becomes too thick, Less radiation passes through. If the sheet becomes too thin, more radiation passes through. The detector reports when the sheet of metal is too thick or too thin. It can automatically stop the conveyor belt when this happens.

Some compounds of promethium are luminescent. Luminescence is the property of giving off light without giving off heat. The light of a firefly is an example of luminescence. Promethium compounds are Luminescent because of the radiation they give off.

Health effects

Like all radioactive materials, promethium must be handled with great care. The radiation it produces can have serious health effects on humans and animals.