Promethium (Pm) is a rare earth element. It is unique because every one of its forms, or isotopes, is radioactive and unstable. This means Promethium does not have a single, fixed number of neutrons. The neutron count depends entirely on which specific, unstable isotope is being discussed.
Atomic Structure and Calculating Neutron Count
To determine the number of neutrons, one must understand the basic structure of the atomic nucleus. Every element is defined by its Atomic Number (Z), which represents the fixed count of protons. For Promethium, the Atomic Number is 61, meaning every Promethium atom contains exactly 61 protons.
The total number of particles in the nucleus is the Mass Number (A). This number represents the combined total of protons and neutrons. To isolate the neutron count, one subtracts the Atomic Number (Z) from the Mass Number (A). The mathematical relationship for finding the number of neutrons (N) is expressed as N = A – Z.
This equation demonstrates that for Promethium, the neutron count always equals the Mass Number minus 61. For instance, an atom with a Mass Number of 150 contains 89 neutrons (150 – 61). The Mass Number changes between isotopes and determines the final neutron count.
Promethium’s Isotopic Range and Specific Neutron Counts
An isotope is an atom of an element that has the same number of protons but a different number of neutrons. This difference results in a different Mass Number for each isotope. Promethium has a vast range of known isotopes, spanning from Promethium-126 up to Promethium-166.
The number of neutrons in Promethium can range from 65 (126 – 61) to 105 (166 – 61). Among the many forms, a few isotopes are notable for their longer half-lives. Promethium-145 is the longest-lived isotope, with a mass number of 145. Applying the formula, this isotope contains 84 neutrons (145 – 61).
Another relatively long-lived isotope is Promethium-147, which has a mass number of 147. This specific form contains 86 neutrons (147 – 61). Promethium-151, found on the heavier end of the practical range, contains 90 neutrons (151 – 61).
The Importance and Application of Promethium-147
Promethium-147 is the only isotope with significant commercial and industrial use. It contains 86 neutrons and 61 protons, resulting in a mass number of 147. It is preferred because its half-life of 2.62 years is long enough to be useful but short enough to be manufactured efficiently from nuclear fuel waste.
The usefulness of this isotope stems from its decay properties. Promethium-147 emits beta radiation (high-energy electrons) as it decays. Crucially, it does not produce significant amounts of penetrating gamma radiation, making it easier and safer to handle than other high-energy radioactive sources.
Promethium-147 is used as a beta particle source in various industrial gauges. It is employed in devices that measure the thickness of materials, such as the inner surface of graphite cladding tubes in nuclear reactors. The isotope’s radiation is also captured to power specialized nuclear batteries, known as beta voltaic cells. These power sources are used for applications needing long-life, low-power energy, including pacemakers and components for space probes.