Why Is Technetium Radioactive?

Periodic Table of Radioactive Elements
Technetium is the lightest radioactive element.

Technetium is a radioactive element, with no stable isotopes. With an atomic number of 43, it is the lightest unstable element. All of the elements surrounding it on the periodic table have at least one stable isotope. What is it about technetium that makes it special? The short answer is that there is no number of neutrons you can put in a technetium atom to form a stable nucleus.

The atomic nucleus consists of protons and neutrons. While the identity of the element is defined by its number of protons (atomic number), an atom can contain different numbers of neutrons (forming different isotopes). For lighter elements, the most stable isotope is usually the atom containing an equal number of protons and neutrons. At first glance, this makes sense, because the mass of the proton and neutron are nearly the same. However, the mass of a proton is ever so slightly greater than the mass of a neutron, so in atoms with larger atomic numbers the mass difference becomes significant. The most stable neutron to proton ratio increases as atoms gain mass, changing from a 1:1 ratio for light elements to more like a 1.3:1 ratio for heavier elements. In the cases of technetium and the next lightest radioactive element (promethium), there is no combination of protons and neutrons that balances out. To make matters more confusing, the mass of an atomic nucleus is actually less than the sum of the mass of the protons and neutrons, because some mass is converted to nuclear binding energy.

Odd and Even Proton Numbers

A technetium atom contains 43 protons, which is an odd number of protons. The evenness or oddness of the atomic number affects the properties of the atomic nucleus. Atoms containing an even number of both protons and neutrons (EE nuclides) tend to be the most stable. Because the protons and neutrons are paired, the nuclei have spin 0. Atoms containing even numbers of protons, but odd numbers of neutrons are less likely to be stable. There are 53 stable nuclides with an even number of protons and odd number of neutrons. Atoms with an odd number of protons and even number of neutrons are even less stable. There are 48 stable nuclides of this type. Atoms containing an odd number of protons and odd number of neutrons are least likely to be stable. There are only five stable nuclides of this type (e.g., deuterium). The unpaired proton and unpaired neutron exert a stronger nuclear force attraction to each other if their spins are aligned, so odd-odd nuclei produce a total spin of at least 1.

The Mattauch Isobar Rule

Although it doesn’t explain the behavior, the Mattauch isobar rule can be used to predict the radioactivity of technetium and promethium. In 1934, Josef Mattauch formulated a rule that says if two adjacent elements on the periodic table have isotopes with the same mass number (isobars), one of the isotopes must be radioactive. Molybdenum and ruthenium both have stable isotopes, so the corresponding isobars for technetium must be unstable. Neodymium and samarium both have stable isotopes, so the isobars for prometium must be unstable. Although it holds true for technetium, there are exceptions for the Mattauch isobar rule. For example, both antimony-123 and tellurium-123 are stable. However, the rule can be applied to most of the periodic table to make predictions about isotope stability.


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