Potassium is a chemical element, which corresponds to the symbol "K" (lat. Kalium) and number 19 in the periodic table of D. I. Mendeleev. From Arabic, the name of this element is translated as "plant ash". The metal has bright alkaline properties, has a white-silver color and is common in nature in salt water and minerals. The chemical activity of the metal is quite high, it is easily oxidized in air. There are a large number of potassium isotopes that are used in various fields of human activity.
What are atomic isotopes?
Before considering the issue of potassium isotopes, we explain the term "isotope". Under the isotopes of an atom of a specific chemical element is meant a set of atoms that have the same number in the periodic table, but differ from each other in the mass of the nuclei. Moreover, the number of protons in all nuclei of isotopes of this element is the same. Only the number of neutrons differs.
The term "isotope" itself originates from the Greek language and consists of two words: isos - equal and topos - place. This term was first used at the beginning of the 20th century by the English scientist Frederick Soddy, who went down in the history of chemistry as a researcher of the radioactivity of atoms and the nature of isotopes of various elements. For his services, Soddy received the Nobel Prize in chemistry in 1921.
Note that most chemical elements have more than one isotope. Only 8 elements of the periodic table have one isotope, for example, sodium or beryllium. The element that has the most stable isotopes is tin; it has 10 of them.
Isotope stability
For the atomic nucleus of a specific chemical element to be stable, it is necessary to observe a certain ratio of neutrons and protons in this nucleus. If this ratio is violated, the isotope becomes unstable and, as a result of the processes of radioactivity, decays into more stable isotopes. The concept of stability is relative, since many isotopes, being unstable, have a long half-life and can be considered relatively stable.
Unstable or radioactive isotopes are converted to more stable atoms. As a result of this process, beta, alpha or gamma radiation is emitted.
Artificially produced radioactive isotopes are used for various research techniques in medicine. The use of natural radioactive isotopes finds itself in the field of geology to determine the age of rocks and living organisms, as well as in the production of atomic energy.
Potassium isotopes
Potassium has 24 different isotopes. These isotopes have a mass of 32 to 56 in atomic mass units (AEM). Considering that potassium occupies the 19th serial number in the periodic table, it means that the nucleus of the potassium isotope contains 19 protons and from 13 to 37 neutrons. Of all these isotopes, only three have a long lifetime. More precisely, we note that two isotopes, 39 K and 41 K, are stable, while the third isotope, 40 K, has a long half-life (1.25 billion years).
The content of potassium and its isotopes in nature
The content of potassium isotopes in nature is 2.4% of the mass of the entire earth's crust, which puts it in 7th place in the list of the most common elements on our planet. Due to its high chemical activity, it is very difficult to obtain pure potassium. In addition, the solubility of potassium is very high, therefore, in many minerals it is contained in small concentrations. Nevertheless, at the bottom of ancient lakes there are large quantities of this element in the form of the following minerals:
- carnallite;
- langbainite;
- polygalite;
- sylvin.
Of these minerals, the extraction of potassium is economically feasible. But potassium does not exist in these minerals as a single isotope. As mentioned above about stable isotopes, potassium has three isotopes, the content of which is 93.3% 39 K, 6.7% 41 K, and 0.012% 40 K.
Relatively stable K40 isotope
A radioactive isotope of potassium 40 K decays either due to electron capture or due to the emission of a positron. In this case, an argon isotope 40 Ar is formed. The long half-life of 40 K (1.248 * 10 9 years) is explained by the spin-forbidden transition of this isotope to another stable element.
In any potassium-containing mineral, this radioactive isotope is in sufficient quantities to demonstrate the phenomenon of radioactivity in school experiments. In humans and animals, this isotope is the main source of their radioactivity. So, in a person weighing 70 kg in the body there are about 4400 nuclei of 40 K, which decay every second.
The radioactive decay of 40 K into 40 Ar is used as one of the most reliable methods for determining the age of rocks. The essence of the potassium-argon method is that when the mountains were converted, they did not contain argon. In the course of time, 40 K atoms decayed to form argon, which was retained in the rock mass. Determining the amount of this noble gas, as well as the percentage of potassium isotopes gives information about the geological age of the rocks.
Other radioactive isotopes
With the exception of the noted 40 K isotope, all other isotopes of this chemical element have half-lives of less than 1 day, while most of the isotopes decay faster than a minute passes. Why do potassium isotopes having masses other than 39-41 AEM decay so quickly? The fact is that the nucleus of the potassium atom contains 19 protons, and the laws of stability of the atomic nucleus say that the number of neutrons should be approximately the same so that the atom can exist for a large amount of time without experiencing radioactive decay. In other words, the closer the number of neutrons in the potassium nucleus to 19, the more stable such an isotope will be.
For example, the 42 K and 43 K isotopes (number of neutrons 23 and 24) have half-lives of 12.3 hours and 22.3 hours, respectively. At the same time, the 33 K and 34 K isotopes (number of neutrons 14 and 15) decay in a few nanoseconds.