What does make a radioactive element radioactive?

Rodolfo Marin Rivera

1The periodic table as we know contains 118 elements,

which are organised according to their atomic number, electron configuration, and chemical properties thanks to discovery done be Dmitri Mendeleev (1869). However, all of them may have some level of radioactivity.

As we know, atoms are made up of small, positively charged nucleus surrounded by a relatively large space (orbitals) occupied by tiny, fast-moving, negatively charged electrons, which are 10,000 to 100,000 times smaller than the atoms. Despite its relatively small size, the nucleus contains over 99.9% of the mass of the atom. A very close approximation about what is going on during the interaction between the electrons and nucleus was given by Bohr (1913) who depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus with attraction provided by electrostatic forces.

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The nucleus is made of positively charged protons and neutral neutrons. The number of protons is equal to the atom atomic number and determines, for instance, what element it is. For example, an atom with 8 protons in its nucleus is an oxygen atom, the eight-element listed in the periodic table. The number of neutrons will simple affect the mass of the atom and determine which isotope of oxygen is. Therefore, every chemical element has one or more isotopes that, for a given element, the number of protons and neutrons may change or not. Isotopes are better known as nuclide. Nuclide refers to any isotope of any element, or in other words, a nucleus with any number of protons and neutrons. Therefore, the radioactivity of a certain element is given by the transformation of a certain nuclide, or by the intensity at which the nuclide may decay. This means that a radioactive nuclide is one that spontaneously undergoes nuclear decay.

3During nuclear reactions (as the big bang), the interaction between a particle (like a neutron) or a photon with a nucleus leads to the formation of another nuclide. This transformation is known as nuclear decay. A nuclear decay stars with an unstable nuclide (parent isotope) that typically spits out a particle and/or a photon while turning itself into something more stable. Therefore, energy is released as part of all nuclear decay. There are three different kinds of decay: alpha (i.e. ejection of neutrons and protons at once, formation of the helium atom), beta (ejection of electrons) and gamma (no particles are emitted or absorbed, and both the numbers of protons and neutrons remains the same). In this last type of decay, at the beginning, neutrons and protons are arranged poorly, in terms of energy. That is to say one or more neutrons/protons occupy a higher energy state than they need to, and that the nucleus is said to be in an excited state. Rearrangement allows the neutrons/protons that are in unusually high-energy states to drop down into lower ones. The lowest energy configuration would be called the ground state. Rearrangement from a higher energy state to a lower one requires the release of energy, which can be done in the form of a high-energy photon, or gamma ray.

Nowadays, about 20 radioactive nuclides are recognised, which will take a very long time to decay. One of the most notable is 40K. Since all isotopes of an element have the same chemical behaviour, and because we humans need potassium in our bodies, we all have some 40K in our bodies. Anything that contains potassium (bananas, Gatorade®, your friends) is naturally radioactive, i.e. of natural radioactive origin (also known as Naturally-Occurring Radioactive Materials, NORM).

All but three of these 20 long-lived radioactive nuclides decay to stable nuclides. The three that don’t are 232Th, 235U and 238U. They all eventually decay to stable isotopes of lead, but along the way, they produce a number of other naturally occurring radioactive nuclides (e.g., 222Rn).

4You must know that from all the ionizing radiation that exist around us, not all of them is related to our activities as human being, because radionuclides exist everywhere in nature (in soil and rocks, air and water, in plants and in our bodies). Some of the isotopes have been there since the creation of Earth, others are being constantly produced or brought in from outer space as cosmogenic radiation.

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