Atoms, Elements, Isotopes & Nuclides

All matter is composed by atoms. Combinations of atoms give us molecules and compounds that consitute all materials we touch and feel every day. At a smaller level, atoms are composed by three subatomic particles: protons and neutrons, which stay in the nucleus of the atom, and electrons, which populate the space between nuclei. The nature of an atom is defined by the number of protons it has. We define all atoms with the same number of protons as being the same chemical element, so we call atoms with one proton hydrogen atoms, with two helium, three litium, and so on. Up to now (2018), 118 elements have been discovered. All elements up to plutonium (number 94) occur in nature, while the last 28 (americium to oganessium) are artificial.

As said, atom nuclei contain also neutrons. Elements can contain different amount of neutrons; e.g. hydrogen can have one to three protons. These “different element” are called isotopes, from the greek “same-place”, because they reside on the same spot on the periodic table of the elements. This is so because isotopes of a given element (usually) have the same chemical properties, so distinguishing among them is (usually) not important to a chemist.

When studying radioactivity and decay processes, isotopes are very important as it happens that for the same element some isotopes are stable, while other are not. Unstable isotopes are said to be “radioactive”, as they are “active” as radium, the first studied radioactive element. Using alwas hydrogens as example, hydrogen-1 (the -1 suffix indicates it has 1 neutron) is stable and is the most common form of hydrogen. Hydrogen-2 (aka deuterium) is also stable, but much less frequent in nature, only 0.02%. Hydrogen-3 (aka tritium) is instead radioactive with a half-life of 12.32 years and decays to helium-3.

Isotopes are soo important here that we give them a second name: nuclides. We consider the terms isotope and nuclide as synonyms.

We can classify nuclides based on their stability. Some isotopes are stable, that means they do not change with time. Hydrogen-1 is stable, so no matter how much we will wait, but 10 atoms of hydrogen-1 will always stay hydrogen-1. Other are unstable, like the mentioned hydrogen-3. That means these nuclides change with time and transforms (aka decay) to other nuclides. A characteristic propriety of an unstable nuclide is its half-life. This is the time (in seconds, days or years) that takes to half of a sample to decay. So, if the half-life of tritium is 12.32 years, it means that if we start now with a sample of 1 g of tritium, in 12.32 years 0.5 g will have decayed to helium-3. After other 12.32 years other 0.25 g would have decayed, and other 0.125 after another 12.32 years, and so on. As you can see the quantity of tritium halves every half-time decreasing the tritium quantitity more and more, but reaching zero more and more slowly.

The “son” of a radioactive nuclide can itself be stable or not. In the second case, the “son” nuclide would decay with its only half-life producing a third nuclide. In the same way a list of nuclides can be generated from a starting “father”, until a stable nuclide is formed. These are know as decay chains or decay series. Famous are the thorium-232 decay chain or the uranium-238.

We can further classify unstable (aka radioactive) nuclides based on their source, or how did they form. Some nuclides have a such long half-life that they formed in the cores of stars and supernova like all other stable nuclides. These are called “primordial” radioactive nuclides. There are 33 primordial nuclides. Most famous are the four shortest-living ones: Th-232, U-238, K-40 and U-235. These elements are radioactive, but as they were already present at the formation of our planet, they can be found in rocks and minerals. Sometimes, the distinction between stable and primordial radioactive nuclides is more formal than practical. For example, Te-128 is considered a primordial radioactive nuclide, but its half-life is 160 trillion times the age of the universe; for any practical purpose, no Te-128 atom has ever decayed. This is essentially true for a lot of the primordial radioactive nuclides.

Apart from primordial nuclides, there are other sources for natural radioisotopes. Some are generated as daughters of primordial isotopes, some by the interaction of stable nuclides with cosmic rays (e.g. C-14), other by geonuclear transmutation, like Np-237 or Pu-239. These all have a shorter half-time, but they are made continuosly every day.

Finally, some radioactive nuclides are artificial, produced in laboratories and nuclear reactors.