The Physics Behind Iron: Why It’s The Most Stable Element

All the elements in the periodic table exist in multiple forms, known as isotopes. These isotopes are classified as stable or unstable, with the unstable isotopes undergoing radioactive decay or fission to become something else. There are, however, hundreds of known stable isotopes that have never been seen to undergo decay, so you might think that no element is more stable than another. Despite this, iron is considered the most stable element, so what is going on?

How Do We Measure An Element’s Stability?

First off, it’s important to note that an element’s stability is not related to its chemistry. Some elements, like caesium are highly reactive, while others, like the noble gases, are immensely hard to get to react with anything. However, this is all about the behavior of electrons orbiting the elements’ nucleus. An atom’s stability, on the other hand, relates to what is happening in the nucleus itself, and whether it will decay to form a different nucleus entirely, releasing radiation in the process, or split into smaller nuclei.

To understand what makes an element stable, it’s easiest to start with the reverse: what makes one unstable?

An atomic nucleus is made of protons and neutrons, together called nucleons. The number of protons defines what element it is, with the number of protons and neutrons combined defining the isotope number. An atom with 6 protons is carbon, and if it also has 6 neutrons it is carbon 12, while 8 neutrons makes carbon 14.

The nucleus is held together by the nuclear force, and is described as having a “binding energy”, the energy required to take it apart into separate protons and nucleons.

Some isotopes are unstable because an imbalance in their number of protons and neutrons gives them an excess of energy. It’s a little like a heavy object on a shelf or table. It has potential energy, which will be released if it falls off. For the object, an external force like an earthquake or a cat might push it over. For the atomic nucleus, internal factors cause it to spontaneously fission into two or more smaller atoms, or to radioactively decay by releasing an alpha or beta particle.

The more unstable a nucleus is, the more quickly this is likely to happen, giving the isotope a shorter half-life. 

Stable isotopes are those that are not known to decay. However, there are currently 251 known isotopes that are stable – that is we have never seen them decay or fission of their own accord. Looked at this way, it might be thought they are all equally stable – no one stands out.

These known stable isotopes come from 80 elements, so some elements have multiple stable isotopes. You might think the element with the most stable isotopes would be awarded the prize of most stable, but you’d be wrong. The fact that tin has 10 stable isotopes, while no other element has more than seven, might be a question at very nerdy trivia nights, but it doesn’t determine the most stable element.

True Stability

Notably, of these supposedly stable isotopes, almost two-thirds could theoretically decay, either through radioactivity or spontaneous fission. We know this because possible transitions have been identified that are energetically possible, like an object falling off a shelf, rather than magically jumping up to one. These decays or splits have never been observed, but that doesn’t mean they don’t happen very rarely, indicating an exceptionally long half-life. The longest known half-life of an unstable isotope belongs to tellurium-128, which has a half-life of 2.2 x 10²⁴ years, or more than a hundred trillion times longer than the current age of the universe. It’s not surprising that if some of the supposedly stable isotopes have even longer half-lives, we’ve missed their decays.

That leaves 90 isotopes that are stable in theory as well as observation. Any decay or fission will leave behind products with more total energy than the isotope itself, so it can’t happen spontaneously.

Therefore, the only way these isotopes can ever turn into something else is under the influence of outside forces. For example, if you bombard a nucleus with neutrons, it may obtain the energy it needs to fission.

This explains why some isotopes can be more stable than others. For some, only a little disruption in the form of extra energy is required to overturn their stability. Others are much more resistant.

Out of all of these, the most stable isotope of all is iron-56.

What Makes Iron So Stable?

Iron-56 has one of the highest binding energies per nucleon of any isotope (second only to nickel-62). If binding energy is imagined as like the glue holding the protons and electrons together, larger atoms have more glue in total, but this is divided by a larger number of nucleons, which have more forces pushing them apart. Having the most glue per proton and neutron makes for maximum stability.

The consequence of this is that in the supremely energetic environment of a supernova or kilonova, the forces available can push nucleons together in many combinations, but iron-56 is the one most likely to stick. This makes iron the most common of the heavier elements in the universe, although still much rarer than those like hydrogen and helium that were formed in the Big Bang. 

The stability of a nucleus is affected by several things. For one, there needs to be a balance between neutrons and protons. Lighter elements are generally most stable when they have similar numbers of each, while heavier elements need more neutrons to overcome the repulsive forces between the positively charged protons.

Just as there are electron “shells” that make atoms less chemically reactive when all shells are full, atomic nuclei have their own shells. These are most stable when there is an even number of both protons and neutrons. Iron has 26 protons, so it is immediately at an advantage compared to the half of the periodic table with an odd number of protons. By having 30 neutrons, iron-56 fulfills the requirement for that to be even as well.

Almost 92 percent of the iron on Earth is iron-56, partly because so many other iron isotopes are unstable and have long since decayed.