Why are some elements rarer than others? Some elements are rarer than others because lighter elements are easier to make, and heavier elements require a huge amount of energy. Also, elements with even atomic numbers are more common than those with odd numbers. And some elements are destroyed inside stars.
This is an interesting question because it isn’t the case that all of the lighter elements are common and all of the heavier elements are rare. Some of the light elements are rare. We should also mention that rare earth metals on Earth are not technically rare; they are just very hard to process. And it is also worth mentioning that some elements are rare on Earth without being rare in the universe.
Let’s begin by looking at why lighter elements are easier to make than heavier elements. The atomic number of an element is the number of protons it contains. Hydrogen has one proton, helium has 2, carbon has 6, iron has 26, and gold has 79. The heaviest element, which is synthetic, is Oganesson, and it contains 118 protons.
Protons have a positive electrical charge, and because like charges repel, they push each other apart. There is another force, called the strong force, which can hold neutrons and protons together when they are extremely close. However, to get the protons close enough together so that the strong force takes over requires a lot of energy. Think of it like two magnets with superglue on them. If you face the south and south ends of the magnets together, you will have a lot of trouble getting them to touch, but if you can push them hard enough, they will touch, and then the superglue will hold them. Because it takes so much energy to bring more than one proton together, heavier elements are only fused in the stars with the highest temperatures and pressures. This is why there are far more lighter elements than there are heavier ones.
Just after the Big Bang, the universe was filled with protons and neutrons. As it cooled, some of these particles combined to form helium nuclei, but most of the protons remained as hydrogen. Within a few minutes, the universe contained mostly hydrogen, a substantial amount of helium, and tiny traces of lithium. Millions of years later, stars formed from these elements. Inside the stars, hydrogen nuclei began fusing into helium, releasing enormous amounts of energy. This is why stars shine. Once all of the hydrogen has been made into helium, if the star is big enough, it will start to fuse helium together and make carbon. In sufficiently massive stars, fusion can continue creating heavier elements, including carbon, oxygen, and silicon. Eventually, the star may develop a core rich in iron-group elements. However, they have to stop when they get to iron. Up to the point of iron, fusing elements together always produces energy, which stops the star from collapsing under its own weight. Fusing iron doesn’t work, though, because it takes energy to fuse iron, rather than produce it. When some massive stars develop iron-rich cores, their cores can collapse, triggering enormous explosions called supernovae. These explosions scatter elements into space. Other types of supernovae also produce large quantities of iron.
We obviously have elements that are heavier than iron, so something must have fused them. Many elements heavier than iron are formed through a different process. Atomic nuclei absorb neutrons, some of which eventually change into protons, creating new elements. This can happen inside aging stars, but the heaviest elements, including gold and platinum, can also form during extraordinary events such as the collision of two neutron stars. Neutron stars don’t collide very often, which is why the really heavy elements are so rare.
However, there is another reason why there are more of some elements than others. We have to look not only at how elements are made, but at how they are destroyed as well. Some surprisingly light elements, including lithium, beryllium, and boron, are relatively rare because nuclear reactions inside stars can easily destroy them. They can react with protons at lower temperatures than many heavier elements. For example, lithium-7, which contains three protons and four neutrons, can absorb another proton and briefly become unstable beryllium-8. This almost immediately splits into two helium nuclei. Stars also don’t produce enough of these elements through ordinary fusion to replace what they destroy.
Another interesting pattern is that elements with even atomic numbers are more abundant than elements with odd atomic numbers. This is partly because protons become more stable when paired inside atomic nuclei. It is also because stars can build heavier elements by adding helium nuclei, each containing two protons and two neutrons. This naturally favors elements with even numbers of protons. Scientists call this the Oddo–Harkins rule, after the two researchers who identified the pattern in the early 20th century.
Some elements are extremely common in the universe but relatively rare on Earth. Helium is a good example. It is the second most abundant element in the universe, but Earth’s supply is limited. Helium is extremely unreactive, so it rarely becomes chemically trapped in minerals. Because its atoms are so light, helium can also escape from Earth’s atmosphere into space. Most of the helium we use comes from underground natural gas deposits, where it has accumulated over millions of years. Once released into the atmosphere, it is very difficult to recover. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Oganesson
https://www.iflscience.com/why-are-the-universes-most-common-elements-so-rare-on-earth-75500
https://www.dongascience.com/en/news/76372
https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements
Photo by Malcoln Oliveira: https://www.pexels.com/photo/close-up-of-iron-ore-texture-in-minas-gerais-34045315/