By Ron B. Davis Jr., Georgetown University
Atoms of one element can indeed change from one type of element to another through two major processes. One process is radiation, in which one or more sub-atomic particles are emitted from a nucleus, often changing the number of protons. The other process is fission, in which a nucleus breaks into two significantly large pieces, creating two new, smaller elements.
The True Structure of Atoms
John Dalton’s famous experiments in the early 1800s provided a compelling argument for the existence of atoms. But Dalton’s take on atoms was that they were truly as simple as the ancient meaning of their name, which means ‘indivisible’. Dalton believed that atoms were something like billiard balls—hard, unchangeable spheres of uniform consistency that could never be changed.
But we have since learned that Dalton’s assumption about simple, indivisible atoms was far too simple. We now know that atoms have structure. Atoms have a clearly defined structure of protons and neutrons in the nucleus, and electrons arranged in energy levels and orbitals that surround the nucleus.
Moreover, it stands to reason that if elements can be built up through nuclear fusion, it should also be possible for them to break down, losing one or more protons to create a completely different kind of atom.
Not All Atoms Are the Same
As scientists began unravelling the mystery of atomic structures in the early 1900s, it started to become clear that not all atoms are created equal. Although it is the number of protons in a nucleus that give each element its identity, the number of neutrons can vary for a given element. The different numbers of neutrons for a given element are called isotopes.
Now, all elements include the possibility of unstable isotopes—we can even make them any time we remove too many neutrons, or add too many neutrons.
But some elements also have several stable isotopes—different ways to form a stable nucleus that will never decay into simpler elements. Other elements have only one stable isotope. Then there are heavy elements whose big atoms are incapable of forming even a single stable isotope. These are elements we usually call radioactive.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
The Chance of Becoming Stable
The ratio of the number of protons to neutrons within any given nucleus is going to go a long way toward helping us predict whether or not it even has a chance of being a stable nucleus.
The larger the number of positively charged protons in the nucleus, the more they must be diluted with a larger and larger ratio of additional neutrons. All the nuclei were imagined as milling about the nucleus randomly, while helping it to stay together. That was called the ‘liquid drop’ model of the nucleus.
But, in fact, look closer and there is evidence for a more defined structure in the nuclei of atoms. This led to the idea of using some sort of shell model for the nucleus, where some numbers are more important than others.
It was Manhattan Project scientist, Maria Goeppert Mayer, who is commonly credited with cracking the code for nuclear stability. Mayer was building on the work of another physicist on the project, Eugene Wigner. Manhattan Project scientists were in the business of making and breaking atomic nuclei that would never decay on their own.
The Magic Number
Wigner had determined that elements with twenty protons or neutrons seemed to have some of the most stable nuclei of all. This discovery perplexed Wigner, who was a firm believer in the liquid drop model.
But Mayer was able to demonstrate that 20 wasn’t the only special quantity that could make an atom’s nucleus particularly stable. She calculated that atoms with 2, 8, 20, 28, 50 and 82 and 126 protons should all be unusually stable. Based on her calculations, Mayer theorized that nuclei did, indeed, have more structure to them, and that the interaction of one nucleon with others in the nucleus was not random.
Not surprisingly, elements with these numbers of protons are some of the most common in the cosmos: helium, oxygen, calcium, nickel, tin and lead, with element 126 perhaps to come in the future. As the story goes, Wigner, when confronted with evidence of more complex nuclear structure was so astounded that he referred to Mayer’s numbers as ‘magic numbers’—a name that has stuck.
What’s more, these magic numbers also apply to neutrons, making some isotopes ‘doubly magic’, and the most stable of all. These include the isotopes helium-4 and oxygen-16, calcium-40 and calcium-48.
The takeaway was that nuclei have a definite internal structure that defines both the most stable and most unstable of atomic nuclei. This made it possible to consider just how unstable nuclei might break down in ways that produce more stable products.
Common Questions about Breakdown of Elements and Nuclear Stability
Maria Goeppert Mayer was one of the Manhattan Project scientists who is credited with cracking the nuclear stability code. Manhattan Project scientists, including Mayer, were working to build or break down the nuclei of stable atoms that would never decay on their own.
Helium, nickel, calcium, tin, lead, and oxygen are among the most common elements in the universe. Thanks to their ‘magic numbers’, these elements enjoy nuclear stability.
Eugene Wigner determined that elements with 20 protons/neutrons have the most stable nuclei. Later, Maria Meyer proved that 20 is not the only quantity that can stabilize the nucleus of an atom. She calculated that atoms with 2, 8, 20, 28, 50, 82, 126 protons are the most stable ones. These numbers are referred to as magic numbers.
