Helium-4

Helium-4 is a stable isotope of the element helium. It is the most abundant form of helium found in nature. In fact, it makes up virtually all the helium found on Earth. This isotope is also a massive component of the entire universe. It accounts for about one quarter of the ordinary matter in the universe by mass. Most of the remaining matter is hydrogen.

To understand helium-4, we must look at its internal structure. The atom consists of a nucleus surrounded by an electron cloud. The nucleus contains two protons and two neutrons. Around this nucleus, two electrons orbit in a cloud. This setup makes helium-4 the second simplest atom, following only hydrogen. Because it has an extra electron, calculating its exact movement is a complex three-body problem. Scientists use numerical approximations to estimate its size and ionization energy. The nucleus is incredibly small, measuring about 1 femtometer in scale.

The stability of helium-4 is truly remarkable. Scientists describe its nucleus as being "doubly magic." This means the two protons and two neutrons are arranged in a very stable way. They occupy specific quantum mechanical levels called 1s orbitals. In these orbitals, the particles exist in pairs. Each pair cancels out the other's intrinsic spin. This specific arrangement is energetically extremely stable. Adding even one more particle would require more energy than is available. This is why no nucleus with five nucleons is stable.

This internal stability affects how helium behaves in the world. The stable electron cloud makes helium chemically inert. This means it is the least reactive of all elements. It also explains why helium atoms do not interact much with each other. This lack of interaction results in the lowest melting and boiling points of all elements.

Helium-4 has a fascinating history tied to the beginning of time. Most helium-4 in the universe is considered primordial helium. This means it was produced during the Big Bang. While stars also produce helium-4 through nuclear fusion, the Big Bang provided the largest amount. On Earth, however, primordial helium is mostly absent. It escaped our planet during the high-temperature phase of Earth's formation. Today, most helium-4 on Earth comes from alpha decay. This happens when heavy elements in the Earth's crust break down.

When helium-4 is cooled to extreme temperatures, it shows strange properties. If liquid helium-4 is cooled below a specific point, it becomes a superfluid. A superfluid is a liquid that flows with almost no friction. It behaves very differently from ordinary liquids. For example, a thin Rollin film can form on the surface. This film can actually climb the sides of an open vessel. This causes the liquid to escape the container. Some scientists theorize that at 0.2 K and 50 atm, it may become a superglass. A superglass is a solid that still shows superfluid behavior.

The stability of helium-4 also shaped the composition of our universe. During the Big Bang, free protons and neutrons were available. As the universe cooled, these particles fused into nuclei. Because helium-4 is so stable, it consumed nearly all the free neutrons. This left very few neutrons to form heavier atoms like lithium or boron. This process also explains why the universe has a specific ratio of hydrogen to helium. Today, that ratio is about three parts hydrogen to one part helium-4 by mass.

Finally, helium-4 is central to how stars work. In the Sun, hydrogen is converted into helium-4 through fusion. This happens because creating helium-4 is highly energetically favorable. It is much easier to produce than helium-3 or other elements. Because the Big Bang produced so much helium-4, it left little energy to make heavier elements immediately. Most heavier elements, including those needed for rocky planets and life, had to be made later in stars. These stars had to be hot enough to fuse helium into heavier elements like carbon.