Why do atoms stay electrically stable?

Science in dialogue

Why don't atomic nuclei burst apart?

Some atomic nuclei actually “burst” apart, namely those that are not stable. The stable cores, on the other hand, remain - as far as the previous level of knowledge - indefinitely. Whether an atomic nucleus is stable or not depends on the number of its core components and their interaction with one another.

Atomic nuclei consist of positively charged protons and electrically neutral neutrons. Two opposing forces are at work there. The electromagnetic interaction drives the core apart, the strong interaction holds it together. The electromagnetic interaction only works between charged particles, i.e. in the nucleus between the protons. Their similar charges repel each other. The electromagnetic interaction has a relatively long range, but is relatively weak. The strong interaction, on the other hand, attracts the core particles to one another. It is very strong, but its range is limited. If the attractive force outweighs the repulsive force in the balance sheet, a nucleus is stable, otherwise it disintegrates and emits radioactive radiation.

The size of the atomic nucleus has a major influence on the stability. If it exceeds a certain radius, a proton only experiences the attractive strong force of the directly neighboring nuclear particles because of its short range. In contrast, the repulsive force of all protons continues to act. As the diameter increases, the balance of forces shifts in favor of a repulsive effect. For this reason, stable isotopes could only be detected among the elements from hydrogen to lead. Isotopes are variants of an element with the same number of protons but different numbers of neutrons. Some isotopes of these elements are stable and some are not. In all previously known elements with a core larger than lead, all isotopes are unstable, so they decay sooner or later. A measure of the stability of an atom is its half-life. For example, the most common and longest-lived uranium isotope U-238 has a half-life of around 4.5 billion years. This means that after this time, half of the uranium has decayed.

The decay of an atom can happen in different ways. In doing so, however, high-energy radiation (ionizing radiation) is always released - in the form of particles and / or as gamma radiation. Some elements that are heavier than lead get rid of the “excess” core particles, for example by releasing alpha particles. These consist of two protons and two neutrons. Even heavier elements - such as uranium - break down into fragments of any size. This process could also be called “bursting”. In addition to the size, the ratio of protons to neutrons also has an influence on the stability of the nucleus. If a single proton disturbs the stability, it transforms into a neutron, if one neutron is superfluous, it becomes a proton. The end products of the decay processes are always stable isotopes of the elements from hydrogen to lead.

Physicists are working on a formula that will make it possible to predict which isotope is stable and which is not. You have to take into account that protons and neutrons also have an internal structure. They each consist of three so-called quarks. The strong interaction has an attractive effect between the quarks. However, their effect extends slightly beyond this, so that core particles also attract one another. The internal structure of the core particles leads to a complicated relationship of forces in the atomic nucleus. Calculations have shown that there could be an "island of stability" for the elements with a mass number (number of protons) from 114 to 118. However, this has not yet been confirmed experimentally. The heaviest element reliably proven so far is the Roentgenium with the mass number 111. It has a half-life of only a few thousandths of a second. It was first detected in 1994 in a particle accelerator of the Society for Heavy Ion Research.

The question was answered by Dr. Ingo Peter from the Society for Heavy Ion Research in Darmstadt.