Atomic nucleus - atomic shell

All elements of the periodic table have an atomic nucleus and an atomic shell in which the electrons are located, which are primarily responsible for the chemical properties of an element. The size ratio of the diameters between the atomic/electron shell and the atomic nucleus is as follows
20.000 - 150.000 : 1, depending on the element.

  • The mass of the electron is
    approx. 9,109 E-31 kg or 0,511 MeV.
  • The mass of the proton is
    approx. 1,673 E-27 kg or 938,272 MeV.
  • Neutrons have a similar mass to protons, at
    approx. 1,675 E-27 kg or 939,565 MeV.

Thus, 99,95 % to 99,98 % of the total mass of an atom is located in the atomic nucleus.

Effects on the atomic nucleus

Isotopes are elements with the same atomic number but different mass numbers, because the number of neutrons varies.

If there is now an uncomfortable ratio between protons and neutrons, this atomic nucleus is called unstable. This instability manifests itself in the fact that the atomic nucleus has a higher ambition to change into a low-energy state. To achieve this, the atomic nucleus decays. In such a decay, a parent nuclide (initial nuclide) subsequently gives rise to decay components (daughter nuclides, energy release and other decay components).

Properties of the nuclear components

The neutrons serve as "glue" in the atomic nuclei by being able to attract the protons as well as themselves (strong interaction).

This gives an atomic nucleus, or from now on called a nuclide or isotope, its stability.

Nuclides are characterised by:

  • The element symbol (for example "C" for carbon)
  • The mass number A (number of protons and neutrons, for example 14)
  • The atomic number Z (number of protons, for example 6 for carbon) 



What are the different types of radioactive decay?

With the prerequisite of the instability of an atomic nucleus, we now go one step further.

Basically, the most important types of decay include (on the left in the picture, the most important radiation particles for radiation protection):

  • Alpha decay (α-particle on the left in the picture)
  • Beta(+) decay
  • Beta(-) decay (β(-) particle on the left in the picture)
  • Gamma - conversion (γ-quant/wave on the left in the picture)
  • Spontaneous fission
  • Cluster decays and emissions of protons or neutrons (n-particle on the left in the picture)


Sources and references:

Source 1: (aufg. 01.04.2021)

Source 2: Grupen, Claus (Springer) (2008), Grundkurs Strahlenschutz, 4. Auflage, Heidelberg.

Source 3: (aufg. 01.04.2021)

Source 4: Vogt, Hans-Gerrit; Schultz, Heinrich (Hanser) (2007), Grundzüge des praktischen Strahlenschutzes, 4. Auflage, München.

Source 5: (aufg. 01.04.2021)