What do radioactive atoms emit

Radioactive decay

What is radioactive decay?

Since atomic nuclei are converted into new nuclei when radioactive radiation is generated, ie the original nuclei “disintegrate”, one speaks of radioactive decay.

Depending on the type of radiation, a distinction is made between the different types of decay α-decay, β-decay and γ-decay.

In order to understand what happens to the atomic nucleus during the respective decays, we need a little background knowledge about the structure of atomic nuclei as well as some terms, which are explained in the following section:

Nuclides and isotopes - identified by mass number and atomic number

As described in the first section, atomic nuclei consist of protons and (except for the hydrogen atom) neutrons.

The number of Protons and neutrons are identified by two numbers in the periodic table:

The Atomic number or. Atomic number Z (= Number of protons)

The Mass number A (Number of protons Z + number of neutrons N → A = Z + N)


So a uranium atom possesses 92 protons in the nucleus (and thus also 92 electrons in the atomic shell).

The number of neutrons in the nucleus can be determined from the mass number, since the mass number is also the number of nucleons (protons and neutrons) in the nucleus.

Therefore: N = A - Z = 238-92 = 146.

So the number of neutrons in the nucleus is N = 146.

For the Basic building blocks of an atom the following symbol notation results:

electronNegligible mass, simply negatively charged
protonMass number 1, simply positively charged
neutronMass number 1, not loaded

Nuclides and isotopes

Each atomic nucleus has a certain number of protons and neutrons.

All atoms one Elements have the same number of protons (i.e. the same atomic or atomic number), but can have different numbers of neutrons (and thus different mass numbers).

An atomic nucleus clearly characterized by mass number and atomic number is called nuclide designated.

Atomic nuclei with the same number of protons but different numbers of neutrons are called Isotopes.


Uranium (see above) occurs in nature both with the mass number 238 (Uranium-238) and (more rarely) with the mass number 235 (Uranium-235) in front. The two isotopes have the same number of protons but different numbers of neutrons.

Since the atomic number is determined by the chemical element, it is often omitted in the symbol notation. A nuclide is clearly characterized by specifying the element and the mass number.

In the running text, the notation used above is usually chosen, in which the mass number is behind the chemical symbol or the element (uranium 238, uranium-238 or U-238).


The mass number 1 corresponds approximately to atomic mass unit u. This is defined as 1/12 of the mass of the carbon isotope C-12. One mole of this carbon isotope has exactly the mass 12g.

The mass number corresponds roughly to the atomic mass in u (this only applies exactly to the carbon isotope C-12).

A detailed explanation of the relationships between Atomic massAmount of substance, Particle number etc. can be found in the subject area thermodynamics in the section on the universal gas equation.

The α-decay

During the α-decay, so-called α-particles (these are double positively charged helium nuclei - symbol notation: ) emitted from the atom with high kinetic energy.


The nucleus of the radium isotope Ra-224 is not stable because the nuclear forces cannot completely cancel out the repulsive forces of the protons with one another. The core therefore tries to move into a more stable state. This happens in several stages. One speaks of a so-called. Decay series, which is composed of the nuclear reaction equations of the individual stages:

Decay series for Ra 224:

Taking into account the emission of α-particles:

A new element is created in each case, with the atomic number decreasing by 2 and the mass number by 4. For the α-decay the following applies:

Core transformation:

The β-decay

When β-decay becomeβ-particle emitted from the core. It refers to Electrons or Positrons.

One distinguishes betweenβ-Decay andβ+-Decay.

a) β-Decay

At the β-Decay transforms a neutron in the nucleus into a proton and an electron.

Reaction equation:

example for a β-Decay:

or taking into account the emission of electrons

Since the nucleus then has one more proton, the atomic number increases by 1, the mass number remains the same.

Core transformation:

β-rays form a current of electrons that shows the same properties as the electric current in metallic conductors. For example, both generate a magnetic field whose field lines lie in concentric circles to the direction of propagation.

b) β+-Decay

In the case of natural and artificially produced radionuclides, radiation can also occur in which particles with the mass of an electron are ejected with a positive charge. It is a matter of Positrons.

When a proton is transformed into a neutron and a positron in the nucleus, the so-called β is created+-Radiation.

Reaction equation:

example for a β+-Decay:

The atomic number decreases by 1, the mass number remains the same.

Core transformation:

There is one more particle ...

The process of the decay of a nuclear neutron into a proton and an electron would violate three conservation laws (energy, momentum and angular momentum conservation law), the validity of which is also generally recognized in the atomic domain.

In 1932 Wolfgang Pauli postulated another particle involved in beta decay, so that all conservation laws are fulfilled. This particle has a very small rest mass (2 eV or less) and no electrical charge. It was neutrino (little neutron) called.

When converting a neutron into a proton and an electron (β- decay) arises Antineutrino, when converting a proton into a neutron and a positron (β+- decay) a neutrino. Neutrinos and antineutrinos have a high penetration capacity and are therefore very difficult to detect. The proof was only possible in 1956, i.e. 24 years later!


In the case of the nuclear reaction equations, neutrinos and antineutrinos have not been given here for reasons of simplification.