Station 3: Building blocks of the core 2
Station 3: Building blocks of the core 2
The core electron hypothesis
According to Ernest Rutherford's atomic model, an atom consists of a small, heavy, positively charged nucleus surrounded by a much larger shell of electrons. The experimental data that Rutherford and his colleagues had obtained from countless scattering experiments supported this model.
In order to explain radioactive β-radiation, however, a number of assumptions had to be made that could not initially be supported by experiments. It was known that β-radiation is nothing other than the already known negatively charged electrons. Experiments showed that these electrons could not originate from the shell of electrons of the atom postulated by Rutherford.
Various scientists, including Rutherford, therefore hypothesised in the 1920s that these electrons came from the nucleus of the atom. It was known that the nuclear charge is approximately half the mass of the atom.
Task
Using this hypothesis, develop a model for the helium nucleus (mass: 4 u, charge: +2) and draw your model.
The atomic nucleus with core electrons
It was generally assumed that the atomic nucleus consists of positively charged particles, which make up the majority of the mass. With a corresponding number of the much lighter and smaller electrons in the nucleus, the charge of the atomic nucleus would be positive overall and just as large as the negative charge of the electrons in the shell of the atom. The relationship between nuclear charge and atomic mass can also be explained in this way.
A model of the atomic nucleus with nuclear electrons
The illustration on the left shows you what a helium atom nucleus, an α-particle, would look like according to the model of nuclear electrons. The blue spheres symbolise the positively charged particles, the red ones the negatively charged electrons. Overall, this results in a nuclear charge of +2 and a mass of around 4 u.
The atomic nucleus without core electrons
The explanation was not provided until 1932 by James Chadwick, one of Rutherford's colleagues:
In 1930, two German scientists had discovered that bombarding light elements, for example beryllium (atomic mass 9 u), with α-particles produces radiation that is unusually penetrating. Chadwick took up these results and carried out experiments to characterise this radiation. He discovered that this radiation consists of particles that cannot be influenced by an electric or magnetic field, just like γ-radiation. However, they have approximately the same mass as the positively charged particles of the atomic nucleus. Chadwick called these particles neutrons.
The discovery of the neutron changed the model of the atomic nucleus. According to this model, the nucleus is composed of positively charged particles and neutrons. Together, these particles make up the absolute majority of the mass of the atom. The number of positively charged particles in the nucleus is equal to the number of electrons in the shell. The number of neutrons in the nucleus, on the other hand, is variable.
