Do you sometimes have an irrepressible thirst for knowledge? Once he’s grabbed you, pull out your smartphone and type a few concise terms into the search bar. After a few microseconds, the first results come in. What a feeling. The world of knowledge is literally in your hands.
Curiosity also led Ernest Rutherford to one of his greatest discoveries: the Rutherford model of the atom, which was named after him. When it was first published between 1909 and 1911, it made the world a little bit easier to understand.
The Rutherford atomic model simply explained
Rutherford was the first scientist to discover that at the center of every atom there is a atomic nucleus must give. According to his findings, almost the entire mass of the atom collects in it. He also found out that the electrons only have a very small mass and are located outside the atomic nucleus.
After this Rutherford atomic model An atom consists of a positively charged nucleus surrounded by negatively charged electrons.
In Figure 1 you can see how the Rutherford atomic model is constructed.
Figure 1: Rutherford model of the atom.
The nucleus carries as many positive charges as necessary to balance the negatively charged electrons. As a result, the atom appears neutrally charged to the outside. Back then, people didn’t know that atomic nuclei were made neutrons and protons exist. The electrons are located within the so-called atomic shell.
The basis is the atomic model according to Thomson
The Rutherford atomic model is based on a theory introduced by Joseph John Thomson in 1904. You may be more familiar with the Thomson atomic model as the «raisin cake model». At that time, the scientist imagined that atoms consist of a positively charged mass in which the negatively charged electrons are evenly distributed.
If you want to read more about Thomson’s atomic model, take a look at the atomic model explanation from the nuclear physics section.
Rutherford wondered whether Thomson’s model of the atom was correct and whether atoms really were made like raisin cakes. Maybe he just didn’t like raisin cake – this is of course a matter of taste. In any case, his curiosity was so great that he wanted to find out the truth.
So, along with his collaborators Hans Geiger and Ernest Marsden, he conducted an experiment that is now known as Rutherford scattering experiment is known.
Rutherford’s scattering experiment
For this experiment, the three scientists first needed a so-called alpha emitters (α emitter). For that they took radiumthe doubly positively charged alpha particles (α-particles) emits.
Alpha particles are formed at radioactive decay from iunstable atomic nuclei. Radium is also unstable and undergoes such radioactive decay, the alpha decay. In this process, a helium-4 atom is parent nucleus (here radium) repelled. The helium-4 atom forms the daughter nucleus and is called the alpha particle.
Alpha radiation is very energetic and can, among other things, knock electrons out of the atomic shell. Therefore it is also called ionizing radiation designated.
If you want to read more on this topic, check out the extremely interesting explanation of alpha radiation.
The researchers aimed this beam of alpha particles at a very thin gold foil (only about 1,000 atoms thick). Now they just had to see what happens. So they used one fluorescent screenwith which they could see the emitted radiation.
You may be wondering why Rutherford chose gold foil. The reason for this is that gold could already be rolled into extremely thin foils back then. In addition, gold atoms have a high atomic mass. This was particularly important for a clear evaluation of the experiment, because the heavier the nucleus of an atom, the more the alpha rays are deflected.
Expected and actual results of the spreading test
Rutherford always had the Thomson model of the atom in mind and therefore expected that none of the alpha particles would be deflected. According to this model is the positive amount of charge distributed over a large area and almost massless. For this reason, the heavy alpha particles should not be able to be deflected, since this would also require heavy mass particles.
After penetrating the gold foil, the alpha particles didn’t just hit a single spot on the screen. The alpha beam was partially deflected, causing the screen to flash at various points. It was particularly surprising that some alpha particles even hit at an angle of more than 90°, that is reflected became.
Thomson’s idea does not correspond to reality, so Rutherford decided to set up a new atomic model.
Conclusions of the scattering experiment
For Rutherford, the results were proof that there is a positive polarity at the center of an atom charge amount must be located. The reason for his assumption was the fact that two positive charges repel each other. The doubly positively charged and heavy alpha particles must therefore have encountered an equally positive mass.
However, only a few alpha particles were thrown back or severely deflected. This led him to the further assumption that the amount of positive charge is concentrated on a very small point – the atomic nucleus. However, Rutherford could not make any statement about the exact distribution of the electrons.
Key statements of the Rutherford atomic model:
- The atomic nucleus is very small compared to the overall atomic size.
- Almost all of the atomic mass is in the nucleus.
- atomic nuclei are positive loaded. the Negatively charged electrons lie outside the atomic nucleus and shield it from the outside.
The problems of the Rutherford atomic model
Science has made great strides with the Rutherford model of the atom, and although there are now more modern models, it is still valid today. However, Rutherford could not explain all aspects that are necessary for a complete understanding of the atomic structure.
For example, Rutherford did not know why atoms are stable. If there is a positively charged atomic nucleus, why doesn’t it then attract all the electrons unchecked? This is how two masses of different charges normally behave. If the electrons fell into the nucleus in this way, there would be no stable atoms.
The Rutherford atomic model has been expanded over the years by the Bohr atomic model, which is directly based on it, and the modern orbital model.
Rutherford atomic model – the most important
- The Rutherford atomic model states that there must be a positively charged nucleus containing almost all of the atom’s mass.
- Rutherford could not make any statement about the positions of the electrons, but he knew that they shield the atomic nucleus from the outside.
- The Rutherford atomic model is based on Thomson’s atomic model and is the forerunner of Bohr’s atomic model.
- The Rutherford atomic model is based on the scattering experiments carried out by Rutherford and his co-workers.
- The Rutherford model of the atom does not explain why the electrons do not collide with the nucleus, even though they are attracted by the opposite charges.
proof
- chemteam.info: On a Diffuse Reflection of the α-Particles. (07/19/2022)
- Edward Andrade (1964). Rutherford and the Nature of the Atom. Double Day New York.
- E Rutherford (1911). The Scattering of α and β Particles by Matter and the Structure of the Atom. Phil. Mag. 6.