Classical Picture of the atom

The very early physicists thought of matter as consisting of an indestructible entity, called an atom. They believed that everything is made up of atoms which are so small that we cannot measure them in any way. The basic idea was that atom was the fundamental unit of matter, extremely small and had all the properties (smell, taste etc.) that is associated with macroscopic sample of that matter.

Plum Pudding Model

However, early experiments by J.J. Thompson and others led to the discovery of electron. The electron was found to be a negatively charged elementary particle and was found to be emitted from metals in discharge tubes(watch this to learn more). The question was, where was it coming from? Thompson argued that the electrons were coming from the atoms of the metal. He conducted experiments with different materials and found that all of them emit electrons. Thus, he proposed that all atoms contain electrons.

The next question was, how are they arranged inside the atom? Also, since atom was electrically neutral there must be some way to balance out the negative charge of electrons. This led to the plum pudding model of the atom:

The basic features of this model are:

  1. Atom is an indestructible unit of matter. However it has internal structure.
  2. ¬†Atoms consist of negatively charged electrons sprinkled here and there on a positively charged ‘mass’, like plums in a pudding.

However, it was found experimentally by Rutherford and his students that almost entire mass of the atom is concentrated in a tiny volume at the centre. The electrons were not embedded in the positive charged mass, as hypothesized. Discovery of protons and neutrons further disproved plum pudding model.

Rutherford’s Model

If not embedded in the positively charged volume, how were the electrons arranged inside? It was known(somewhat) that electrons had negligible size, if any. The gold foil experiment by Geiger-Marsden (guided by Rutherford) proved that the positive charge, whatever it was, was concentrated in a small space as well. Since electrons had negligible mass as compared to the atom, it must be the case that this positively charged volume had almost all the mass of the atom. Protons and neutrons were discovered already to be a part of the atom as well, how did they fit into the picture?

Rutherford explained it with a model analogous to the solar system:

  1. Atom is the fundamental unit of matter. However, it has an internal structure and is made up of three kinds of particles: electrons, protons and neutrons.
  2. 99% of the mass of the atom is concentrated in the small region at the center called the nucleus. The nucleus consists of protons (positively charged) and neutrons (neutral). Both of them are almost equally massive and stay in place.
  3. The electrons are negatively charged and are bound to the nucleus by Coulomb force. This force acts as a centripetal force, just like gravity in the solar system. As a result, electrons revolve around the nucleus in circular orbits.
  4. The nucleus occupies a tiny amount of space. The electrons are tiny too, so 99% of the atom is nothing but empty space. However, since electrons move really fast, the atom appears to be solid.

This picture of atom had some peculiar problems, the most important of which was as follows:

Any classical charged particle in an orbit around another particle would emit electromagnetic radiation(Bremsstrahlung), losing energy in the process. This would ultimately lead to the electron falling spirally inwards and into the nucleus (in less than a fraction of a second). If this is so, how come atoms are stable?

Also, why are atoms of different elements different? Why are electrons arranged differently in different atoms? These questions were resolved by Niels Bohr in 1913 along with Rutherford in what we call as the Bohr’s Model. It was a semiclassical model that incorporated some ideas from quantum physics, but more on that in the future.

Atomic Physics: Structure of the atom-1
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