Microcosmology: Atom In Jain Philosophy & Modern Science: [1.2.2.2] Atom in Modern Science - Structure of Atom - Bohr's Theory of Atom

It was noticed that some of the properties of the elements begin to repeat themselves after a definite number of steps when arranged in a natural sequence. Since each step along the sequence of elements corresponds to one additional electron, the observed periodicity must be due to the recurrent formation of certain stable configuration of atomic electrons, or "electronic shells".

In 1913, Neils Bohr, a Danish physicist, won the Nobel prize by showing that electrons do not revolve round the nucleus of an atom at any random distance, but in definite orbits or shells which are at specific distances from the nucleus. A certain number of electrons, but no more, can be accommodated in each of these shells thus:

Thus, an atom with 3 electrons will have 2 in shell no. 1 and 1 in shell no. 2. If the atom has more electrons than the combined capacity of the first two shells, viz. 10, then the third shell begins to fill up and so on.(See Appendix Atomic Table)

Bohr also discovered that whenever an atom absorbs energy, its electrons jump to one of the outer shells. They return to the inner shells (ground state) by emitting the energy absorbed earlier and normally stay as close to the nucleus as they can.

We can now, having the picture of an atom, turn our attention to the nature of forces, which bind together the atoms of different elements into complex molecules of innumerable chemical compounds. The chemical bond between the neighbouring atoms in molecule is due to the interaction of the electronic shells and the forces involved are comparatively small. The distinction between the atomic nuclei and the electronic shells of different elements at once gives a proper explanation of the various physical and chemical properties of the elements and also of the fact that they are the ultimate units of matter. The chemical properties of atom are, however, controlled by the nucleus. If one wants to change the chemical properties of an atom, one has to change the nucleus and this requires energies about a million times greater.