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Microcosmology : Atom 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 : Shell number
1 2 3 4 5...... Maximum number of electrons 2 8 18 32 50......
Thus, an atom with 3 electrons will have 2 in shell no. 1 and l 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.!
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.
CONSTITUENTS OF ATOM
With the creation of the planetary model of atom by E. Rutherford, 'proton' and 'electron' emerged as the smallest units of matter. The questions asked by the physicists were: Are these
1. See Atomic Table in the appendix.
