In
1913, Niels Bohr proposed a model which was an improvement
over Rutherford’s nuclear model. Bohr proposed that
an electron moves around the nucleus in a well defined circular
path. He set down following two main postulates to explain
the stability of atom particularly hydrogen atom
(i)
An electron can have only a definite circular path around
the nucleus with specific energy values. This circular path
he called orbit or energy level
(ii)
Electron may go to next higher energy level (orbit) when
given a definite amount of energy. In other words, an electron
absorbs energy when it goes to higher energy level from
a lower energy level.
Contrary
to this, electron will emits out a definite amount of energy
when it comes from a higher energy level to lower energy
level. If E2 is energy of an electron in higher
energy level and E1 is energy of electron in
lower energy level, then energy released DE will be expressed
as,
DE
= E2 – E1
If
the electron remians in the same orbit, the energy would
neither be released nor absorbed. These orbits will, therefore,
were called stationary orbits or stationary states.
| Niels
Bohr (1885-1962). He was a Danish physicist. He
was awarded the Nobel Prize in Physics in 1922 |
Although
Bohr model could explain a number of aspects related to
hydrogen atom but it could not explain stability of atoms
having more than one electron. After the nature of electron
was studied in detail, it was found that an electron cannot
remain in a fixed circular orbit as envisaged by Bohr. Bohr
model was rejected on this ground.Based on the nature of
electron, concept of circular orbit was modified and a three
dimensional shell with definite energy came into existence.
These shells are similar to circular path/energy levels
given by Bohr. These shells are represented by letters K,
L, M, N etc. Each shell is associated with a definite energy.
The energies of these shells go on increasing as we move
away from the nucleus. The maximum number of electrons which
can be accommodated in each shell is given by 2n2
where n can take values 1, 2, 3….etc. Thus, the first
shell can have two electrons whereas the second shell can
have 8 electrons. Similarly the maximum number of electrons
present in third and fourth shells would be 18 and 32, respectively.
Each shell could be further sub-divided into various sublevels
of energy called subshells. These subshells are denoted
by letters s, p, d, f, etc about which you would study in
your higher classes.
