Bohr’s Model of Atom Various models of the atom have been proposed
by eminent scientists over the years. These models have increased our understanding
of atomic structure. Among these models, Rutherford suggested that
electrons revolve around the nucleus in well defined orbits. But there was a problem with
this. The motion of electrons in Rutherford’s model
was unstable because any charged particle moving in a circular path emits electromagnetic
radiation, thus the electrons would lose energy and fall into the nucleus making the atom
highly unstable. Then which would be a better model of an atom? To overcome the objections against Rutherford’s model of atom, Niels Bohr in 1913 proposed a new atom model. What does this model propose? It first proposes that electrons revolve around
the nucleus in specific orbits and these orbits are associated with definite energies and
are called shells or energy levels. These orbits or shells are represented by
the letters K, L, M, N.. or by the numbers n=I,2, 3, 4….
The maximum number of electrons that can be accommodated in a particular orbit is 2n2
where ‘n’ is the number of the orbit. Thus k shell would have 2 electrons, L shell – 8
electrons, M shell – 18 electrons, N shell – 32 electrons and so on. That said, what
does this model next propose?It next proposes that the maximum number of electrons that
can be accommodated in the outermost level is 8.
And finally it says that, the orbit closest to the nucleus has minimum energy and the
orbit farthest has maximum energy. Well then, does an electron radiate energy
by itself? Since electrons move in a particular orbit,
they do not radiate energy by themselves. What happens instead is, when an atom absorbs
energy, the electrons get excited and jump into the next higher energy level.
The electron can radiate energy and return to its original state or drop down to the
next energy level. Bohr’s model works well for simple atoms and
is easy to understand. It is one among the atomic structures still in use today, the
other being the quantum mechanical model. Things to remember: In 1913 a Danish physicist Niels Bohr proposed
an atomic model known as Bohr’s atomic model. In an atom, the electrons revolve around the
nucleus in definite energy levels called orbit or shells.
The maximum number of electrons that can be accommodated in a particular orbit is 2n2.
The maximum number of electrons that can be accommodated in the outer most shell is 8.
The orbit closest to the nucleus has minimum energy and the orbit farthest has maximum
energy. Electrons are excited to the higher energy
levels by absorbing energy and return to lower energy levels by radiating energy.