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Rutherford’s atomic model

Atom is composed of a positively
charged nucleus surrounded by
negatively charged electrons, just like
planets revolving around the sun. Most of

the mass in an atom is concentrated in the
nucleus and a large fraction of its volume
is empty space. As an atom is neutral, the

numbers of the positive charges in the
nucleus (the number of protons) must be
the same as that of electrons.

Figure 8. Rutherford’s Atomic Theory

Bohr’s Atomic Theory

In 1913, Bohr applied Placnk‟s quantum theory to put formward his postulate so as to
explain the atomic stability and the hydrogen atomic spectrum.

Bohr’s Postulates
a. Electrons travel around the nucleus in a certain stationary

course called an orbit or a shell. Athough electrons experience

acceleration, they do not emit or absorb energy in the orbit, hece
their energy remains constant.

b.Electrons can move from one orbit to another by emitting or
absorbing energy.

In general, the Bohr‟s atomic model could explain the atomic stability

and the hydrogen atomic spectrum well. However, it had weaknesses
Figure 9. Niels Bohr
among them:

- The model could only explain the hydrogen atomic spectrum
accurately, and failed to explain atomic spectra that were more

complex than those of hydrogen.

- The assumption that electrons travel around the nucleus in a
circular orbit was not completely true, for ellipse shapes are

possible.
- The model cannot explain the thin lines in the hydrogen

atomic spectrum because Bohr only considers electron as Figure 10. Bohr’s Atomic Theory
particles. The next atomic theory uses the dualism

characteristic of electron, i.e. electron as a particle and a wave. The characteristic causes the

position of the electrons inside an atom to be indeterminate and can only be described by a
probability.

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