FURTHER REFINEMENT OF THE ATOMIC STRUCTURE
The atom is now known to consist of a tiny nucleus made up of protons and neutrons. The electrons are
arranged in areas around the nucleus called electronic shells. Electrons in different shells have different
energies so a shell is also called an energy level.
EVIDENCE OF THE EXISTENCE OF ELECTRONIC SHELLS FROM EMISSION SPECTRA
When the vapour of an element is given energy by passing an electric charge through it or by heating it
strongly, the vapour glows. The same is true for a compound of an element.
Emission spectra can be classified as follows:
1. CONTINUOUS SPECTRUM
When white light is passed through a prism it is separated into its constituent colours (frequencies in
energy). The spectrum produced by the splitting of white light is a continuous series of colours emerging
into each other and called a continuous spectrum.
A continuous spectrum is one in which ranges of radiation of different frequencies pass smoothly from
one point to the next without a break.
2. LINE SPECTRUM
When the above experiment is repeated using an atomic vapour, a number of coloured lines separated by
dark spaces is seen in the spectrum produced. Each of these lines of colours corresponds to a light of
different and definite energy. This is referred to as a line spectrum or discontinuous spectrum.
A line spectrum is a set of radiation of different frequencies separated from each other by dark spaces.
The pattern of lines or light produced by any given element is always the same and hence this is used to
identify the particular element or detect small quantities of it in the presence of other elements.
DEDUCTIONS FROM THE LINE SPECTRUM OF ATOMS/ELEMENTS
1. The set of lines which are converging shows a transition from one energy level to another. This implies
that electrons in atoms are arranged in discrete or definite energy levels called orbits.
2. The separation between energy levels decreases as we move towards higher energy levels. This suggests
that the energy that an electron needs in order to move (or exist) in a particular orbit depends on the
radius of the orbit. An electron in an orbit distant from the nucleus requires higher energy than an electron
in an orbit closer to the nucleus.
3. Electronic transition occurs between different lines of colours with different energy levels. This shows
that different types of orbits in which electron exist are present.
4. An electron does not emit energy when it is moving in a particularly energy level.
5. An electron moving in the lowest energy level is said to be in the ground state and does not emit energy.
6. The electron can only change its energy state (move to higher orbit) by absorbing energy to do work
against the attraction of the nucleus. The electron is then said to be excited. But this state is however,
unstable and the electron may fall back to the ground state giving off the energy that was used in exciting
it.
7. The amount of energy (E) emitted or absorbed by an electron is exactly equal to the energy difference
between the energy levels of the two states.
i.e E = E2 – E1 E = h
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