You would recall from the previous lesson that according to Neil Bohr, energy is radiated
by electron only when it transverses the energy levels of the atom. This energy is in the form
of photons when it moves from a higher energy level of n=2 to the lowest energy level, n=1.
The radiation emitted gives a spectral line of a given wavelength.
As the electron jumps from the other higher energy levels of n=3, 4 and so on, there are
multiples of the quanta of energy radiated respectively. Each of these different energies gives
a corresponding spectral line of definite wavelength. Consequently, we have series of
spectral lines for the hydrogen atom for the electron dropping from all the higher energy levels (n=9,
8, 7, 6, etc) to the lowest energy level, n=1. This series is called the
Lyman series (named after Lyman who first noticed them) and is located in the ultraviolet region of the
electromagnetic spectrum. When there is transition between higher levels and the energy level, n=2, the
transition series is called Balmer series (located in the visible region, i.e. ROYGBIV);
Ritz-Paschen series is the term given to the transition between the higher levels and the energy level of
n=3; (located in the infra-red region)
Brackett series when the n=4 (located in the far
infra-red region); and Pfund series when n=5 (also found in the far infra-red region of the spectrum).
Nature of Light
Since the emission energy of electron is in the form of light, it is apt to find out the real
nature of light. There are two schools of thought that attempt to explain just what light is.
One school of thought led by Sir Isaac Newton argued that light is made of packets of
energies called corpuscles and that this explains the phenomenon of reflection whereby these tiny
packets or particles are bounced back from objects that they meet as well as refraction which is
the bending of light. The other school of thought disagreed with this proposition. They
suggested that light is of the nature of electromagnetic wave. This set of scientists was led by Sir
William Huygens. Both schools are correct in that light is both particulate and of wave nature.
Quantum theory
Max Planck (1900) and Albert Einstein (1905) proved the schools of thought of Sir
Isaac Newton and Sir William Huygens right by indicating that besides wave properties of light,
it also has properties of particles (i.e. they are particulate) and that energy emitted is in
discrete units called quanta (sing. Quantum) and are not a continuous stream. Read more about
the black body radiation experiment as well as other works that afforded foundation to
this proposition. Later on, these units, quanta became known as photons.
Modern Chemists tend to bend towards a wave atomic model because of the ability of
this model to explain several behaviours of atoms:
Terms Associated with Wave
Wave: is a disturbance that travels from one point to the other without causing any
structural malformation to the medium.
Amplitude: This is the maximum displacement of a particle from the point of rest. At the
peak is the crest while the lowest displacement possible from rest is the trough.
Wavelength: This is the distance between two successive crests or troughs. It is denoted by ë
Frequency: This is another important characteristic that is often measured in a wave
motion and is the number of oscillations or waves passing a given point per unit time.
It is denoted by υ or simply the letter, f.
Velocity: This is the distance (or displacement) that is covered by the wave per unit time.
For light wave, the velocity is denoted by the letter, c and the value is 3.0 x
108 ms-1
Relationship between the velocity, frequency and wavelength
The relationship is given by the equation:
Where c is the velocity of light
The energy of the photon, E, emitted or absorbed by a substance is proportional to the
frequency of light:
where h is called Planck's constant and the value is 6.63 x
10-34 Js
Worked Problem
If the energy difference between two electronic energy levels is 6.63kJ
mol-1, calculate the frequency of the electromagnetic radiation emitted when the electron drops from the higher
to the lower energy level (h = 6.63 x
10-34 Js)
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