DEVELOPMENTS LEADING TO THE BOHR’S MODEL OF ATOM
Historically, results observed from the studies of interactions of radiations with matter have
provided immense information regarding the structure of atoms and molecules. Neils Bohr utilized
these results to improve upon the model proposed by Rutherford. Two developments played a
major role in the formulation of Bohr’s model of atom. These were:
i. Dual character of the electromagnetic radiation which means that radiations possess both
wave like and particle like properties, and
ii. Experimental results regarding atomic spectra which can be explained only by assuming
quantized electronic energy levels in atoms.
Wave Nature of Electromagnetic Radiation
James Maxwell (1870) was the first to give a comprehensive explanation about the interaction
between the charged bodies and the behaviour of electrical and magnetic fields on macroscopic
level. He suggested that when an electrically charged particle moves under acceleration,
alternating electrical and magnetic fields are produced and transmitted. These fields are transmitted
in the forms of waves called electromagnetic waves or electromagnetic radiation.
The light that we can see with our eyes, visible light, is an example of electromagnetic radiation.
Because electromagnetic radiation carries energy through space, it is also known as radiant energy.
There are many types of electromagnetic radiation in addition to visible light. These different
forms-such as the radio waves that carry music to our radios, the infrared radiation (heat) from a
glowing fireplace, and the X-rays used by a dentist-may seem very different from one another, but
they all share certain fundamental characteristics. These radiations are characterized by the
properties, namely, frequency (ν) and wavelength (λ). The SI unit for frequency (ν) is hertz (Hz,
s–1), after Heinrich Hertz. It is defined as the number of waves that pass a given point in one second.
Wavelength should have the units of length and as you know that the SI units of length is meter
(m). Since electromagnetic radiation consists of different kinds of waves of much smaller
wavelengths, smaller units are used.
In a vacuum all types of electromagnetic radiations, regardless of their wavelength, travel at the
same speed, i.e., 3.0 × 108 m s–1 (2.997925 × 108 m s–1, to be precise). This is called speed of light
and is given the symbol ‘c‘. All have wavelike characteristics similar to those of waves that move
Page 1 of 5
, through water. The frequency (ν), wavelength (λ) and velocity of light (c) are related by the
equation:
c=νλ
The other commonly used quantity is the wavenumber, defined as the number of wavelengths per
unit length. Its units are reciprocal of wavelength unit, i.e., m–1. However commonly used unit is
cm–1. Why do different forms of electromagnetic radiation have different properties? Their
differences are due to their different wavelengths, which are expressed in units of length. The
Figure below shows the various types of electromagnetic radiation arranged in order of increasing
wavelength, a display called the electromagnetic spectrum.
As progress in the science field was happening, Maxwell’s suggestion about the wave nature of
electromagnetic radiation was helpful in explaining the phenomena such as interference,
diffraction etc. However, he failed to explain various other observations such as the nature of
emission of radiation from hot bodies, photoelectric effect i.e. ejection of electrons from a metal
compound when electromagnetic radiation strikes it, the dependence of heat capacity of solids
upon temperature, line spectra of atoms (especially hydrogen).
Black Body Radiation
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Historically, results observed from the studies of interactions of radiations with matter have
provided immense information regarding the structure of atoms and molecules. Neils Bohr utilized
these results to improve upon the model proposed by Rutherford. Two developments played a
major role in the formulation of Bohr’s model of atom. These were:
i. Dual character of the electromagnetic radiation which means that radiations possess both
wave like and particle like properties, and
ii. Experimental results regarding atomic spectra which can be explained only by assuming
quantized electronic energy levels in atoms.
Wave Nature of Electromagnetic Radiation
James Maxwell (1870) was the first to give a comprehensive explanation about the interaction
between the charged bodies and the behaviour of electrical and magnetic fields on macroscopic
level. He suggested that when an electrically charged particle moves under acceleration,
alternating electrical and magnetic fields are produced and transmitted. These fields are transmitted
in the forms of waves called electromagnetic waves or electromagnetic radiation.
The light that we can see with our eyes, visible light, is an example of electromagnetic radiation.
Because electromagnetic radiation carries energy through space, it is also known as radiant energy.
There are many types of electromagnetic radiation in addition to visible light. These different
forms-such as the radio waves that carry music to our radios, the infrared radiation (heat) from a
glowing fireplace, and the X-rays used by a dentist-may seem very different from one another, but
they all share certain fundamental characteristics. These radiations are characterized by the
properties, namely, frequency (ν) and wavelength (λ). The SI unit for frequency (ν) is hertz (Hz,
s–1), after Heinrich Hertz. It is defined as the number of waves that pass a given point in one second.
Wavelength should have the units of length and as you know that the SI units of length is meter
(m). Since electromagnetic radiation consists of different kinds of waves of much smaller
wavelengths, smaller units are used.
In a vacuum all types of electromagnetic radiations, regardless of their wavelength, travel at the
same speed, i.e., 3.0 × 108 m s–1 (2.997925 × 108 m s–1, to be precise). This is called speed of light
and is given the symbol ‘c‘. All have wavelike characteristics similar to those of waves that move
Page 1 of 5
, through water. The frequency (ν), wavelength (λ) and velocity of light (c) are related by the
equation:
c=νλ
The other commonly used quantity is the wavenumber, defined as the number of wavelengths per
unit length. Its units are reciprocal of wavelength unit, i.e., m–1. However commonly used unit is
cm–1. Why do different forms of electromagnetic radiation have different properties? Their
differences are due to their different wavelengths, which are expressed in units of length. The
Figure below shows the various types of electromagnetic radiation arranged in order of increasing
wavelength, a display called the electromagnetic spectrum.
As progress in the science field was happening, Maxwell’s suggestion about the wave nature of
electromagnetic radiation was helpful in explaining the phenomena such as interference,
diffraction etc. However, he failed to explain various other observations such as the nature of
emission of radiation from hot bodies, photoelectric effect i.e. ejection of electrons from a metal
compound when electromagnetic radiation strikes it, the dependence of heat capacity of solids
upon temperature, line spectra of atoms (especially hydrogen).
Black Body Radiation
Page 2 of 5
