Chemistry - Developments leading to the Bohr’s atomic model

Developments leading to the Bohr’s
atomic model : 

At the time when different models of atomic structure were being put forth, some results obtained from the studies of interactions of radiation with matter required to be correlated to atomic structure. Niels Bohr utilized these results to get over the drawbacks of Rutherford atomic model. These results were : 

(1) wave particle duality of electromagnatic radiation.
(2) line emission spectra of hydrogen.

Wave particle duality of electromagnetic radiation : 

A dilemma was posed by electromagnetic radiation in the world of science. Phenomena such as diffraction and interference of light could be explained by treating light as electromagnetic wave. On the other hand, the black-body radiation or photoelectric effect could not be explained by wave nature of light, and only accounted for by considering particle nature of light. The only way to resolve the dilemma was to accept that light has dual behaviour. When light interacts with matter it behaves as a stream of particlescalled photons, when light propagates, it behaves as an electromagnetic wave.

Electromagnetic wave

a. Characteristics of electromagnetic wave : 

Figure shows a schematic representation of an electromagnetic wave. Various parameters used to describe the different types of electromagnetic radiation are wavelength, frequency, wavenumber, amplitude, and velocity.

i. Wavelength (λ) : The distance between two consecutive crests or troughs is called wavelength. It is represented by the symbol λ which is a greek letter (lambda). The SI unit for wavelngth is metre (m).

ii. Frequency (ν) : The number of waves that pass a given point in one second is called frequency. It is represented by the greek letter nu, (ν). The SI unit of frequency is Hertz (H2 or s-1).

iii. Wavenumber (ν) : Wavenumber is the number of wavelengths per unit length. Wavenumber is represented by the symbol (nu bar) (ν). The commonly used unit for wavenumber is reciprocal centimeter (cm-1), while the SI unit is m-1. Wavenumber is related to the wavelength by an expression ν = 1 / λ

iv. Amplitude (A): Amplitude of a wave is the height of the crest. Square of the amplitude denotes the intensity of the radiation. Different regions of electromagnetic radiation have different values of frequency or wavelengths. Thus the radiofrequency region is around 106

Hz, microwave region is around 1010 Hz, infrared region is around 1013 Hz, ultraviolet region is around 1016 Hz.In vacuum, the speed of all the types of electromagnetic radiation is the same, which is 3.0 × 108 m s-1 (2.997925 × 108 m s-1 to be precise). This is called speed of light, and is denoted by the symbol ‘c’. The parameters wavelength (λ ), frequency (ν ) and the speed of light (c) are related by the expression : c = ν λ

Different regions of electromagnetic radiation have different values of frequency or wavelengths. Thus the radiofrequency region is around 106 Hz, microwave region is around 1010 Hz, infrared region is around 1013 Hz, ultraviolet region is around 1016 Hz. In vacuum, the speed of all the types of electromagnetic radiation is the same, which is 3.0 × 108 m s-1 (2.997925 × 108 m s-1 to be precise). This is called speed of light, and is denoted by the symbol ‘c’. The parameters wavelength (λ ), frequency (ν ) and the speed of light (c) are related by the expression : c = ν / λ .

b. Particle nature of electromagnetic
radiation : 

In the year 1900, Max Plank put forth his quantum theory to explain black- body radiation. According to this theory, the energy of electromagnetic radiation depends upon the 

frequency and not the amplitude. Plank gave the name ‘quantum’ to the smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic radiation. The energy 

(E) of one quantum of radiation is proportional to its frequency (ν) and given by
                                    E = h v

The proportionality constant ‘h’ is called Plank’s Constant. Later its value was found out to be 6.626 × 10-34 J s.In the year 1905, Albert Einstein explained the photoelectric effect using Plank’s quantum theory. In doing so he considered electromagnetic radiation as a stream of photons of energy hν . A photon has zero rest mass.

Line emission spectrum of hydrogen : 

When a substance is irradiated with light it absorbs energy. Atoms, molecules or ions, which have absorbed radiation are said to be ‘excited’. Heating can also result in an excited state. When an excited species gives away the 

absorbed energy in the form of radiation, the process is called emission of radiation. The recorded spectrum of this emitted radiation is called ‘emission spectrum’.