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Atomic Models

As a prelude to Bohr’s theory, we should have an introduction to
  1. nature of radiations,
  2. atomic spectra, and
  3. quantum theory.


Ordinary light, X-rays, and γ-rays are called electromagnetic radiations, and they have wave characteristics. These radiations are called electromagnetic because when they pass through a point in space, they produce oscillating electric and magnetic fields at that point. In 1873, James Clark Maxwell, a Scottish physicist, showed that a static charge or a charge with uniform velocity sets up electric and magnetic fields which give rise to an energy density in space associated with the electric and magnetic fields, but the energy density remains constant. On the other hand, if we were to change the velocity of the charged particle, the energy density varies and then gives rise to electromagnetic waves.
There are three fundamental characteristics associated with wave motion: (i) wavelength (λ), (ii) frequency (ν), and (iii) velocity (c).
  1. Wavelength: Consider a wave profile as shown in the below fig. The distance between the two successive crests or troughs is known as wavelength (λ). It is measured in cm or Angstrom unit (Å).
    1 Å = 10–8 cm = 10–10 m
    Sometimes it is also measured in nanometer (1 nm = 10–9 m).
  2. Frequency: The number of waves that passes through a given point in one second is called its frequency (number of waves per second). Frequency (ν) is expressed in cycles per second (cps) or Hertz (Hz).
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  1. Velocity: The distance traveled by a wave in one second is its velocity.
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Atomic spectra

When white light is passed through a prism, it is separated into light of seven colors (VIBGYOR) or radiations of different wavelengths. The pattern obtained by splitting or sorting out of radiations into its component wavelengths is called a spectrum. The spectrum of white light when analyzed by spectrometer (an instrument that indicates the wavelengths/frequencies of individual components of a radiation) is a continuous spectrum, suggesting that white light is made up of all possible wavelengths or frequencies of radiations. If a gas is heated, it emits light. When this emitted light is analyzed in a spectrometer, the spectra obtained consist of a series of well-defined sharp lines, each line corresponding to a definite wavelength or frequency. These line spectra are characteristic of atoms.

Quantum theory

According to quantum theory, a body cannot emit or absorb energy in the form of radiation of continuous energy; energy can be taken up or given out as whole number multiples of a definite amount known as a quantum. Light is imagined to consist of a stream of particles called photons. If E is the energy of a photon, its quantum for a particular radiation of frequency ν s–1 is given by quantum theory as
E =
where h is a universal constant known as Planck’s constant (h = 6.626 × 10–27 erg-s or 6.626 × 10–34 Js).
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