# Bohr's Atomic Model

The Bohr Model has an atom consisting of a small, positively-charged nucleus orbited by negatively-charged electrons. Here's a closer look at the Bohr Model, which is sometimes called the Rutherford-Bohr Model

Niels Bohr proposed the Bohr Model of the Atom in 1915. Because the Bohr Model is a modification of the earlier Rutherford Model, some people call Bohr's Model the Rutherford-Bohr Model.

The modern model of the atom is based on quantum mechanics. The Bohr Model contains some errors, but it is important because it describes most of the accepted features of atomic theory without all of the high-level math of the modern version. Unlike earlier models, the Bohr Model explains the Rydberg formula for the spectral emission lines of atomic hydrogen.

The Bohr Model is a planetary model in which the negatively-charged electrons orbit a small, positively-charged nucleus similar to the planets orbiting the Sun (except that the orbits are not planar). The gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively-charged nucleus and the negatively-charged electrons.

### Postulates of the Bohr Model

1. Electrons revolve round the nucleus only in certain circular orbits. These orbits are associated with definite energies and are called Energy Shells or Energy Levels.

2. Electrons orbit the nucleus in orbits that have a set size and energy.

The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit.

3. Only those orbits are permitted in which angular momentum (mvr) of electron is an integral multiple of h/2Ï€ i.e.

mvr = nh/2Ï€

where, m = mass of electron, n = number of orbit in which electron is present, v = velocity of the electron, r = radius of the orbit, h = Planck’s constant.

3.As long as the electron remains in a particular orbit, it does not lose or gain energy. This means that energy of an electron in a particular orbit remains constant. That is why, these orbits are also called as Stationary States. This orbital rotation without emitting energy follows the Newtonian law; i.e. the force of attraction between the nucleus and electron is equal to the centrifugal force of moving electron.

Radiation is absorbed or emitted when an electron moves from one orbit to another.

When energy some external source is supplied to the electron, it may jump to some higher energy level by absorbing a definite amount of energy. When electron jumps back to the lower energy level it radiates same amount of energy in the form of light radiations.

### Drawbacks of the Bohr Model

- It violates the Heisenberg Uncertainty Principle because it considers electrons to have both a known radius and orbit.
- The Bohr Model provides an incorrect value for the ground state orbital angular momentum.
- It makes poor predictions regarding the spectra of larger atoms.
- It does not predict the relative intensities of spectral lines.
- The Bohr Model does not explain fine structure and hyperfine structure in spectral lines.
- It does not explain the Zeeman Effect.

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