List of Atomic Models (With Drawbacks) | Material Engineering

Here is a list of three atomic models:- 1. Thomson’s Atomic Model 2. Rutherford’s Atomic Model 3. Bohr’s Atomic Model. Also learn about the drawbacks of each model.

1. Thomson’s Atomic Model:

Atom is a sphere filled with positively charged matter. The positive charge is distributed uniformly, with sufficient electrons embedded in it to balance positive charge. The radius of the sphere of an atom is about 10^-10 m. An atom is electrically neutral, so that total positive charge on it is equal to the total negative charge on it. The electrons possess vibratory, motion about their equilibrium position and cause emission of light.

Drawbacks of the Model:

Thomson’s model did not explain:

i. The scattering of α-particles incident on metal foil

ii. The emission of spectral lines by the atoms

This model of atom had, therefore, to be superseded.

2. Rutherford’s Atomic Model:

On the basis of α-particles scattering through large angle, Rutherford proposed the following structure of atom:

i. An atom have small central part called the nucleus, in which the whole mass and positive charge are concentrated. The nucleus size is about 10^-15m.

ii. As the atom is electrically neutral, so an equal number of electrons (negatively charged particles) are distributed in the space around nucleus to balance the positive charge of nucleus.

iii. Electrons are not stationary. They revolve around the nucleus in circular path in the same way as planets go around the sun. By this way Rutherford explained the stability of atom. He assumed that the electrons are revolving at a high speed around the nucleus in close circular orbits. The force of attraction between nucleus and electrons are balanced by the centrifugal force acting on the electron.

Drawbacks of Rutherford’s Atomic Model:

i. A revolving electron around the nucleus must be continuously accelerating due to the change in direction of motion at every point. According to electromagnetic theory, an accelerating charge particle, a revolving around the nucleus possesses centripetal acceleration and there would be continuous loss in energy of the revolving electron. Thus, the electron must move along a spiral path of decreasing radius and ultimately falls into the nucleus when its whole rotational energy is exhausted. This would result in annihilation of atom.

ii. According to electromagnetic theory the revolving electron must radiate continuous radiations of all increasing frequencies. But it is observed that elements emit spectral lines of fixed frequencies and not all the frequencies.

iii. In Rutherford model, there is no discussion of distribution of electrons.

3. Bohr’s Atomic Model:

Bohr’s extended the above model further by applying Planck’s quantum theory.

He made the following assumptions in his theory:

i. An atom consists of a small heavy central part called nucleus in which whole of the mass and positive charge of the atom are concentrated. The negatively charged electrons revolve around the nucleus in close circular orbits. The centripetal force required for rotation is provided by electrostatic force of attraction between positively charged nucleus and the negatively charged electrons.

Let m = mass of electron revolving around the nucleus

z = atomic number

e = charge on electron

v = orbital velocity of electron

r = radius of the orbit

Ze = charge on the nucleus

ε₀ = permittivity of free space = 8.854 × 10^-12 F/m

The electrostatic force of attraction between nucleus and the electron,

The necessary centripetal force = mv²/r

Hence, according to Bohr’s theory;

ii. The electrons revolve only in certain specific non-radiating orbits defined as stationary orbits. When the electrons revolve in these orbits there is no loss of energy. Bohr found that electron can only occupy an orbit for which the angular momentum of moving electron is an integral multiple of h/2π, where h is Planck’s constant. Thus, for any given orbit,

Momentum of electron = mv

Angular momentum of electron = mv r

According to the above postulate of Bohr’s theory, we have-

mvr = n. (h/2π)

where n is any positive integer having values (n = 1, 2, 3 ……………………. etc.) and is called principal quantum number, n = 1, for innermost orbit.

iii. When an atom jumps from one orbit of higher energy to another orbit of lower energy, the energy is released in the form of radiations. The amount of energy released is the difference of energies of electron in the two orbits. Thus, an atom may emit or absorb energy photons only when one electron of atom jumps from higher to lower orbit or from lower to higher orbit respectively.

Let, E₁ = energy of electron in inner or lower orbit

E₂ = energy of electron in outer or higher orbit

v = frequency of radiation emitted, when electron jumps from higher orbit to lower orbit.

Now, according to this postulate of Bohr’s theory, we have-

hv = E₂ – E₁

Drawback of Bohr’s Theory:

i. It gives no information regarding the arrangement of distribution of electrons in the atom.

ii. This theory does not explain wave nature of electrons.

iii. The theory does not explain why only circular orbits have been considered where as elliptical orbits are also possible.

iv. This theory does not explain the fine structural lines even for hydrogen.

v. It does not give any information about the relative intensities of spectral lines.

vi. This theory explains the atomic spectra only for simple atoms like hydrogen, etc.