Various Models of the Atom

1. Thompson’s Atomic Model:

Thompson proposed that an atom consists of homologous sphere of positive charges which are embedded in negatively charged electrons

2. Rutherford Model of Atom:

Lord Ernest Rutherford, in 1911, used positively charged particles called Alpha particles to bombard a thin-gold foil. He found that most of the alpha particles passed through the foil, while a few of them were deflected back.

Rutherford explained his findings as follows:

a. Since most of the alpha particles passed through without being deflected, the atoms in the goldfield must contain a large space in which negative particles move.

b. Since very few of the alpha particles were deflected, the deflection might have been caused by a small region of the atom which consists of positive particles. The positive particles might have collided with the alpha particles leading to the deflection.

c. Among the particles deflected, a few were deflected at large angles. This shows that the positive particles in the atom must be quite heavy.

Rutherford concluded by proposing a nucleus theory of an atom. According to his theory, the atom consists of a small nucleus which contains protons and neutrons while electrons revolve around the nucleus.

Rutherford proposed a planetary model of the atom with the following assumptions:

• Protons with positive charges are concentrated in a small nucleus.
• The nucleus is centrally located in an atom.
• The electrons are located at the periphery of the nucleus and they render the atom electrically neutral.
• Electrons revolve around the nucleus of an atom.
• There is a wide gap between the nucleus of an atom and its orbiting nucleus.

Limitations of Rutherford’s Atomic Model:

(i) It predicts that light of a continuous range of frequencies will be emitted whereas experiments reveal line spectra instead of continuous spectra.

(ii) He stated that atoms are unstable – electrons quickly spiral into the nucleus, but generally atoms are stable.

3. Bohr’s Postulate of Atomic Model:

Bohr, in his model of atoms, suggested that:

(i) Electrons occupy fixed orbits around the nucleus of an atom, when an electron is in one of these orbits, it does not radiate energy

(ii) When an electron jumps from a higher energy level to a lower energy level, it emits an indefinite amount of radiation (quanta).

(iii) An electron absorbs radiation when it jumps from a lower energy level to a higher level.

(iv) When such a jump occurs, a single photon of light energy is emitted and is given by:

hf = E_u – E_L

where:

• E_u = energy at the upper level
• h = Planck’s constant
• f = frequency of emitted light or radiation
• E_L = energy at the lower level

Success of Bohr’s Atom:

Bohr’s model was a success in:

(i) In calculating the radius of the allowed orbit and total energy of the electrons in each allowed orbit of the hydrogen atom.

(ii) In Predicting the spectral line of glowing hydrogen atom.

(iii) In his Explanation of the quantized energy state of a photon in the photoelectric effect.

Failure or Limitations of Bohr’s Atom:

(i) It could not explain bright spectral lines of complex atoms like oxygen.

(ii) It views electrons as particles moving around the nucleus at a definite orbit.

(iii) Experiments have shown that electrons can behave like waves or particles.

4. Electron Cloud Model:

An atom consists of a positively charged nucleus surrounded by electrons in a cloud of different energy levels. The location of electrons depends on how much energy the electron possesses.

Electrons with the lowest energy are found in the energy level close to the nucleus while those with the highest energy level are found on the outermost energy levels, farther from the nucleus.

The electrons change from one energy level to another level depending on whether they gain or lose energy.