Answer to Question #276990 in General Chemistry for Sohan

Question

Postulates the solar system atom model with its limitation Explain it

Answer 1

Neil Bohr’s Atomic Model is an improvement of Rutherford’s Atomic Model and was presented by both Niels Bohr and Ernest Rutherford in 1913. This post will discuss about what is Neil Bohr’s Atomic Model, how it different from Rutherford’s Atomic Model, its three postulates and limitations.

According to Niels Bohr, his Atomic Model explained the existence of an Atom which consists of a small, dense Nucleus which is surrounded by orbiting electrons much like the structure of the Solar System, where the attraction between the particles is due to electrostatic force rather than gravity.

Bohr’s model of the hydrogen atom is based on three postulates: (1) an electron moves around the nucleus in a circular orbit, (2) an electron’s angular momentum in the orbit is quantized, and (3) the change in an electron’s energy as it makes a quantum jump from one orbit to another is always accompanied by the emission or absorption of a photon. Bohr’s model is semi-classical because it combines the classical concept of electron orbit (postulate 1) with the new concept of quantization (postulates 2 and 3).

Limitations:

(i) It does not explain the spectra of atoms having more than one electron.

(ii) Bohr’s atomic model failed to account for the effect of the magnetic field (Zeeman Effect) or electric field (Stark effect) on the spectra of atoms or ions. It was observed that when the source of a spectrum is placed in a strong magnetic or electric field, each spectral line further splits into a number of lines. This observation could not be explained on the basis of Bohr’s model.

(iii) De Broglie suggested that electrons like light have a dual character. It has particle and wave characters. Bohr treated the electron only as a particle.

(iv) Another objection to Bohr’s theory came from Heisenberg’s Uncertainty Principle. According to this principle “It is impossible to determine simultaneously the exact position and momentum of a small moving particle like an electron”. The postulate of Bohr, that electrons revolve in well-defined orbits around the nucleus with well-defined velocities is thus not tenable.