- Definition of Bohr’s Atomic Model: Bohr’s atomic model is defined as a theory that describes the structure of atoms with electrons orbiting a small, central nucleus in circular paths.
- Structure of the Atom: In Bohr’s model, the atom consists of a nucleus with protons and neutrons, surrounded by electrons in specific orbits.
- Quantum Theory and Energy Levels: Electrons move in fixed orbits with set energy levels, and they absorb or emit energy when transitioning between these levels.
- Photon Emission: When an electron drops to a lower energy level, it releases energy as a photon, calculated using E = hυ = hc/λ.
- Limitations of Bohr’s Model: The model mainly explains hydrogen atoms, lacks rules for electron transitions, and does not account for all quantum numbers.
It is introduced by Danish Physicist Niels Bohr in the year of 1913. According to this model the atom consists of small nucleus at the center and electrons which rotates in circular orbits surrounding the nucleus – similar to solar system. But here, the force of attraction is provided by electrostatic forces rather than gravitational forces. The nucleus is positively charged and electrons are negatively charged. Further Niels Bohr illustrated that the positively charged nucleus consists of protons and neutrons. The protons are positively charged and neutrons does not have any charges. Niels Bohr introduced the quantum theory to overcome the drawbacks of Rutherford’s atomic model. According to this theory –
- Electrons revolve around the nucleus in specific orbits, each with a certain energy level. These orbits are called stationary orbits. The orbit closest to the nucleus has the lowest energy level, while outer orbits have higher energy levels. An electron can stay in a particular energy level without losing energy. When energy is added to the atom, the electron jumps to a higher energy orbit.
Conversely, when an electron jumps from a higher energy orbit to a lower energy orbit, it releases energy in small packets called quanta or photons. The energy of a photon is given by the formula:
Where,
‘h’ is the plank’s constant,
‘υ’ is the frequency of light (in Hz),
‘c’ is the speed of light (in m/sec),
‘λ’ is the wave length of light emitted (in meter).
- The centripetal force from the electrostatic attraction between the positively charged nucleus and the negatively charged electron balances the centrifugal force of the electron moving in circular orbits.
- The angular momentum of electron moving in circular orbits is the integral multiple of
Where, n is an integer called quantum number. - The radius of the orbit is proportional to n2 and the velocity of electron is inversely proportional to n. These assumptions led to results which have been found correct of being tested.
This model also has some deficiencies, which are listed below:
- It is applied to one electron atom i.e. hydrogen atom. It cannot be easily extended to explain the more complicated atoms.
- It does not give any rule or restriction regarding the transition of an electron from one orbit to another orbit.
- It introduced only one quantum number n. Whereas, experimental evidence concerning fine structure of spectral line suggests the more additional quantum numbers.
- The quantitative explanation of chemical bonding cannot be explained by the Bohr’s atomic model.
