- 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.
Danish physicist Niels Bohr introduced this model in 1913 to explain atomic stability and the line spectrum of hydrogen. It pictured the atom as a small positively charged nucleus with electrons in allowed circular paths, superficially similar to a solar system. Electrostatic attraction supplied the inward force. The original model did not include neutrons, which were discovered later. Bohr combined the nuclear picture of Rutherford’s atomic model with quantum postulates. The circular paths are part of this historical model; modern quantum mechanics describes electrons with wavefunctions and orbitals.
- The model permits an electron to occupy only certain stationary orbits, each with a discrete energy. The lowest allowed state is the ground state. An atom absorbs energy when an electron changes to a higher state.
When it changes to a lower state, it emits a photon whose energy equals the difference between the two levels. The photon energy is a quanta of electromagnetic radiation and follows:
Where,
‘h’ is Planck’s constant,
‘υ’ is the frequency of light in hertz,
‘c’ is the speed of light in metres per second,
‘λ’ is the wavelength of the emitted or absorbed light in metres.
- Within the model, electrostatic attraction between the positive nucleus and negative electron provides the centripetal force required for circular motion. No separate outward force is needed in an inertial-frame description.
- The model restricts the orbital angular momentum to an integral multiple of
Where, n is a positive integer called the principal quantum number. - For a one-electron hydrogen-like ion, the allowed radius is proportional to n2 divided by nuclear charge Z, while the model’s electron speed is proportional to Z divided by n. These relations are not general rules for multielectron atoms.
The Bohr model made successful hydrogen-like predictions but has important limits:
- It works best for hydrogen and hydrogen-like ions with one electron. It does not accurately describe general multielectron atoms.
- Its postulates give allowed energies and transition frequencies but do not calculate transition probabilities or selection rules.
- It uses only the principal quantum number n and cannot account fully for fine structure, electron spin or the other quantum numbers of modern atomic theory.
- A quantitative theory of general chemical bonding requires quantum mechanics rather than the literal circular orbits of the Bohr’s atomic model.
