Bohr Model: Introduction
Bohr model of atom was proposed by Neil Bohr in 1915. It came into existence with the modification of Rutherford’s model of atom. Rutherford’s model introduced nuclear model of atom, in which he explained that a nucleus (positively charged) is surrounded by negatively charged electrons. Bohr modified this atomic structure model by explaining that electrons move in fixed orbital’s (shells) and not anywhere in between and he also explained that each orbit (shell) has a fixed energy level. Rutherford basically explained nucleus of an atom and Bohr modified that model into electrons and their energy levels.
Bohr’s model consists of a small nucleus (positively charged) surrounded by negative electrons moving around the nucleus in orbits. Bohr found that an electron located away from the nucleus has more energy, and electrons close to the nucleus have less energy.
Postulates of Bohr’s Model of an Atom
- In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called as orbits or shells.
- Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.
- The energy levels are represented by an integer (n=1, 2, 3…) known as quantum number. This range of quantum number starts from nucleus side with n=1 having the lowest energy level. The orbits n=1, 2, 3, 4… are assigned as K, L, M, N…. shells and when an electron attains the lowest energy level it is said to be in the ground state.
- The electrons in an atom move from lower energy level to higher energy level by gaining the required energy and an electron moves from higher energy level to lower energy level by losing energy.
Limitations of Bohr’s Model of an Atom
- Bohr’s model of an atom failed to explain Zeeman Effect (effect of magnetic field on the spectra of atoms).
- It also failed to explain the Stark effect (effect of electric field on the spectra of atoms).
- It violates the Heisenberg Uncertainty Principle.
- It could not explain the spectra obtained from larger atoms.
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