Chemistry · Structure Of Atom · NEET
There are four main ideas. (1) The electron moves around the nucleus only in certain fixed circular orbits, called stationary states or allowed energy states. (2) As long as the electron stays in one orbit, its energy stays constant and it does NOT lose energy. (3) The electron gives out or takes in energy only when it jumps from one orbit to another; the energy of that light is the difference between the two orbits (Bohr's frequency rule, delta-E = E-high minus E-low = h times frequency). (4) The angular momentum of the electron is quantised: mvr can only be n times h over 2 pi (n = 1, 2, 3...). Learn these four; NEET asks them directly.
This is Bohr's biggest fix over Rutherford. In Rutherford's model, a moving (accelerating) electron should keep radiating energy, spiral in, and the atom would collapse in a fraction of a second. Bohr simply POSTULATED (assumed as a rule) that in a stationary state the electron does NOT radiate energy. So it keeps its energy and stays in its orbit forever unless it jumps. Bohr did not prove why; he just made it a rule that matched experiments. For NEET, remember: stationary state = no energy loss = stable atom.
'Stationary' does NOT mean the electron is standing still. The electron is still moving fast in a circle. 'Stationary state' means the ENERGY of the electron is fixed and unchanging in that orbit. So the state (the energy level) is steady, not the electron. These orbits are also called 'allowed energy states' or 'orbits'. Do not confuse 'stationary state' with a still electron - that is a common trap.
Only during a JUMP between two orbits, never while sitting in one orbit. If the electron jumps from a higher orbit to a lower orbit, it EMITS (gives out) light. If it jumps from a lower orbit to a higher orbit, it ABSORBS (takes in) light. The energy of that photon equals the energy gap: delta-E = E2 - E1 = h times nu (frequency). This is Bohr's frequency rule and it explains the sharp line spectrum of hydrogen.
Bohr's model has four postulates. The FOURTH one is the quantisation of angular momentum: mvr = n times h over 2 pi, where n = 1, 2, 3... This says the electron can only be in orbits where its angular momentum is a whole-number multiple of h/2 pi. This single rule is what forces the orbits to be fixed and gives the fixed radius and fixed energy. NEET often tests this formula separately, so learn it well.
Bohr's postulates work well for hydrogen and for hydrogen-like ions that have only ONE electron, such as He+ , Li2+ and Be3+ (single-electron species). For these you just include the atomic number Z. The model FAILS for atoms with two or more electrons (like helium, He) because it ignores electron-electron repulsion and the wave nature of the electron. So Bohr = one-electron species only.
If the radius of the second Bohr orbit of the He+ ion is 105.8 pm, what is the radius of the third Bohr orbit of the Li2+ ion?
In a hydrogen atom, the de Broglie wavelength of an electron in the second Bohr orbit is: [Bohr radius a0 = 52.9 pm]
Try the real previous-year questions from this chapter — each with the answer and a full solution.
When an electron jumps between two orbits, the frequency of the light emitted or absorbed is given by delta-E = E-higher minus E-lower = h times nu. So the light frequency depends only on the energy gap between the two stationary states. This rule explains why hydrogen gives sharp spectral lines.
The electron is moving fast in a circular orbit. 'Stationary' only means its energy is fixed and does not change while it stays in that orbit. It does not mean the electron is at rest.
To fix Rutherford's model. Classical physics said a circling electron must radiate energy and spiral into the nucleus, so the atom would collapse. Bohr assumed as a rule that stationary-state orbits do not radiate, which keeps the atom stable and matches experiments.
Hydrogen and hydrogen-like ions with only one electron, such as He+ , Li2+ and Be3+. It fails for multi-electron atoms because it ignores electron-electron repulsion and the wave nature of matter.
Bohr uses fixed circular orbits with a definite path for the electron. The quantum mechanical model says we cannot know the exact path (Heisenberg uncertainty) and uses orbitals - regions of probability. Bohr also ignores the wave nature of the electron, which the quantum model includes.
