Chemistry · Structure Of Atom · NEET
There are four simple postulates. (1) The electron moves around the nucleus only in certain fixed circular paths called orbits or stationary states. (2) While the electron stays in one orbit, its energy stays constant and it does NOT lose energy (that is why the atom is stable). (3) Each orbit has a fixed energy, so orbits are also called energy levels, labelled by n = 1, 2, 3... (4) The angular momentum of the electron is quantised: it can only be a whole-number multiple of h/2π, written as mvr = nh/2π. Energy is emitted or absorbed only when the electron jumps from one orbit to another.
Here m is the mass of the electron, v is its speed, and r is the radius of the orbit. The product mvr is called angular momentum. Bohr said this value is not free to be anything. It can only be n times h/2π, where n = 1, 2, 3... and h is Planck's constant. So angular momentum is 'quantised' (comes in fixed steps). This single rule is what fixes the allowed orbits and separates Bohr's model from earlier models. For NEET, remember n is a positive whole number and never zero or a fraction.
According to older physics, a moving (accelerating) charge should keep radiating energy, spiral inward, and crash into the nucleus. Bohr fixed this by a bold postulate: when the electron is in an allowed orbit (a stationary state), it does NOT radiate energy at all. So it stays in that orbit forever unless something makes it jump. This is a rule Bohr assumed to match experiments; it is not derived. That is why the hydrogen atom is stable. This point is a common NEET 'true/false statement' trap.
Light is emitted or absorbed ONLY when the electron jumps between two orbits, never while it stays in one orbit. If the electron jumps from a higher orbit (higher energy) to a lower orbit, it emits a photon. If it jumps from a lower orbit to a higher orbit, it absorbs a photon. The energy of that photon equals the energy gap: ΔE = E(higher) − E(lower) = hν. This is the 'Bohr frequency condition' and it explains the line spectrum of hydrogen.
No. 'Stationary' does not mean the electron is not moving. The electron is still revolving fast around the nucleus. 'Stationary state' means the STATE (the orbit and its energy) does not change with time and does not lose energy. So the energy is stationary (fixed), not the electron. NEET often uses this wording to confuse students, so read the statement carefully.
The Bohr model works well only for hydrogen and for hydrogen-LIKE ions that have just one electron, such as He⁺, Li²⁺ and Be³⁺. It fails for atoms with two or more electrons because it ignores electron-electron repulsion. So in NEET, if a question asks you to apply Bohr formulas, the species must be single-electron. This is why the next topic covers radius and energy for hydrogen-like ions.
In a hydrogen atom, the de Broglie wavelength of an electron in the second Bohr orbit is: [Given that Bohr radius a₀ = 52.9 pm]
Try the real previous-year questions from this chapter — each with the answer and a full solution.
n is the principal quantum number and it can only be a positive whole number: n = 1, 2, 3, 4... It labels the orbit or energy level. n = 1 is the closest orbit to the nucleus (lowest energy, most stable). n can never be 0, negative, or a fraction.
Yes. This is the central postulate. Angular momentum mvr can only take fixed values equal to nh/2π, where n = 1, 2, 3... So it changes in fixed steps, not continuously. This is what 'quantised' means.
In the Bohr model they mean nearly the same thing. An orbit is the fixed circular path of the electron; an energy level is the fixed energy value of that orbit. Since each orbit has one fixed energy, the words are used together. (Note: an orbit is different from an 'orbital' in the modern quantum model — see the related topic.)
Yes, this is its biggest success. When the electron jumps from a higher orbit to a lower orbit, it emits a photon of a fixed energy (ΔE = hν), giving a sharp line. Different jumps give different lines, which matches the observed hydrogen spectrum (Lyman, Balmer, etc.).
Because it gives simple, exact formulas for radius, energy, and spectral lines of one-electron species (H, He⁺, Li²⁺). Many NEET numericals are solved fastest using Bohr formulas. You should learn the postulates first, then the radius/energy formulas in the next topic.
