Chemistry · Structure Of Atom · NEET
It means energy is not smooth or continuous. An atom or molecule can only emit (give out) or absorb (take in) energy in fixed small amounts called quanta. It cannot handle a value between two allowed amounts. So energy comes in whole packets, like coins of a fixed value, not like water you can pour in any amount.
NCERT compares quantisation to standing on a staircase. You can stand on step 1, step 2, or step 3, but you cannot stand in the space between two steps. In the same way, an atom can only have certain allowed energy values (the steps) and nothing in between. A ramp would be continuous energy; a staircase is quantised energy.
A quantum is the smallest packet of energy that can be emitted or absorbed. Planck showed its energy depends on the frequency of the radiation: E = hv, where h is Planck's constant (6.626 x 10^-34 J s) and v (nu) is the frequency. For light, one such packet is also called a photon. Higher frequency means a bigger energy packet.
Continuous energy means any value is allowed, with no gaps (like the slope of a ramp). Quantised energy means only certain fixed values are allowed, with forbidden gaps between them (like steps). Classical physics assumed energy was continuous, but black body radiation could only be explained if energy was quantised, so Planck introduced the quantum idea in 1900.
Hot objects glow and give out radiation over many wavelengths. Old wave theory could not correctly predict how much radiation came out at each wavelength (the black body radiation problem). Planck fixed this by assuming atoms emit or absorb energy only in discrete chunks (quanta) of size E = hv, not continuously. This assumption matched the experiment and started quantum theory.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Max Planck proposed it in 1900 to explain black body radiation. He suggested atoms and molecules can emit or absorb energy only in discrete packets called quanta, each of energy E = hv.
E = hv, where h = 6.626 x 10^-34 J s (Planck's constant) and v is the frequency of the radiation. Using c = v x lambda, it can also be written as E = hc/lambda.
A quantum is the smallest packet of any energy. When that packet is a packet of light or electromagnetic radiation, it is specially called a photon. So a photon is a quantum of light energy.
The idea appears wherever a system can only have certain allowed energy levels, such as electrons in atoms. It is the basis of Bohr's model and the modern quantum mechanical model, so it is very important for the whole Structure of Atom chapter in NEET.
It is the starting point for Planck's equation, the photoelectric effect, and Bohr's model of the hydrogen atom. NEET often tests the fact that energy is not continuous and the formula E = hv, so understanding this concept unlocks many later questions.
