what is meant by energy levels of an atom are quantized?

Answer 1

It means that there are certain discrete energy levels that an atom can attain, by promoting electrons to higher-energy orbitals. The energy state of an atom is not continuously variable: each possible orbital to which an electron can be promoted has a certain energy level. As an electron "drops" from a high-energy orbital to a lower-energy one, a discrete amount of energy is released in the form of a photon. Whether this is visible light, IR, UV, gamma or X ray depends on the photon's energy (and therefore its wavelength).

Answer 2

Quantized Energy Levels

Answer 3

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RE:
what is meant by energy levels of an atom are quantized?

Answer 4

quantized : comes in discrete quantities. eg one shoe, two shoes, three shoes... one ball, two balls, three balls.

It turns out that to make theory fit the experimental data it was necessary to have 'energy' come in discrete lumps too. Hence quantized. 'Levels' may be a hang-over from the Bohr model of the atom that had little point like electrons in fixed orbits around a central nucleus, we still say energy level but have abandoned the Bohr model.

[I've not been absolutely strict with my usage of 'energy' in order to emphasize the idea that quantized means that we have to measure/count using integers]

history and depth here http://physicsworld.com/cws/article/print/373 and many other places too.

Answer 5

the energy is available only in discrete packets. like steps on a ladder. energy can only exist on the steps not between. this is why electrons don't collide into the nucleus of the atom. the nucleus is between possible energy levels of the electrons.