# What does quantised mean?

"The energy levels of the electron in a hydrogen atom are quantised".

What do they mean by the term quantised?

• My answer here might help physics.stackexchange.com/q/538930 – anna v Mar 29 at 4:55
• I'm not convinced this is a question about physics and not the English language, since the answer can be found in a dictionary.and the word has been used in this sense outside of its origin in quantum physics. – ACuriousMind Mar 29 at 6:50
• @ACuriousMind that definition comes at it from the wrong direction. It implies we get to decide how much we consider measurable. Really it's as simple as counting whole apples as opposed to counting half apples or bushels of apples. An apple is an apple regardless of what you're doing. – candied_orange Mar 29 at 12:50

What we mean by quantise is discrete. You know for example that the energy of a free particle is a continuous value of the impulse $$E = \frac{p^2}{2m}$$ The energy of the electrons in the hydrogen atom is not a continuous function of anything, instead takes on discrete values depending on an integer $$n$$ which we call energy level. The energy levels of the hydrogen atom are evaluated in the quantum theory and are given by $$E_n = -\frac{E_0}{n^2}$$ where $$E_0 = 13.6$$ eV. There's no way for an electron in the hydrogen atom to have an energy between two successive levels like $$E_1$$ and $$E_2$$.