I know that when we say an atom is excited, we actually mean that one of its electrons either moves to another energy level or to another orbit. But when we say that an atom is in a certain energy level, what happens to its electrons? If the system (nucleus+electrons) are both described as having the same energy, how can electrons within the system be in different energy states than the atom as a whole?
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In general one is talking about a system, the whole atom, which consists of neutrons and protons in the nucleus and electrons.
There is a lowest energy state of an atom which is called the ground state.
Energy can be added to an atom so that the state of an electron in the atom changes, which in turn means the state of the atom has changed, and the energy of the atom has increased.
A way of describing this change is to say that an electron has moved to a new energy level.
An illustration of the energy changes being related to the whole atom is that when the energy of an atom decreases with the emission of a photon (an electron dropping in energy level) the atom recoils because momentum has to be conserved, a phenomenon which is explained in this post, Does an atom recoil when photon radiate?.
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Yes I understand, but my concern was in case an atom had more than one electron, what determines its (the atom's) energy level? Is it the electron on the outermost shell? – Girl_ Feb 21 '22 at 08:28
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1@Girl_ the energy of a system is the sum of the energies of its components. The energy of the atomic system includes the energy levels of all the electrons. When an electron gets into an excited state, higher in energy by a difference $\Delta E$, the total energy of the atomic system simply increases by $\Delta E$. As to the title of your main question, I have never heard an atom referred to as "at energy level 1". As far as I know, this is not a wording that is used, so it is hard to tell what it means indeed. – Barbaud Julien Feb 21 '22 at 08:41