energy of the photon = 18 ev
energy of the atom after it absorbs the photon,
=(-20 +18) ev = -2ev
the electron moves from level n=1 to n=4.
possible energies of the emitted photons: 3.0 ev, 5.0 ev, 8.0 ev, 10 ev, 15 ev, 18 ev
An electron is a negatively charged subatomic particle that can be both sure to an atom or free.
An electron that is sure to an atom is one of the 3 number one types of particles inside the atom -- the opposite  are protons and neutrons. Collectively, electrons, protons and neutrons form an atom's nucleus.
The hydrogen atom, as an example, has simply one electron and one proton. The uranium atom, alternatively, has 92 protons, and therefore, 92 electrons.
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