Blow a nucleus up to the size of a pea and the outermost electrons would sit a hundred metres away, with almost nothing in between. It is the proton count that names the element: going from 1 to 20 swaps the element outright, and the shell configuration and valence-electron count step along with it. Given a pump of energy, an electron climbs to a higher level and falls back, and every fall adds another coloured line to the spectrum, because the emitted photon carries exactly the gap between the two levels. Adding or removing neutrons changes only the mass number A; the element keeps its name.
Blow a nucleus up to the size of a pea and the outermost electrons would sit a hundred metres away, with almost nothing in between. It is the proton count that names the element: going from 1 to 20 swaps the element outright, and the shell configuration and valence-electron count step along with it. Given a pump of energy, an electron climbs to a higher level and falls back, and every fall adds another coloured line to the spectrum, because the emitted photon carries exactly the gap between the two levels. Adding or removing neutrons changes only the mass number A; the element keeps its name.
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