The 1st electron shell of an atom can hold up to 2 electrons.
Subsequent shells hold up to 8 electrons.
An atom is most stable when its outermost, or valence, shell is filled.
A helium atom has 2 protons,
2 neutrons,
and 2 electrons.
The electrons fill its one electron shell.
A nitrogen atom has 7 protons,
7 neutrons,
and 7 electrons.
It has 3 vacancies in its valence shell.
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