P+N+E=21
P=E
N=33,333%.(N+P+E)
⇔2P+N=21
⇒N=33,333%.21
⇔2P+N=21
⇒N=7
⇔P=E=Z=7
⇒N=7
Cấu hình e (Z=7): 1s22s22p3
=> Có 2 lớp electron.
Ta có: \(\left\{{}\begin{matrix}p+e+n=21\\p=e\\n=33,33\%\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}p=e=7\\n=7\end{matrix}\right.\)