CHe: 1s22s22p63s23p63d104s24p65s2
=> Z=38=P=E
Mà 2Z-N=31=>N=2Z-31=2*38-31=45
=>A=Z+N=83
\(hpt:\left\{{}\begin{matrix}e=p=12\\p+e-n=31\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}p=e=12\\n=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}Z=12\\A=X+N=12+7=19\end{matrix}\right.\)