\(\left\{{}\begin{matrix}2z+n=13\\z\le n\le1,5z\end{matrix}\right.\)
\(\Leftrightarrow z\le13-2z\le1,5z\Leftrightarrow3\le\frac{13}{z}\le3,5\)
\(\Leftrightarrow\frac{13}{3}\ge z\ge\frac{26}{7}\Leftrightarrow4,3\ge z\ge3,7\Leftrightarrow z=4\)
=> X là \(^9_4Be\)
\(\Rightarrow1s^22s^2\)