-Nguyên tử khối là 35
`=>X` là Clo
`=>p=e=17`
Ta có
`p+e+n=52`
`=>n=52-p-e=52-17-17`
`=>n=18`
Vậy `p=e=17;n=18`
\(Tacó:\left\{{}\begin{matrix}P=E\\P+E+N=52\\M=A=P+N=35\end{matrix}\right.\\ \Rightarrow\left\{{}\begin{matrix}P=Ẻ=17\\N=18\end{matrix}\right.\)