$2p+n=84$
Ta có:
$\dfrac{2p+n}{3,5}\le p\le \dfrac{2p+n}{3}$
$\Leftrightarrow 24\le p\le 28$
$\Rightarrow p\in\{24;25;26;27;28\}$
$p=24\Rightarrow n=36$ (loại)
$p=25\Rightarrow n=34$ (loại)
$p=26\Rightarrow n=32$ (TM)
$p=27\Rightarrow n=30$ (loại)
$p=28\Rightarrow n=28$ (TM)
Vậy $p=e=26; n=32$ (Fe)