Energy of hydrogen like species \(=-13.6\times\frac{Z^2}{n^2}\)
Whereas, Z = Atomic number and n = Shell n shell represents n = 4
∴ Energy of Be+3ion(E) \(=-13.6\times\frac{4^2}{4^2}\) = -13.6 eV (∵ ZBe = 4)
∴ Potential energy(P.E.) = 2 x E = 2 x (-13.6) = -27.2 eV