Bohr's theory is useful in calculating energy of electron in one electron species. Energy of electron in 2nd orbit of ${\mathrm{He}}^{+}$ ion is

${\mathrm{E}}_{\mathrm{n}}=\frac{-{\mathrm{Z}}^2}{{\mathrm{n}}^2}{\mathrm{E}}_1$  
${\mathrm{E}}_1=13.6\mathrm{ev}/\mathrm{atom}$ (magnitude)
$=2.18\times {10}^{-18}\mathrm{J}/\mathrm{atom}$
$=2.18\times {10}^{-11}\mathrm{ergs}/\mathrm{atom}$
$=313.6\mathrm{kcal}/\mathrm{mole}$
${\mathrm{He}}^{+}\mathrm{ion}$
Z = 2 
Given n = 2
${\therefore {\mathrm{E}}_{\mathrm{n}}=\frac{-2^2}{{\text{2}}^2}{\mathrm{E}}_1}_{ }$
${\mathrm{E}}_{\mathrm{n}}=-{\mathrm{E}}_1$