Energy levels of the hydrogen atom:
E_n = -me⁴/(2ħ²n²) = -13.6 eV/n²

Ground state wave function of the hydrogen atom:
ψ₁₀₀(r,θ,φ) = (πa₀³)^-1/2 exp(-r/a₀).
a₀ = ħ²/(me²) = 52.92 pm

Hydrogenic atoms:
E_n  = -Z²*13.6 eV/n²
a₀ = 52.92/Z pm

The spectrum of hydrogen:
hf = 13.6 eV(1/n_f² - 1/n_i²),  
1/λ = (13.6 eV(/hc))(1/n_f² - 1/n_i²) = R(1/n_f² - 1/n_i²).