An element X has a hydrogen-like spectrum and its lines have wavelengths four times shorter than those of atomic hydrogen. Name the element X.

For hydrogen-like atom,1λ′=RZ21n12−1n22 (i)For hydrogen atom,1_λ=R1n12−1n22 (ii)Dividing equation (ii) by equation (i), we getλ′λ=R1n12−1n22RZ21n12−1n22λ′λ=1Z2According to the problem, λ′=λ4λ′4λ′=1Z2∴Z=2The atomic number of helium atom is 2.Hence, required element is Helium.