Quantum theory was postulated by :

The energy of the electron in the first orbit of $He^{+}$ is $-871.6 \times 10^{-20} \ J$. The energy of the electron in the first orbit of hydrogen would be

Calculate the energy released when an electron in a hydrogen atom transitions from the $3^{rd}$ energy level to the $2^{nd}$ energy level.

$A$ bulb is rated at $150 \text{ W}$, converting $8\%$ of its energy into light. If the energy of one photon is $4.42 \times 10^{-19} \text{ J}$, how many photons are emitted by the bulb per second?

When $Z$ is doubled in an atom,which of the following statements are consistent with Bohr's theory?

The wavenumber associated with the maximum energy in the Balmer series will be......