At what condition is the energy of an electron in an atom considered to be zero?

Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nanosecond range. If the radiation source has the duration of $2 \, ns$ and the number of photons emitted during the pulse source is $2.5 \times 10^{15}$,calculate the energy of the source.

Which of the following equations gives the angular momentum of an electron in a stationary orbit?

The energy of the second Bohr orbit of the hydrogen atom is $-3.4 \ eV$. The energy of the fourth Bohr orbit of the $He^{+}$ ion will be: (in $eV$)

According to classical theory,the proposed circular path of an electron in Rutherford atomic model will be:

The electron in a hydrogen atom,upon absorbing $12.75 \ eV$ of energy,will jump to the ....... orbit.