The most probable radius (in $pm$) for finding the electron in $He^{+}$ is

$A$ certain transition in the hydrogen spectrum from an excited state in one or more steps gives rise to a total of $10$ lines. The number of lines lying in the visible spectrum is:

Which of the following is correct regarding Bohr's theory?

For a hydrogen atom with $n = 1$,the radius of the Bohr orbit is approximately $0.530 \, \mathring{A}$. What will be the radius for the first excited state $(n = 2)$ in $\mathring{A}$?

Which of the following represents the velocity of an electron in a hydrogen-like atom?

In which transition is minimum energy emitted?