The true statement(s) about chromium (atomic number $=24$) is/are

$1$. It contains $24$ neutrons 
$2$. Its valence electrons lies in N-shell 

$3$. ${}_{24}{}^{54}\mathrm{Cr}$ is isotonic with ${}_{25}{}^{60}\mathrm{Mn}$

The correct alternative/s is 

Isoelectronic species are

How many neutrons are present in the tritium nucleus?

An atom contains $2$ electrons in K shell, $8$ electrons in L shell and $4$ electrons in M shell. The element is

How many valence electrons does calcium(atomic number $=20$) has? 

The number of electrons in the chlorine atom is

Which is not true with respect to cathode rays?