The number of valence electrons in $Cl^{-}$ ion are

Which one of the following is the correct electronic configuration of sodium?

$(a)$ Explain the Bohr$-$Bury rules for the distribution of electrons into different shells.
$(b)$ Draw the electronic structure of element $X$ with atomic number $17$ and element $Y$ with atomic number $16$.

Identify the most stable atom from the following. Also give a reason for your answer.
$Na_{11}^{23}, Cl_{17}^{35}, Al_{13}^{27}, Ar_{18}^{40}$

Define the term valency. How is valency different from valence electrons? Find the valency of oxygen and fluorine.

$A, B$ and $C$ are three metals with atomic numbers $4, 11$ and $13$ respectively. Arrange the metals in the increasing order of their valency.