$A$ helium atom has $2$ electrons in its valence shell,but its valency is not $2$. Explain.

$(a)$ Name the three subatomic particles of an atom.
$(b)$ An atom of an element has $5$ electrons in the $L$ shell.
$(i)$ What is the atomic number of the element?
$(ii)$ State its valency.
$(iii)$ Identify the element and write its name.

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

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

$(a)$ Which of the following electronic configurations are not possible? Give reasons.
$(i)$ $X: 2, 8, 4$
$(ii)$ $Y: 3, 8, 2$
$(iii)$ $Z: 2, 8, 9$
$(b)$ Write electronic configurations of the following elements and predict their valencies:
Fluorine: $9$,Aluminium: $13$,Argon: $18$

$Na^{+}$ has completely filled $K$ and $L$ shells. Explain.