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

What do you understand by an octet of electrons in the valence shell?

An atom with $3$ protons and $4$ neutrons will have a valency of

$(a)$ Explain Bohr$-$Bury rules for the distribution of electrons into different shells.
$(b)$ Write the electronic configuration of an atom of sulphur. Also,draw a schematic diagram of its atom showing the distribution of electrons in its shells.

If $K$ and $L$ shells of an atom are full,then what would be the total number of electrons in the atom?

$(a)$ 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: $18$