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

$(a)$ 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.

The number of electrons in an element $X$ is $15$ and the number of neutrons is $16$. Which of the following is the correct representation of the element?

The atomic numbers of calcium and argon are $20$ and $18$ respectively,but the mass number of both these elements is $40$. What is the name given to such a pair of elements?

Write the symbols of two isotopes of uranium.

$(a)$ How many neutrons are present in $C-14$ isotope of carbon?
$(b)$ How many protons does $He^{2+}$ ion possess?
$(c)$ How many maximum electrons can be filled in the $3^{rd}$ orbit of an atom?
$(d)$ How many electrons does $He^{2+}$ ion possess?