Name any two noble gases.

The atomic numbers of three elements $A, B$ and $C$ are $9, 10$ and $13$ respectively. Which of them will form a cation?

An atom of an element has full $K$ and $L$ shells and has $5$ electrons in its $M$ shell. What is its atomic number?

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

$(a)$ 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 electron distribution in an aluminium atom is