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

Differentiate between atomic mass and mass number.

The number of electrons in the outermost '$L$' shell of an atom is $5$.
$(a)$ Write its electronic configuration.
$(b)$ What is its valency and why?

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

State reasons for the following statements:
$(i)$ Some elements possess fractional atomic mass.
$(ii)$ Isotopes of an element have similar chemical properties.
$(iii)$ Noble gases are inert.

The ratio of the radii of a hydrogen atom and its nucleus is $\sim 10^5$. Assuming the atom and the nucleus to be spherical,
$(i)$ What will be the ratio of their sizes (volumes)?
$(ii)$ If the atom is represented by the planet Earth $(R_e = 6.4 \times 10^6 \, m)$,estimate the size of the nucleus.