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?

List applications of any three isotopes in various fields.

Give reasons for the following:
$(a)$ Isotopes of an element are chemically similar.
$(b)$ An atom is electrically neutral.
$(c)$ Noble gases show least reactivity.
$(d)$ Nucleus of an atom is heavy and positively charged.
$(e)$ Ions are more stable than atoms.

An atom with atomic number $9$ and mass number $19$ will have the following constituents:

If an ion $M^{3+}$ contains $10$ electrons and $14$ neutrons, what are the atomic number and mass number of the element $M$?

Why is the atomic mass of chlorine taken as $35.5\, u$ and not a whole number like $35\, u$ or $36\, u$? Explain.