Why do Helium,Neon and Argon have a zero valency?

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

Elements with valency $1$ are

$(a)$ Explain the Bohr$-$Bury rules for the distribution of electrons into different shells.
$(b)$ Draw the electronic structure of element $X$ with atomic number $17$ and element $Y$ with atomic number $16$.

Sulphur dioxide $(SO_{2})$ is a colourless,pungent-smelling gas and is a major air pollutant.
$(a)$ Write the electronic configuration of its constituent elements,sulphur and oxygen.
(Given: $_{16}^{32}S, _{8}^{16}O)$
$(b)$ Write the valency of sulphur and oxygen.
$(c)$ Are sulphur and oxygen isotopes of the same element? Explain your answer.

Write five applications of isotopes in various fields.