According to the Bohr-Bury scheme,the maximum number of electrons that can be accommodated in a given shell is given by the formula:

An atom of an element $X$ may be written as ${ }_{4}^{9} X$.
$(a)$ What does the figure $9$ indicate?
$(b)$ How many neutrons does this element $X$ contain?

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.

$A$ helium atom has $2$ electrons in its valence shell,but its valency is not $2$. Explain.

State whether the following statement is true or false:
The relative number and kinds of atoms are varying in a given compound.

An atom with $3$ protons and $4$ neutrons will have a valency of: