Calculate the number of neutrons present in the nucleus of an element $X$ which is represented as $^{31}_{15}X$.

What is the valency of $(a)$ aluminium in aluminium sulphate,and $(b)$ sodium in sodium sulphate?

Which of the $Na^+$ and $He$ has completely filled $K$ and $L$ shells? Give a reason to support your answer.

State whether the following statement is true or false:
The relative atomic masses of all elements have been found with respect to an atom of oxygen-$16$.

Write the charge and mass of an electron.

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.