The chemical symbol for sodium is

Write the atomicity of the following:
$(i)$ Sulphur molecule
$(ii)$ Phosphorus molecule

State whether the following statement is true or false:
The elements having a valency of $1$ are called monovalent.

Write the molecular formulae of all the compounds that can be formed by the combination of the following ions: $Cu^{2+}, Na^{+}, Fe^{3+}, Cl^{-}, SO_{4}^{2-}, PO_{4}^{3-}$.

Which of the following represents $1 \, u$?

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