Compute the difference in masses of $10^{3} \, \text{mol}$ each of magnesium atoms and magnesium ions. (Mass of an electron $= 9.1 \times 10^{-31} \, \text{kg}$)

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

The chemical formula of the oxide of an element $X$ is $X_2O_5$. Write the chemical formula of its chloride.

Classify each of the following on the basis of their atomicity:
$(a) F_2$
$(b) NO_2$
$(c) N_2O$
$(d) C_2H_6$
$(e) P_4$
$(f) H_2O_2$
$(g) P_4O_{10}$
$(h) O_3$
$(i) HCl$
$(j) CH_4$
$(k) He$
$(l) Ag$

$(a)$ In this crossword puzzle (Fig),names of $11$ elements are hidden. Symbols of these are given below. Complete the puzzle.
$1. Cl$
$2. H$
$3. Ar$
$4. O$
$5. Xe$
$6. N$
$7. He$
$8. F$
$9. Kr$
$10. Rn$
$11. Ne$
$(b)$ Identify the total number of inert gases,their names,and symbols from this crossword puzzle.

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