The molecular formula of aluminium sulphate is

$(a)$ An atom is considered electrically neutral. Explain.
$(b)$ State the valency of Helium.
$(c)$ If an atom has an atomic mass of $4$ and $2$ protons in its nucleus,how many neutrons does it have?

$A$ chlorine atom is electrically neutral,but a chloride ion is a charged ion. Explain.

Find the ratio by mass of the combining elements in the following compounds. (You may use Appendix $-III$)
$(a)$ $CaCO_{3}$ $(b)$ $MgCl_{2}$
$(c)$ $H_{2}SO_{4}$ $(d)$ $C_{2}H_{5}OH$
$(e)$ $NH_{3}$ $(f)$ $Ca(OH)_{2}$

Complete the following crossword puzzle by using the names of the chemical elements based on the data provided in the table.

Across	Down
$2$. The element used by Rutherford during his $\alpha$-scattering experiment.	$1$. $A$ white lustrous metal used for making ornaments and which tends to get tarnished black in the presence of moist air.
$3$. An element which forms rust on exposure to moist air.	$4$. Both brass and bronze are alloys of this element.
$5$. $A$ very reactive non-metal stored under water.	$6$. The metal which exists in the liquid state at room temperature.
$7$. Zinc metal when treated with dilute hydrochloric acid produces a gas of this element which when tested with a burning splinter produces a pop sound.	$8$. An element with the symbol $Pb$.

As per the law of definite proportion,carbon and oxygen combine in the ratio of $3:8$. Compute the mass of oxygen gas that would be required to react completely with $15\,g$ of carbon.