The average atomic mass of a sample of an element $X$ is $16.2 \, u$. What are the percentages of isotopes $_8^{16}X$ and $_8^{18}X$ in the sample (in $\%$)?

Composition of the nuclei of two atomic species $X$ and $Y$ are given as under:

	$X$	$Y$
Protons $=$	$6$	$6$
Neutrons $=$	$6$	$8$

Give the mass numbers of $X$ and $Y$. What is the relation between the two species?

Complete the following table.

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of the Atomic Species
$9$	-	$10$	-	-	-
$16$	$32$	-	-	-	sulphur
-	$24$	-	$12$	-	-
-	$2$	-	$1$	-	-
-	$1$	$0$	$1$	$0$	-

Summarize the rules for writing the distribution of electrons in various shells for the first eighteen elements.

If bromine atom is available in the form of,say,two isotopes $_{35}^{79}Br$ $(49.7 \%)$ and $_{35}^{81}Br$ $(50.3 \%),$ calculate the average atomic mass of bromine atom. (in $,u$)

Compare the properties of electrons,protons,and neutrons.