Identify the most stable species in the following set of ions giving reasons.
$(a) \ \stackrel{+}{C} H_{3}, \stackrel{+}{C} H_{2} Br, \stackrel{+}{C} HBr_{2}, \stackrel{+}{C} Br_{3}$
$(b) \ \stackrel{\Theta}{C} H_{3}, \stackrel{\Theta}{C} H_{2} Cl, \stackrel{\Theta}{C} HCl_{2}, \stackrel{\Theta}{C} Cl_{3}$

(N/A) $\stackrel{+}{C} H_{3}$ is the most stable species because the $-I$ effect of $Br$ intensifies the positive charge and hence destabilizes the species. Further,the more the number of $Br$ atoms,the less stable is the species. Thus,the stability of these species decreases in the order:
$\stackrel{+}{C} H_{3} > \stackrel{+}{C} H_{2} Br > \stackrel{+}{C} HBr_{2} > \stackrel{+}{C} Br_{3}$
$(b)$ The $-I$ effect of the $Cl$ atom disperses the negative charge and thus stabilizes the species. Further,the more the number of $Cl$ atoms,the more is the dispersal of the negative charge and hence the more stable is the species. Thus,$\stackrel{\Theta}{C} Cl_{3}$ is the most stable species. The stability order of other species decreases as:
$\stackrel{\Theta}{C} Cl_{3} > \stackrel{\Theta}{C} HCl_{2} > \stackrel{\Theta}{C} H_{2} Cl > \stackrel{\Theta}{C} H_{3}$