Question

Assertion -
Sodium and aluminium have atomic numbers $11$ and $13$, respectively. They are separated by one element in the periodic table, and have valencies $1$ and $3$ respectively. Chlorine and potassium are also separated by one element in the periodic table (their atomic number being $17$ and $19$ respectively) and yet both have valency $1$.
Reason -
$Cl$ requires one electron to complete its octet and its valency is $1$ while $K$ has one electron in its outermost shell which it can easily lose to complete its octet and can attain the most balanced state after forming an octet. So, both have valency $1$.
A. Both assertion and reason are correct and reason is correct explanation of assertion
B. Both assertion and reason are correct and reason is not correct explanation of assertion
C. Assertion is correct but reason is incorrect
D. Both assertion and reason are incorrect

Answer 1

Hint: Each element has a characteristic atomic number, which also shows the number of electrons in that element. The most stable state is attained when atoms have a fully filled outermost shell, that is, a noble gas configuration. This is related to the valency of the element.

Complete step by step answer:
Let us first look at the electronic configurations of both sodium and aluminium. The atomic number of sodium is $11$ , which gives us a configuration is $[Ne]3{s^1}$ and that of aluminium is $13$ , which gives us a configuration of $[Ne]3{s^2}3{p^1}$ .
As we know, elements which can attain the nearest noble gas configuration, that is, the configuration of Neon in this case, tend to be most stable. To obtain Neon’s configuration, sodium needs to lose one electron and aluminium needs to lose three, hence representing their valencies as the same as well.
Now let us look at the second statement in the assertion. It talks about chlorine and potassium. The electronic configuration of chlorine is $[Ne]3{s^2}3{p^5}$ . Thus, in its outermost shell, it has a total of seven electrons ( $2$ in $s$ and $5$ in $p$ ). As we know, it needs to attain noble gas configuration to be stable. The nearest noble gas (Argon) is just one electron away, rather than neon, which is seven electrons away. Naturally, gaining one electron to achieve Argon’s configuration is easier than losing seven electrons to achieve Neon’s configuration. Thus, chlorine has a valency of $1$ .
The configuration of potassium is $[Ar]4{s^1}$ . Thus, the nearest noble gas is Argon, and it can attain this configuration while losing one electron from its outermost shell, giving potassium too a valency of $1$ .
This is exactly what the reason statement explains, and thus, it gives us the correct explanation for the assertion statement.

So, the correct answer is Option A .

Note: Valency is defined as the number of electrons an atom needs to gain or lose in order to achieve a stable electronic configuration. Note that valency of an element is never more than four, since no atom is capable of losing more than four or gaining more than four electrons. It will always choose the path which requires less energy.