Question

The formula of the compound is ${A_2}{B_5}$. The number of electrons in the outermost orbits of A and B respectively:
A.6 and 3
B.5 and 6
C.5 and 2
D.2 and 3

Answer 1

We know that valence electrons are those electrons that are seen in the outermost shell present in an atom. In simple words, we could tell that electrons that could be either gained or lost during a chemical reaction are known as valence electrons. We can calculate the number of valence electrons by using the formula of the compound. The formula of the compound gives the oxidation states of the elements present in the compound. With the help of the oxidation state, we can determine the electrons present in the elements.

Complete step by step answer:
We can say the valency as the number of electrons which are gained or lost by an atom to control the stability for attaining the electronic configuration of the noble gas which is nearest to the element.
We know that the formula of the compound is ${A_2}{B_5}$. We can also write the compound as ${A^{5 + }}{B^{2 - }}$.
We can see that the oxidation state/valency present on A is $+ 5$ and the number of outermost electrons present orbital is five electrons.
We can see the oxidation state/valency present on B is $- 2$ and it is deficient of two electrons. So, the number of outermost electrons present in the valence shell of element B is calculated as,
Number of valence electrons = $\left( {8 - valency} \right)$
Here, eight represents the total number of electrons that would present in the outermost shell of an atom.
Let us substitute the valency/oxidation state of element B in the expression to calculate the number of valence electrons.
Number of valence electrons = $\left( {8 - 2} \right)$
Number of valence electrons = $6$
The number of valence electrons present in element B is six.
So, the number of valence electrons present in A is five and the number of valence electrons present in B is six.

So, the correct answer is Option B.

Note: Some of the elements exhibit variable valency. We can define variable valency as a state that is generally seen in transition elements. It is found in the second last shell of these elements and is not completely occupied with electrons. We have to know that few elements has variable valency due to differences in electronic configurations.
We could consider the example of iron: Iron reacts with oxygen to give iron (II) oxide and iron (III) oxide. In iron (II) oxide, the oxidation state/valency of iron is +2 and in iron (III) oxide, the oxidation state/valency of iron is +3. We can term as variable valency.