Question

Electronic configuration of element T is 2,8,7.
What is the valency of element T?
A.1
B.3
C.5
D.7

Answer 1

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 can determine the valency using the octet rule, using periodic tables and also based on the chemical formula.

Complete step by step solution:
We can say valency is the ability of atoms of a single element to react and join with a specific number of atoms of another element.
The given electronic configuration is $2,8,7$. We know that the first shell contains two electrons whereas in the second shell eight electrons could be present, in this second shell two electrons would be present in s orbital and six electrons in p orbital.
The left seven electrons should be present in the third shell. The third shell would contain eighteen electrons. In this, $2$ electrons are present in s orbital and five electrons are present in p orbital, where one electron is unpaired. The element that has seven valence electrons with atomic number seventeen is chlorine and the valency of the element is one as it gains one electron to complete the octet configuration.
Therefore, the option (A) is correct.

Note: The difference between the octet and the number of valence electrons would also give the valency of an element. We know that the valency is the number of electrons that is necessary to finish its octet. So, we can calculate the valency of the element T as,
Valency=$8 - {\text{number of valence electrons}}$
In the given configuration, the number of valence electrons is seven. Let us substitute this in the formula to calculate valency.
Valency=$8 - {\text{number of valence electron}}$
Valency=$8 - 7$
Valency=$1$
The valency of the element T is one.
Therefore, the option (A) is correct.