Question

:Three elements X, Y and Z have atomic numbers 7, 8 and 9 respectively. Write the formula of the compound formed when X combines with Z.

Answer 1

To answer this question, you must recall the method to write the electronic configurations of any element. Majority of the physical and chemical properties of an element are determined by their valence electrons.

Complete answer:
To write the electronic configuration of the given elements, we must be familiar with the laws that determine the electronic configuration of an element.
The Hund’s rule of maximum multiplicity states that the pairing of electrons in an orbital does not start until all the orbitals in the sub- shell are singly filled.
The Pauli’s exclusion principle states that any two electrons in the same atom cannot have the exact same quantum number, i.e., each electron has a different set of quantum numbers associated to it.
The Auf- bau principle states that atomic orbitals are filled in the increasing order of the energy level of the orbitals given by $\left( {n + l} \right)$
The electronic configuration of element X is- $X:{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{3}}}$
The electronic configuration of element Y is- $Y:{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^4}$
The electronic configuration of element Z is- $Z:{\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^5}$
From the electronic configurations, the valencies of elements X and Z can be calculated to be 3 and 1 respectively. Thus, the molecular formula for the compound formed by these two elements will be $X{Z_3}$

Note:
It should be noted that the difference between oxidation number and valency is well understood and the two are not confused. Valency of an element gives the combining capacity of the element. For valence electrons less than 4, the valency is equal to the number of valence electrons and for valence electrons more than 4, the valency is given by subtracting the number of electrons from 8.