If X represents an element of atomic number 9 and Y an element of atomic number 20, the compound formed by these elements would be:
A. ionic with formula $Y{{X}_{2}}$
B. covalent with formula $Y{{X}_{2}}$
C. ionic with formula YX
D. covalent with formula YX

Hint: The electron configuration is the term coined to the distribution of atom or molecular electrons in atomic or molecular orbitals. A compound in form when there is a bond between them, it can be ionic or covalent bond.

Complete answer: As given in question, if X represents an element of atomic number 9 and Y an element of atomic number 20, the compound formed by these elements would be:
Let us say that the atomic number is represented by Z.
Z = 9 as given in question and it is represented by X.
Electronic configuration of X is $1{{s}^{2}}2{{s}^{2}}2{{p}^{5}}$, therefore element X needs one electron to complete its octet.
Z = 20 as given in question and it is represented by Y.
Electronic configuration of Y is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}$ , therefore element Y can lose two electrons to achieve its octet.
Hence, element X needs one electron to complete its octet whereas element Y can lose two electrons to achieve its octet so they will form a compound with formula $Y{{X}_{2}}$ .
According to the periodic table, element X having atomic number 9 is Fluorine (F) whereas element Y having atomic number 20 is Calcium (Ca).
Bonds in $Ca{{F}_{2}}$ have about 89% ionic character. The ionic character of a bond is a function of electronegativity difference. The greater the difference in electronegativities the more ionic is the bond. Fluorine is the most electronegative element, and has the greatest attraction for the shared pairs of electrons. Therefore, the bonds are highly polar and the charges on Ca and F in $Ca{{F}_{2}}$ are approaching their hypothetical limits, +2 and -1.
Hence, if X represents an element of atomic number 9 and Y an element of atomic number 20, the compound formed by these elements would be ionic with formula $Y{{X}_{2}}$.
So, the correct answer is “Option A”.

Note: Always remember, the ionic character of a bond is a function of electronegativity difference. The greater the difference in electronegativities the more ionic is the bond.
A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding.