Question

Classify as either mostly ionic or mostly covalent

RbF,CaS,PO,BeTe,BeSe,BeS

Beryllium Sulfide

Hydrogen Chloride

Magnesium Sulfide

Potasium Flouride

Magnresium Selenide

Strontium Telluride

If you could explain how to tell the difference between them(ionic and covalent) too that would be great!

Answer 1

No chemical bond is completely ionic or completely covalent.

An ionic bond is formed by complete transfer of electrons from one atom to the other. However, every ionic bond has some covalent nature which is given by Fajan's Rule: According to Fajan's Rule, the covalent character of ionic compounds depends on 3 factors:

1. Polarising power of the cation: In simple terms it means how well can the cation distort the electron cloud of the anion. The polaizing power is directly proportional to the charge on the cation, higher the charge more strongly can the cation attract the electron cloud of the anion. It also depends inversely on size of the cation. Smaller size means the distribution of charge on the surface of the cation will be more dense and hence higher polarisation.
2. Polarisability of anion: Polarisability of anion simply means how easily does the anion lets the cation distort its electron cloud. This depends directly on size. Larger the size of the anion, more dispersed will be its electron cloud and it will be easier to distort.
3. Pseudo inert gas configuration of cations: Some cations such as Cu⁺ and Ag⁺ show higher polarising power as compared to Na⁺, this is due to 18 electrons in their valence shell which is the electronic configuration of the noble gas in a period above them.

From this we can solve the above question:

The compounds given in the question are RbF, CaS, BeTe, BeSe, BeS, HCl, MgS, KF, MgSe and SrTe (I haven't included PO since such a compound doesn't exist)

Now to tell if a two elecemts will form ionic or covalent bonds we need to consider their nature, metal has a tendency to give electrons and non-metals have a tendency to take electrons, hence if the bond is formed between a metal and non-metal it will primarily be ionic with trasfer of electrons taking place from metal to non-metal. If bond is between is between two non-metals, since both have a tendency to take electrons, they will mutually share the electrons forming a covalent bond.

Lets look at the nature of bonds your question:

1. RbF : Primarily a ionic bond. Also according to Fajan's Rule for higher covalent character we need small cation and large anion unlike the case in RbF. Hence this compound will be mostly ionic.
2. CaS: Primarily an ionic bond. Altough the charge density on cation is high the medium size of anion makes it difficult for polarisation and increased covalent character. Hence this compound is mostly ionic.
3. BeTe: Cation has a high charge density and size of anion is large hence this compound will have more covalent character than ionic.
4. BeSe: Again as above cation has high charge density and anion is large. This compound will also have covalent character but lesser than BeTe.
5. BeS: Altough the size of anion here is not that large, the high charge density of cation causes more covalent chaacter in this compound than ionic.
6. HCl: This compound is primarily covalent however due to difference in electronegativity the bond is polarized and it acts as an ionic compound in aqueous solutions.
7. MgS: The size of the cation and anion are almost the same. This compound is ionic.
8. KF: This compound is ionic since small size of anion does not allow polarisation.
9. MgSe: The charge to size ratio of cation is small and hence polarisation wont be effiecient and this compound will be ionic.
10. SrTe: Due to large size of cation the polarisation will be small and hence this compound will be majorly ionic.