In which of the following pairs of compounds are both members of the pair ionic compounds? Explain

Question

a. Pl3 and HBr
b. H2S and NCl3
c. LiF and H20
d. LiI and NiO

DrBob222 you said you would choose the answer d and I should know how to figure this out. I don't understand the pair ionic compounds. Can you please explain it to me in lay-man terms. I'm slooow
Thanks

Answer 1

The idea of predicting which is ionic, which covalent, which polar covalent depends upon some man made definitions and how to interpret them.

Look up the value for the electronegativity (EN) for each element. If the difference is about 1.9 or so it is about 50% covalent/50% ionic bond. Generally, if it is more than 1.9 we call it an ionic bond; if less than 1.9 we call it a covalent bond. This number of 1.9 varies with instructors so you should know what your prof uses.(I know some use 1.7.) Also, some profs like to point out that a difference of zero is a pure covalent bond(H2, O2, N2 are example) while values between zero and 1.9 are not strictly covalent but not ionic either so those are called polar covalent. To show the examples above, here are the EN values of some of them.
P = 2.19
I = 2.66
so PI3 is not ionic so I need not even worry about HBr; however, to show it is covalent too H = 2.1 and Br = 2.96 diff = <1

b. Both are covalent. You can go through the drill to confirm that.

c. Li is about 1; F is 4.0; difference is 3 so it clearly is ionic. H2O, however, is not. H = 2.1; O = about 3.5 for diff of about 1.4 or so. This is not ionic but it is polar covalent.

d. Li = about 1
I = about 2.66
diff = about 1.7

Ni = about 1.9
O = about 3.5
diff = about 1.4

Strictly speaking none of these meet the criteria of BOTH being ionic by the criteria I use. I looked at them and knew the answer couldn't be a,b,or c so I figures d without looking up the values. You need to take the above information, use the tables your prof is using as well as his/her definition of ionic/covalent bond (the difference of EN values) and re-evaluate each based on the values your prof used.
Note: If your prof uses 1.7 and the EN values in his/her table varies a little (Li = 1 and I = 2.7 diff = 1.7
Ni = 1.8 and 3.5 diff = 1.7
that would make d the answer.