First, you
need to compare the electronegativities of the elements that compose the
molecule.
The ionic or covalent character of a compound depends on how evenly the
electrons are shared and this is determined by the relative electronegativities
of the elements that constitute the compound.
If the difference between the electronegativities is large, then the compounds
will have a greater ionic character.
If the electronegativities are very similar the compound is mainly covalent.
Intermediate differences in their electronegativities, will drive to a mixed
character of the bond.
Electronegativities:
Cl:3.0
Na: 0.9---> Difference 3.0 - 0.9 = 2.1
Mg: 1.2----> Difference 3.0 - 1.2 = 1.8
Al: 1.5 ----> Difference 3.0 - 1.5 => 1.5
Si:1.8 ---> Difference 3.0 - 1.8 => 1.2
Here the
anion is the same (Cl -), so the difference is made by the cation.
You can
eliminate the SiCl4, because the difference in electronegativities between Si
and Cl are so low that it is not considered ionic, but covalent. You can
predict it because both Si and Cl are nonmetal, and the bond between two non
metal compounds is covalent.
Then we
have to analyze NaCl, MgCl2 and AlCl3, which indeed are ionic.
The charge density
of the cation will distort the ionic character of the compound. The higher the
density charge of the cation the larger its polarizing effect (its ability to attract
electron cloud from the anion toward itself and thus make the bond less ionic
and more covalent).
The charge density
is the ratio charge to size.
Then, the
charge density is the determinant factor.
The bigger
the charge of the cation (Na+, Mg2+, Al3+), the larger the charge density. The
smaller the size, the larger the charge density-
Both higher
charge and smaller size of the cation Al3+, leads to its largest polarizing
effect among NaCl, MgCl2 and AlCl3. In fact, that is the order or increasing covalent
character, NaCl is less covalent than MgCl2 and this is less covanlent than
AlCñ3
So you can
predict that SiC4 is a covalent copound, NaCl and MgCl2 are the compounds of the list with the most ionic
character, and AlCl3 is the ionic compound with less ionic or more covalent
character
And the answer
to the question is AlCl3
