Among the following metal chlorides, which compound shows the maximum covalent character: MgCl2, FeCl2, SnCl2 or AlCl3?

Introduction / Context:
This chemistry question tests your understanding of how to compare the covalent and ionic character of different metal chlorides using Fajans rules. Being able to predict whether a bond is more ionic or more covalent is important for explaining melting points, solubility, and volatility of inorganic compounds.

Given Data / Assumptions:

• The compounds to compare are MgCl2, FeCl2, SnCl2, AlCl3 and CaCl2.
• All are metal chlorides with chloride ions Cl- as the common anion.
• We assume normal oxidation states: Mg2+, Fe2+, Sn2+, Al3+, Ca2+.
• Covalent character is judged qualitatively using Fajans rules.

Concept / Approach:
Fajans rules state that the covalent character of an ionic bond increases when the cation has high charge and small size, and when the anion is large and easily polarised. A small, highly charged cation strongly polarises the electron cloud of the anion, pulling it towards itself and creating sharing of electrons similar to covalent bonding. Since the anion is the same in all options (chloride), we mainly compare the size and charge of the metal cations to decide which compound is most covalent in nature.

Step-by-Step Solution:
1) List cations: Mg2+, Fe2+, Sn2+, Al3+, Ca2+ all combined with Cl- ions.2) According to Fajans rules, greater positive charge and smaller ionic radius make a cation more polarising.3) Among the given cations, Al3+ has a +3 charge and is quite small in size compared with Mg2+, Fe2+, Sn2+ and Ca2+.4) Because Al3+ is highly polarising, AlCl3 shows strong polarisation of chloride ions and therefore has the maximum covalent character among the listed chlorides.5) The other cations with +2 charge are less polarising, so their chlorides are comparatively more ionic than AlCl3.

Verification / Alternative check:
We can cross check this conclusion with physical properties. AlCl3 has a relatively low melting point and exists as a covalent molecular solid or dimeric Al2Cl6 in the vapour phase. In contrast, typical ionic salts like MgCl2 and CaCl2 have high melting points and exist as extended ionic lattices. This supports the prediction from Fajans rules that AlCl3 has more covalent character than the other metal chlorides listed in the options.

Why Other Options Are Wrong:
MgCl2: Magnesium ion Mg2+ has lower charge and a larger radius than Al3+, so MgCl2 is more ionic and less covalent.
FeCl2: Iron(II) ion Fe2+ has the same charge as Mg2+ but is not as small and highly charged as Al3+, so FeCl2 has less covalent character than AlCl3.
SnCl2: Tin(II) ion Sn2+ is larger and has lower charge density than Al3+, so SnCl2 is less covalent compared to AlCl3.
CaCl2: Calcium ion Ca2+ is a typical alkaline earth metal cation with large size and +2 charge, giving mainly ionic character and therefore not the most covalent among the options.

Common Pitfalls:
Many students look only at the metal being a transition element or a p block metal and ignore the charge and size together. Some think that FeCl2 or SnCl2 must be more covalent because transition metals often form coloured compounds. However, for covalent character prediction Fajans rules focus on charge density, not only on block of the periodic table. Another common error is to confuse covalent character with simple electronegativity difference, but here the more subtle factor is polarising power of the cation since all anions are chloride ions.

Final Answer:
The compound with the maximum covalent character among the given metal chlorides is AlCl3.