Final answer:
CsCl is the most ionic compound among LiCl, NaCl, CsCl, and AlCl3 because it has the largest difference in electronegativity between the bonding atoms, which is indicative of a stronger ionic bond.
Step-by-step explanation:
To determine which of the compounds LiCl, NaCl, CsCl, and AlCl3 is the most ionic, you must consider the differences in electronegativity between the cations and anions. Ionic character increases with an increase in the difference in electronegativity between the bonded atoms. Hence, you need to look for the compound with the largest difference in electronegativity.
Lithium (Li) has an electronegativity value of 1.0, sodium (Na) 0.9, cesium (Cs) 0.7, and aluminum (Al) 1.5. Chlorine (Cl) has an electronegativity value of 3.0. Therefore, the differences in electronegativity are 2.0 for LiCl, 2.1 for NaCl, 2.3 for CsCl, and 1.5 for AlCl3.
With these values in mind, CsCl has the largest difference in electronegativity, thus it can be inferred that CsCl is the most ionic out of the listed compounds. Additionally, since cesium is larger than lithium and sodium, the outermost electron is further away from the nucleus and more readily transferred to chlorine, making the ionic bond stronger in CsCl compared to LiCl and NaCl.