Which of these compounds has the greatest ionic character?

Step 1: Analyze the question We need the compound with the greatest ionic character. Ionic character depends on the electronegativity difference between the bonded atoms. The larger the difference, the more ionic the bond.

Step 2: Concept (Fajans' rules)

• A large electronegativity difference gives a more ionic bond.
• A large, weakly polarising cation (low charge density) gives less covalent distortion of the anion, so the bond is more ionic.
• A small, highly charged cation (like Al³⁺ or B³⁺) polarises the anion strongly, which gives more covalent character.

Step 3: Compare the options All the compounds contain fluorine, the most electronegative element (4.0). So we look at the other element:

| Compound | Other element | Electronegativity | Difference with F | |---|---|---|---| | AlF₃ | Al | 1.5 | 2.5 | | BF₂ (as written) | B | 2.0 | 2.0 | | CF₄ | C | 2.5 | 1.5 (covalent molecule) | | CsF | Cs | ~0.7 | ~3.3 | | NaF | Na | 0.9 | 3.1 |

Cesium is the least electronegative element in the list (it is far down Group 1), so CsF has the largest electronegativity difference. Cs⁺ is also very large with low charge density, so it hardly polarises F⁻. This makes CsF the most ionic.

Step 4: Answer CsF has the greatest ionic character.

Tip: For ionic character, pick the metal furthest down and to the left of the periodic table (Group 1, bottom) paired with the most electronegative nonmetal (F or O). Cs and F are the extreme pair. "Cs + F = most ionic."