Which of the following pairs of elements has the greatest difference in electronegativity?

Analyze the question: We need the pair with the largest electronegativity difference. Sodium is the same in every pair, so the answer depends on which halogen is most electronegative.

Concept: Electronegativity is an atom's ability to attract shared electrons. Down a group, it decreases because atomic size and shielding increase. Across a period, it increases. The halogens (Group 17) therefore follow this order:

F > Cl > Br > I

Pauling values:

| Element | Electronegativity | |---|---| | Na | 0.93 | | F | 3.98 | | Cl | 3.16 | | Br | 2.96 | | I | 2.66 |

Differences with Na:

• Na and F: 3.98 − 0.93 = 3.05
• Na and Cl: 3.16 − 0.93 = 2.23
• Na and Br: 2.96 − 0.93 = 2.03
• Na and I: 2.66 − 0.93 = 1.73

Answer: Na and F has the greatest difference, so the correct option is A. This is why NaF is the most ionic of these compounds.

Tip: When one element is fixed in every pair, just pick the other element that is farthest away on the scale. Fluorine is the most electronegative element in the periodic table, so any pair with F and a metal will usually give the biggest difference.