Ionic or covalent? Ask the electronegativity difference

Back to the tug-of-war. What happens to the electrons depends on the difference between the two atoms' pulls:

• Equal pull (difference 0): electrons shared equally. Non-polar covalent, like $\text{Cl}_2$.
• Somewhat unequal (small difference): still shared, but unevenly. Polar covalent, like $\text{HCl}$.
• Hugely unequal (large difference): the stronger puller takes electrons completely. Ions form, and the bond is ionic, like $\text{NaCl}$.

Rule of thumb: a difference of more than about 1.8 means ionic. Less than that means covalent (polar, unless the difference is 0).

It's a sliding scale, not a cliff. Books draw the line anywhere from about 1.7 to 2.0, so treat a difference close to it as borderline.

The Pauling values are given in the question. You'll be asked to:

• predict whether the bonding between two elements is ionic or covalent;
• explain why one compound is ionic and another is covalent;
• pick the pair of elements that forms the most ionic (or most covalent) bonding.

1. Find the two elements joined by the bond, and write down their Pauling values.
2. Subtract: bigger value minus smaller value.
3. Ignore the subscripts in the formula: the difference is per bond, so $\text{CaCl}_2$ uses 3.2 − 1.0, never 2 × 3.2.
4. Place the difference on the scale: 0 is non-polar covalent, small is polar covalent, above about 1.8 is ionic.
5. For "most ionic", pick the largest difference.
6. In a written answer, quote both values and the difference, then link it: "large difference, so electrons transferred, so ionic".