The Ionic Bond
Electrostatic Attraction, Electronegativity Differences, and Formula Deduction.
1. The Definitive Definition
Examiner Requirement
"The electrostatic attraction between oppositely charged ions."
2. The Bonding Continuum
Bonding is rarely "100% Ionic." It exists on a spectrum governed by the Difference in Electronegativity (\(\Delta\chi\)).
Covalent
\(\Delta\chi < 1.8\)
Electron Sharing
Ionic
\(\Delta\chi > 1.8\)
Electron Transfer
Common Pitfall: Aluminum Chloride (\(AlCl_3\))
Often assumed to be ionic (Metal + Non-metal). However, its \(\Delta\chi\) is only ≈ 1.5. It acts as a covalent molecule, subliming at low temperatures.
Reason: The high charge density of \(Al^{3+}\) polarizes the \(Cl^-\) cloud (Fajans' Rules).
3. Formula Deduction
Worked Example: The Cross-Over Method
Problem: Deduce the formula for Calcium Phosphate given ions: \(Ca^{2+}\) and \(PO_4^{3-}\)
1. Cross the charges to the opposite subscript:
- Ca takes the 3.
- PO₄ takes the 2.
2. Result:
\( Ca_3(PO_4)_2 \)
Check Your Understanding
Q1. Define the Ionic Bond.
Q2. Why does AlCl₃ sublime at low temperatures?