Electronegativity is one of the most important concepts in chemistry because it determines how atoms share electrons in chemical bonds. For IB Chemistry students, understanding electronegativity makes topics like molecular structure, intermolecular forces, and reaction mechanisms far easier. This article explains exactly how electronegativity affects bond polarity and how you can apply it confidently in your exams.
What Is Electronegativity?
Electronegativity is the ability of an atom to attract electrons in a chemical bond.
The most electronegative element is fluorine, assigned a value of 4.0 on the Pauling scale. Generally:
- Electronegativity increases across a period (left → right)
- Electronegativity decreases down a group
These trends arise from changes in nuclear charge and atomic radius, two concepts that strongly impact how electrons are pulled within a bond.
What Is Bond Polarity?
Bond polarity refers to how unevenly electrons are shared between two bonded atoms.
If one atom attracts electrons more strongly, the bond becomes polar.
A polar bond has:
- A partial negative charge on the more electronegative atom (δ–)
- A partial positive charge on the less electronegative atom (δ+)
A non-polar bond forms when atoms attract electrons equally.
How Electronegativity Determines Polarity
Bond polarity depends on the difference in electronegativity (ΔEN) between the two atoms.
General rule:
- ΔEN = 0 → non-polar covalent
- ΔEN small (≈ 0.1–1.7) → polar covalent
- ΔEN large (> 1.7) → ionic bond (electrons transferred rather than shared)
For example:
- H and Cl: ΔEN ≈ 0.9 → polar covalent
- C and H: ΔEN ≈ 0.4 → mostly non-polar
- Na and Cl: ΔEN ≈ 2.1 → ionic
These values aren’t strict boundaries but helpful guidelines that appear frequently in IB exam questions.
Why Polarity Matters
Polarity affects almost every major chemistry topic:
- Intermolecular forces: Polar molecules attract via dipole–dipole forces.
- Solubility: “Like dissolves like” — polar dissolves polar.
- Boiling points: Polar compounds show stronger forces and higher boiling points.
- Reactivity: Bond polarity influences reaction mechanisms, such as nucleophilic attack.
- Molecular shape and dipole moment: VSEPR and polarity work hand-in-hand.
This is why examiners test electronegativity so frequently—it links directly to molecular behaviour.
Example: Which Bond Is More Polar?
Consider the following bonds:
- C–O
- C–Cl
- C–H
Electronegativity values (Pauling scale):
- C = 2.5
- O = 3.5
- Cl = 3.0
- H = 2.1
Calculate ΔEN:
- C–O: 3.5 – 2.5 = 1.0
- C–Cl: 3.0 – 2.5 = 0.5
- C–H: 2.5 – 2.1 = 0.4
Ranking (most → least polar): C–O > C–Cl > C–H
This type of ranking question appears constantly in IB Paper 1 and Paper 2.
Common IB Mistakes to Avoid
- Forgetting molecular geometry: A molecule can contain polar bonds but still be non-polar if the shape is symmetrical (e.g., CO₂).
- Memorizing instead of reasoning: Use ΔEN to justify polarity—it earns marks on structured questions.
- Confusing bond polarity with molecular polarity: They are related but not identical.
- Assuming ionic/covalent boundaries are rigid: The ΔEN values give trends, not absolute rules.
FAQs
Why do noble gases have no electronegativity values?
Most noble gases do not form stable compounds, so they do not regularly share electrons in bonds. If bonding does occur, it is rare, and therefore assigning electronegativity values is not meaningful for most cases.
Can a molecule with polar bonds be non-polar overall?
Yes. If bond dipoles cancel because of symmetrical geometry (like in CO₂ or CCl₄), the molecule becomes non-polar. IB Chemistry often tests this by combining electronegativity knowledge with VSEPR shapes.
Do metals have low electronegativity?
Generally, yes. Metals have low electronegativity because they lose electrons easily, forming cations. This is why metal–non-metal pairs typically form ionic bonds with large electronegativity differences.
Conclusion
Electronegativity governs how electrons are shared in a bond, and understanding this concept allows you to determine whether a bond is polar or non-polar. This skill is foundational for mastering intermolecular forces, molecular geometry, solubility, and reaction mechanisms. By focusing on electronegativity differences and practicing comparisons, you’ll strengthen your understanding across multiple IB Chemistry topics.
