2.2.6Additional lessons
Bond Polarity — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 27 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Electronegativity
Different atoms attract shared electrons to different extents.
Definition
- Definition Electronegativity is the ability of an atom to attract the shared electrons in a covalent bond.
Scale
- Scale The Pauling scale gives fluorine the highest value, 4.0, and values generally rise across a period.
Down a group
- Down a group Electronegativity decreases down a group because the bonding pair is further from the nucleus and more shielded.
Data booklet
- Data booklet Values are given in the data booklet, for example H 2.2, C 2.6, N 3.0, O 3.4, F 4.0 and Cl 3.2.
Part 2 of 6Polar and Non-Polar Bonds
Polar and Non-Polar Bonds
Non-polar
- Non-polar If two atoms have equal electronegativity, as in Cl–Cl, the electrons are shared equally.
Polar
- Polar If the electronegativities differ, the shared pair is pulled towards the more electronegative atom, as in H–Cl.
Partial charges
- Partial charges The more electronegative atom carries a partial negative charge δ− and the other atom a partial positive charge δ+.
Dipole
- Dipole The separation of partial charges forms a bond dipole, often drawn as an arrow pointing towards δ−.
| Bond | Electronegativity difference | Type |
|---|---|---|
| H–H | 0 | non-polar |
| C–H | 0.4 | very slightly polar |
| H–Cl | 1.0 | polar |
| H–O | 1.2 | polar |
| H–F | 1.8 | highly polar |
Part 3 of 6The Bonding Continuum
1
Two atoms of identical electronegativity share electrons equally, giving a purely covalent bond.
2
As the electronegativity difference grows, the electron pair is pulled further towards one atom, and the bond becomes more polar.
3
When the difference is very large, the electron is effectively transferred and the bonding is best described as ionic.
4
There is no sharp border between these categories, so bonding is described as a continuum.
5
The greater the difference in electronegativity, the greater the ionic character of the bond.
Part 4 of 6Consequences of Polarity
Reactivity
- Reactivity Polar bonds create positive and negative sites that attract reacting species.
Physical properties
- Physical properties Polar bonds often make a molecule polar, which affects boiling point and solubility in water.
Intermolecular forces
- Intermolecular forces Polar bonds can lead to dipole-dipole attractions and hydrogen bonding between molecules.
Prediction
- Prediction Always identify which atom is more electronegative before labelling δ+ and δ−.
Part 5 of 6Worked Example
Question Using the data booklet values, state which bond is more polar, C–Cl or C–O, and show the direction of the dipole in each.
- ElectronegativitiesC 2.6, Cl 3.2, O 3.4.
- Difference for C–Cl is 3.2 − 2.6 = 0.6, and for C–O is 3.4 − 2.6 = 0.8.
- In both bonds the more electronegative atom (Cl or O) takes δ− and carbon takes δ+.
Answer
- C–O is more polar (0.8 compared with 0.6);
- in each bond the dipole points from carbon towards the more electronegative atom.
Part 6 of 6Exam Checklist & Tips
2.2.6 – Key Points for the Exam
- Electronegativity The ability of an atom to attract the bonding pair of electrons in a covalent bond.
- Polar bond A covalent bond with unequal sharing of electrons due to different electronegativities.
- Dipole A separation of positive and negative charge, shown with δ+ and δ−.
- Ionic character The extent to which electrons in a bond are transferred rather than shared.
Common Mistakes
- Saying that a polar bond means a polar molecule; the shape must also be considered.
- Labelling δ+ on the more electronegative atom.
- Treating electronegativity difference as having an exact cut-off between covalent and ionic.
- Treating C–H as strongly polar; its electronegativity difference is small.
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