1.5.2Physical Chemistry
Bond Polarity — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 28 of 102
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 4Electronegativity
Definition
- Definition Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond.
Pauling scale
- Pauling scale Values run up to 4.0 for fluorine, which is the most electronegative element, followed by oxygen, nitrogen and chlorine.
Across a period
- Across a period It increases because nuclear charge rises while shielding stays about the same and the atomic radius gets smaller.
Down a group
- Down a group It decreases because extra shells increase shielding and distance, so the nucleus attracts the bonding pair less.
Common values
- Common values Hydrogen 2.1, carbon 2.5, nitrogen 3.0, chlorine 3.0, oxygen 3.5 and fluorine 4.0 are the values most often used for comparison.
Part 2 of 4Polar Bonds and Molecules
Polar Bonds and Molecules
Polar bond
- Polar bond When the two atoms differ in electronegativity, the shared pair is closer to the more electronegative atom,
- which becomes δ− while the other becomes δ+.
Non-polar bond
- Non-polar bond Bonds between identical atoms, as in Cl₂ and O₂, share electrons equally and have no dipole.
Polar molecule
- Polar molecule A molecule is polar if its dipoles do not cancel because the shape is asymmetric, as in H₂O, NH₃ and CH₃Cl.
Cancelling dipoles
- Cancelling dipoles Symmetrical molecules such as CO₂, BF₃ and CCl₄ contain polar bonds, but the dipoles cancel, so the molecules are non-polar.
| Molecule | Polar bonds? | Molecule polar? |
|---|---|---|
| CCl₄ | Yes | No (symmetrical) |
| CHCl₃ | Yes | Yes |
| CO₂ | Yes | No (linear) |
| H₂O | Yes | Yes |
Part 3 of 4Deciding Polarity
1
Compare the electronegativities of the bonded atoms to see whether a bond dipole exists.
2
Mark the δ+ and δ− ends on each bond, with the more electronegative atom carrying the partial negative charge.
3
Work out the shape of the molecule using electron pair repulsion.
4
Check whether the dipoles are arranged symmetrically and so cancel, or whether they add up to a net dipole.
5
State the conclusion, giving the reason in terms of both the bonds and the shape.
Part 4 of 4Exam Checklist & Tips
1.5.2 – Key Points for the Exam
- Electronegativity The power of an atom to attract the bonding pair of electrons in a covalent bond.
- Dipole A separation of partial positive and negative charge across a bond.
- Polar bond A covalent bond with an uneven distribution of the shared electrons.
- Partial charge A small charge such as δ+ or δ−, less than a full unit.
Common Mistakes
- Assuming a molecule with polar bonds is always polar, which ignores cancellation in symmetrical shapes.
- Saying electronegativity increases down a group, when it decreases.
- Placing the δ+ on the more electronegative atom instead of the δ− .
- Describing the bond in Cl₂ as polar although both atoms are identical.
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