2.2.6Additional lessons
Bond Polarity — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 30 of 172
Part 1 of 5Electronegativity
Unequal sharing of electrons gives a polar bond.
Definition
- The ability of an atom to attract the shared electrons in a covalent bond towards itself.
Across a period
- It increases from left to right because nuclear charge rises while the shielding stays about the same.
Down a group
- It decreases because the bonding electrons are further from the nucleus and more shielded.
Highest value
- Fluorine has the highest electronegativity at 4.0 on the Pauling scale.
Part 2 of 5Polar and Non-Polar Bonds
Polar and Non-Polar Bonds
Equal sharing
- Identical atoms, such as in Cl₂, share electrons equally and the bond is non-polar.
Unequal sharing
- The more electronegative atom attracts the pair more and gains a partial negative charge, δ−.
Partial positive
- The other atom carries δ+, because it is partially stripped of electron density.
Dipole
- A polar bond is drawn with an arrow pointing towards the more electronegative atom.
Part 3 of 5Size of the Difference
1
Subtract the electronegativity values of the two bonded atoms to find the difference.
2
A difference of zero, or close to it, gives a non-polar covalent bond.
3
Larger differences give more polar bonds with larger partial charges.
4
At very large differences the electron is effectively transferred and the bond is described as ionic.
5
Real bonds form a continuum, so there is no sharp boundary between polar covalent and ionic.
| Bond | Electronegativity values | Difference |
|---|---|---|
| H–F | 4.0 and 2.2 | 1.8 |
| H–O | 3.4 and 2.2 | 1.2 |
| H–Cl | 3.2 and 2.2 | 1.0 |
| H–N | 3.0 and 2.2 | 0.8 |
Part 4 of 5Uses of Polarity
Reactivity
- Polar bonds give sites where reagents attack, such as the δ+ carbon in C–Cl.
Intermolecular forces
- Polarity creates the dipole–dipole forces and hydrogen bonds between molecules.
Ranking
- Compare differences to rank bonds, so H–F is the most polar of those in the table.
Part 5 of 5Exam Checklist & Tips
2.2.6 – Key Points for the Exam
- Electronegativity The tendency of an atom to attract bonding electrons.
- Polar bond A covalent bond with unequal sharing of electrons.
- Dipole A separation of partial positive and negative charge.
- Partial charge A charge smaller than a full unit, written δ+ or δ−.
Common Mistakes
- Saying that electrons are transferred completely in a polar covalent bond.
- Drawing the dipole arrow towards the less electronegative atom.
- Using atomic radius instead of electronegativity to judge polarity.
- Believing electronegativity increases down a group.
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