1.7.4Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Electronegativity — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 24 of 36 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 6Electronegativity
Electronegativity is a property of atoms in bonds, not an energy measured in kJ mol⁻¹.
Definition
- Definition Electronegativity is the ability of an atom in a covalent bond to attract the bonding pair of electrons towards itself.
Pauling scale
- Pauling scale Values range from 0.7 for caesium to 4.0 for fluorine, which is the most electronegative element.
Across a period
- Across a period It increases because nuclear charge rises and atomic radius falls, so the bonding pair is attracted more strongly.
Down a group
- Down a group It decreases because the bonded atoms are larger and the shared pair is further from the nucleus and more shielded.
Section 2 of 6Bond Polarity
Bond Polarity
Polar bond
- Polar bond If two bonded atoms differ in electronegativity, the pair of electrons is closer to the more electronegative atom.
Partial charges
- Partial charges The atom with the higher value carries δ− and the other atom δ+, which is shown above the atoms in a diagram.
Non-polar bond
- Non-polar bond Atoms with equal or very similar values share electrons evenly, as in Cl–Cl and, almost exactly, in C–H.
Values to know
- Values to know Typical values are H 2.1, C 2.5, N 3.0, O 3.5, F 4.0 and Cl 3.0, with sodium at 0.9 and magnesium at 1.2.
| Element | Na | Mg | H | C | N | Cl | O | F |
|---|---|---|---|---|---|---|---|---|
| Electronegativity | 0.9 | 1.2 | 2.1 | 2.5 | 3.0 | 3.0 | 3.5 | 4.0 |
Section 3 of 6From Bond Polarity to Bonding Type
1
Calculate the difference in electronegativity between the two atoms.
2
A difference of zero or close to zero means a pure or nearly non-polar covalent bond.
3
A moderate difference means a polar covalent bond with partial charges.
4
A very large difference, as in NaCl at 2.1, indicates electron transfer and ionic bonding.
5
Treat the types as a continuum, since there is no sharp border between polar covalent and ionic.
Section 4 of 6Polar Molecules
Polar Molecules
Dipole
- Dipole A molecule has an overall dipole only if the bond polarities do not cancel out because of its shape.
Polar example
- Polar example Water, ammonia and chloromethane are polar because their polar bonds point in different directions without cancelling.
Non-polar example
- Non-polar example CO₂, CCl₄ and BF₃ contain polar bonds but are non-polar overall because of their symmetrical shapes.
Consequences
- Consequences Polar molecules attract each other with permanent dipole forces and are deflected by a charged rod.
Section 5 of 6Worked Example
Question Use Pauling values to compare the polarity of the bonds in H–Cl, C–O and C–H.
- H–Clchlorine 3.0 minus hydrogen 2.1 gives 0.9.
- C–Ooxygen 3.5 minus carbon 2.5 gives 1.0.
- C–Hcarbon 2.5 minus hydrogen 2.1 gives 0.4, so it is almost non-polar.
Answer C–O (1.0) is the most polar, then H–Cl (0.9), with C–H (0.4) the least polar.
Section 6 of 6Exam Checklist & Tips
1.7.4 – 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 in which the electrons are shared unequally.
- Dipole A separation of partial positive and negative charge.
- Pauling scale The most common scale of electronegativity, with fluorine the highest at 4.0.
- Partial charge A fractional charge, written δ+ or δ−, on an atom in a polar bond.
Common Mistakes
- Saying a molecule is polar just because it contains polar bonds, without checking symmetry.
- Writing δ+ on the more electronegative atom.
- Explaining the trend down a group only by increased distance, without shielding.
- Treating electronegativity values as having units.
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