1.7.1Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Evidence of Covalent Bonding — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 22 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.
Part 1 of 5Covalent Bonds
Covalent bonding usually occurs between non-metal atoms, with no complete electron transfer.
Definition
- Definition A covalent bond is the electrostatic attraction between the nuclei of two atoms and a pair of electrons shared between them.
Orbital overlap
- Orbital overlap A bond forms when atomic orbitals overlap to make a molecular orbital with high electron density between the nuclei.
Multiple bonds
- Multiple bonds Double and triple bonds involve two or three shared pairs, bringing the nuclei closer and increasing bond strength.
Dative bonds
- Dative bonds Both electrons of the pair come from one atom, as in NH₄⁺ formed when NH₃ donates its lone pair to H⁺.
Part 2 of 5Experimental Evidence
Experimental Evidence
Electron density maps
- Electron density maps Maps of H₂ or Cl₂ show a high density of electrons between the two nuclei, unlike the near-zero value between ions.
HCl map
- HCl map The contours bunch closer to the chlorine nucleus, showing the shared pair is pulled towards the more electronegative atom.
Bond lengths
- Bond lengths The C–C, C=C and C≡C bond lengths are about 0.154, 0.134 and 0.120 nm, so more shared pairs bring atoms closer.
Bond enthalpies
- Bond enthalpies The same three bonds have average values of 347, 612 and 838 kJ mol⁻¹, showing greater strength with more pairs.
Part 3 of 5Physical Properties of Simple Molecular Substances
1
Covalent substances often exist as discrete molecules such as H₂O, CO₂ and CH₄.
2
The atoms within a molecule are held by strong covalent bonds.
3
The molecules themselves are held together only by weak intermolecular forces.
4
Melting or boiling overcomes only these weak forces, so simple molecular substances usually have low melting and boiling points.
5
They do not conduct electricity because they contain no mobile charged particles.
Part 4 of 5Beyond Simple Molecules
Beyond Simple Molecules
Giant covalent
- Giant covalent Diamond, graphite and silicon dioxide have covalent bonds throughout the structure and so very high melting points.
Solubility
- Solubility Non-polar covalent substances tend to dissolve in non-polar solvents, whereas many polar ones dissolve in water.
Charge clouds
- Charge clouds Shared electrons are better pictured as charge clouds in constant motion than as pairs fixed in position.
Bond polarity
- Bond polarity Unequal sharing arises when the atoms differ in electronegativity, leading to partial charges.
| Bond | Length / nm | Average enthalpy / kJ mol⁻¹ |
|---|---|---|
| C–C | 0.154 | 347 |
| C=C | 0.134 | 612 |
| C≡C | 0.120 | 838 |
Part 5 of 5Exam Checklist & Tips
1.7.1 – Key Points for the Exam
- Covalent bond The electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.
- Molecular orbital The region formed when two atomic orbitals overlap, which can hold the shared pair.
- Dative covalent bond A covalent bond in which both shared electrons come from one atom.
- Bond length The distance between the nuclei of two bonded atoms.
- Bond enthalpy The energy needed to break one mole of a particular covalent bond in gaseous molecules.
Common Mistakes
- Saying that covalent bonds are broken when a molecular substance boils;
- the intermolecular forces are broken instead.
- Drawing the arrow of a dative bond in the wrong direction; it points from donor to acceptor.
- Assuming the shared pair in HCl lies exactly halfway between the two atoms.
- Confusing bond enthalpy with bond length when comparing C–C and C=C.
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