2.2.1Additional lessons
Covalent Bonds — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 25 of 172
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.
Section 1 of 5What a Covalent Bond Is
Covalent bonding is the sharing of electrons rather than their transfer.
Shared pair
- Shared pair Two atoms each contribute an electron, and the pair is shared between their nuclei.
Electrostatic attraction
- Electrostatic attraction The bond is the attraction of both positive nuclei to the negatively charged shared electron pair.
Non-metals
- Non-metals Covalent bonding normally occurs between atoms of non-metallic elements, which both need electrons.
Octet rule
- Octet rule Atoms of period 2 elements usually share electrons until each has eight in its outer shell.
Section 2 of 5Balance of Forces
1
As two atoms approach, each nucleus begins to attract the electrons of the other atom.
2
The energy of the system falls as attraction increases and the atoms move closer together.
3
At very short distances the nuclei and the electron clouds repel each other strongly.
4
The bond length is the separation at which attraction and repulsion balance and the energy is lowest.
5
Supplying energy equal to the bond enthalpy pulls the atoms apart again.
Section 3 of 5Bond Length and Bond Enthalpy
Bond Length and Bond Enthalpy
Bond length
- Bond length The distance between the nuclei of two bonded atoms, usually quoted in picometres.
Bond enthalpy
- Bond enthalpy The energy needed to break one mole of a given bond in the gaseous state.
Relationship
- Relationship Shorter bonds between the same pair of atoms are generally stronger and have higher bond enthalpies.
Atom size
- Atom size Larger atoms form longer, weaker bonds, so bond enthalpy falls down a group.
| Bond | Bond enthalpy / kJ mol⁻¹ |
|---|---|
| H–H | 436 |
| Cl–Cl | 242 |
Section 4 of 5Molecular Formulae and Examples
Hydrogen
- Hydrogen Two H atoms share one pair, giving H₂ with a full first shell of two electrons.
Chlorine
- Chlorine Two Cl atoms share one pair, so each reaches eight outer electrons in Cl₂.
Water
- Water Oxygen shares one pair with each of two hydrogen atoms in H₂O.
Methane
- Methane Carbon shares four pairs with four hydrogen atoms in CH₄.
Section 5 of 5Exam Checklist & Tips
2.2.1 – Key Points for the Exam
- Covalent bond The electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
- Bond length The distance between the nuclei of two covalently bonded atoms.
- Bond enthalpy Energy needed to break one mole of a bond in gaseous molecules.
- Octet rule The tendency of atoms to have eight electrons in their outer shell.
Common Mistakes
- Saying the bond is the shared electrons alone; it is their attraction to both nuclei.
- Thinking that covalent bonds form only between identical atoms.
- Believing that breaking bonds releases energy; it requires energy.
- Confusing bond length with atomic radius.
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