A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Theory and principles
CSCA Chemistry · Lesson 13 of 21Chemical bonds and intermolecular forces
Liaisons chimiques et forces intermoléculaires · الروابط الكيميائية والقوى بين الجزيئات
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Atoms bond to reach a stable arrangement of electrons. The type of bond, and the forces between molecules, decide melting points and solubility.
Key ideas
- Ionic bond: electrons transferred from a metal to a non-metal (NaCl).
- Covalent bond: electron pairs shared between non-metals (H₂O, CO₂); polar if the atoms differ in electronegativity.
- Metallic bond: metal ions in a sea of electrons, so metals conduct.
- Between molecules: van der Waals forces and hydrogen bonds (H bonded to N, O or F).
Formulas to know
- Ionic compounds: high melting points, conduct when molten or dissolved
- Hydrogen bonds: H₂O, NH₃ and HF boil unusually high
- 'Like dissolves like': polar dissolves in polar
Worked example
Why does water (H₂O) boil at 100 °C while H₂S boils at about −60 °C?
- Compare: both are small, bent, polar molecules
- Difference: O is very electronegative → hydrogen bonds between H₂O molecules
- Result: more energy is needed to separate H₂O molecules
Answer: Hydrogen bonds between water molecules
Practice questions
1. Which compound is ionic?
CO₂ · CH₄ · MgO ✓ · HCl
Metal + non-metal.
2. How many shared electron pairs are in N₂?
1 · 2 · 3 ✓ · 4
N≡N is a triple bond.
3. Which forms hydrogen bonds between its molecules?
CH₄ · NH₃ ✓ · H₂S · CO₂
H is bonded to N.