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💡 Different-element partners
When atoms of different elements share electrons — hydrogen with chlorine, or hydrogen with oxygen — they build a molecule of a compound.
How does a water molecule form?
- Oxygen needs 2 electrons; each hydrogen needs 1 .
- Two hydrogen atoms each share an electron with one oxygen atom .
- This gives the water molecule, written H₂O (2 hydrogen + 1 oxygen).
In hydrogen chloride, hydrogen and chlorine each need one electron, so they share one pair and bond as H—Cl — a covalent compound.
⏸▶ Pause and Ponder
- 10. Show the formation of the following molecules: (i) Carbon dioxide (CO₂) (ii) Hydrogen sulfide (H₂S) (iii) Ammonia (NH₃)
- (i) CO₂ — C is 2, 4 (needs 4); each O is 2, 6 (needs 2). Carbon shares two pairs with each oxygen → two double bonds , O=C=O.
- (ii) H₂S — S is 2, 8, 6 (needs 2); each H needs 1. Sulfur shares one pair with each hydrogen → H—S—H.
- (iii) NH₃ — N is 2, 5 (needs 3); each H needs 1. Nitrogen shares one pair with each of three hydrogens → three single bonds .
- 11. Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain. → Neon is 2, 8 — its valence shell already has a complete octet . It is stable, so it has no need to lose, gain or share electrons.
Important Definitions
- Molecule of a compound — a molecule made of atoms of different elements, e.g. H₂O, HCl.
