Simple molecules and covalent bonds
Introduced| English | Meaning |
|---|---|
| covalent bond/ˈkəʊvələnt bɒnd/ | A chemical bond involving a shared pair of electrons attracted to the nuclei of bonded atoms. |
| molecules/ˈmɒlɪkjuːlz/ | Groups of atoms joined by covalent bonds that can exist as discrete particles. |
| simple molecular/ˈsɪmpl məˈlekjʊlə/ | simple molecular |
Sharing, not giving
- Two non-metals can't both grab electrons — so instead they share them.
- Shared pairs of electrons form covalent bonds 共价键, holding atoms together in molecules 分子.
A covalent bond is:
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Covalent bonding shares electrons.
In a covalent bond, atoms ______ a pair of electrons.
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Sharing lets both atoms fill their outer shells.
Covalent bonds
- A covalent bond is a shared pair of electrons between two atoms.
- Sharing lets both atoms reach a full outer shell.
A model of a water molecule: atoms share electrons in covalent bonds
Covalent bonding (sharing)
Step through it: two non-metal atoms overlap and share a pair of electrons — counting for both — so each reaches a full outer shell. That shared pair is the covalent bond; O₂ shares two pairs (a double bond).
Which is a simple molecule?
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CO₂ is a small covalent molecule.
Match each idea about simple molecules.
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Strong covalent bonds hold atoms inside a molecule, but only weak forces act between molecules.
Simple molecules
- Simple molecular 简单分子 substances are small molecules (e.g. H₂O, CO₂, O₂).
- The bonds inside each molecule are strong, but the forces between molecules are weak.
Why a molecule has its shape
Shared electron pairs repel one another and spread out as far apart as they can. Four bonding pairs and no lone pairs gives a tetrahedral shape, like methane (CH4).
Simple molecular substances have low melting points because:
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Only the weak intermolecular forces need breaking.
Simple molecular substances do not conduct electricity because they have:
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No charged particles are free to move.
Their properties
- Low melting and boiling points — only the weak forces between molecules need breaking.
- They don't conduct electricity (no free electrons or ions).
It's the weak forces between molecules that melt easily — not the bonds. The covalent bonds inside a molecule are strong; melting only separates the molecules, breaking the weak forces between them. That's why simple molecules melt at low temperatures.
You've got it
- a covalent bond is a shared pair of electrons (lets atoms fill their outer shells)
- simple molecules have strong internal bonds but weak forces between molecules
- so they have low melting points and don't conduct electricity