Chemical Bonds (C2.1)
Why do atoms bond?
Atoms bond to achieve a full outer electron shell (stable configuration like a noble gas). They do this by transferring or sharing electrons.
Ionic bonding
Ionic bonds form between a metal and a non-metal. The metal atom loses one or more electrons (becomes a positive cation); the non-metal atom gains electrons (becomes a negative anion). Opposite charges attract — electrostatic attraction holds the ions together.
Example: NaCl (sodium chloride)
- Na (2,8,1) loses 1 electron → Na⁺ (2,8) — full outer shell
- Cl (2,8,7) gains 1 electron → Cl⁻ (2,8,8) — full outer shell
Example: MgO (magnesium oxide)
- Mg loses 2 electrons → Mg²⁺
- O gains 2 electrons → O²⁻
Ionic compounds form a giant ionic lattice — millions of ions arranged in a regular 3D pattern. The strong electrostatic forces between all ions explain high melting points.
Covalent bonding
Covalent bonds form between non-metals. Atoms share pairs of electrons. Each shared pair is one covalent bond.
Examples:
- H₂O: oxygen shares 2 electrons (one with each H)
- CO₂: carbon shares 4 electrons with oxygen (two double bonds)
- N₂: nitrogen shares 3 electrons with each other (triple bond, N≡N)
- CH₄ (methane): carbon shares one electron with each of four H atoms
Metallic bonding
In metals, atoms lose their outer electrons into a delocalised sea of electrons shared across all atoms. The positive metal ions are held together by attraction to the delocalised electrons.
This explains:
- High melting points — strong attraction between ions and sea of electrons
- Electrical conductivity — delocalised electrons can flow
- Thermal conductivity — electrons transfer energy
- Malleability — layers of ions can slide without breaking bonds (sea of electrons reforms)
Summary comparison
| Feature | Ionic | Covalent (simple) | Metallic |
|---|---|---|---|
| Between | Metal + non-metal | Non-metals | Metals |
| Transfer or share? | Transfer | Share | Delocalised (share) |
| Conducts electricity? | When molten/dissolved | No | Yes |
| Melting point | High | Low (simple molecules) | High |
Common exam errors
- Saying ionic compounds conduct when solid — they don't (ions fixed in lattice); only when molten or in solution.
- Drawing dot-and-cross diagrams with ions sharing electrons — ionic bonds involve transfer, NOT sharing.
- Forgetting that metallic bonding involves delocalised electrons, not shared pairs.
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