CDS Chemistry · Chemical Bonding
Ionic and Covalent Bonds
How ionic bonds (electron transfer) differ from covalent bonds (electron sharing), what makes a compound more ionic, and why plastics last so long.
Why this matters
Two CDS questions: which compound is the most ionic (2024) and why plastics do not degrade (2025). Both turn on knowing what kind of bond holds the substance together.
Concept 1 of 1: Ionic and covalent bonds
Definition
The two bonds:
- Ionic: electrons transferred from a metal to a non-metal (NaCl, K₂S). High melting points; conduct when molten or dissolved.
- Covalent: electrons shared between non-metals (H₂O, CH₄, SiO₂, NCl₃). Usually low melting points (except network solids); do not conduct.
- Ionic character is greatest with a large, singly charged cation (K⁺, Cs⁺) and an electronegative anion.
- A small, highly charged cation (Be²⁺, Al³⁺) distorts the anion's electrons, so its compounds lean covalent (Fajans' rule).
- Plastics are long chains held by strong covalent C–C and C–H bonds that microbes cannot break, so they do not degrade easily.
| Property | Ionic | Covalent |
|---|---|---|
| How the bond forms | Electron transfer | Electron sharing |
| Typical partners | Metal + non-metal | Non-metal + non-metal |
| Melting point | High | Usually low |
| Conducts when molten | Yes | No |
| Example | K₂S (highly ionic) | SiO₂, NCl₃, plasticsQ |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Chemical Bonding · Ionic and Covalent Bonding
A big, singly charged cation gives the most ionic compound
Plastics last because of covalent bonds
SiO₂ is covalent despite its high melting point
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (1)
Ionic and covalent bonds5 rows
| Property | Ionic | Covalent |
|---|---|---|
| How the bond forms | Electron transfer | Electron sharing |
| Typical partners | Metal + non-metal | Non-metal + non-metal |
| Melting point | High | Usually low |
| Conducts when molten | Yes | No |
| Example | K₂S (highly ionic) | SiO₂, NCl₃, plasticsQ |
Watch out for (3)
- A big, singly charged cation gives the most ionic compound→ Ionic and covalent bonds
- Plastics last because of covalent bonds→ Ionic and covalent bonds
- SiO₂ is covalent despite its high melting point→ Ionic and covalent bonds
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.