Metals and Non-metals
Ionic compounds
Learn how ionic compounds form and why they behave as they do.
"Why do elements react at all?" The noble gases (helium, neon, argon) hardly react. Their outer shell is full: 8 electrons (2 for helium). Other atoms react to reach a full outer shell like them.
Sodium has electrons 2, 8, 1. The easiest way to a full shell is to give away that 1 electron. It becomes Na⁺ (2, 8), a positive ion. Chlorine has 2, 8, 7. It needs one more, so it takes 1 electron and becomes Cl⁻ (2, 8, 8), a negative ion. One gives, one takes.
The opposite charges then attract each other strongly. Sodium chloride is not made of separate molecules: it is a huge lattice of Na⁺ and Cl⁻ ions held together. Compounds made this way, by transferring electrons, are ionic (or electrovalent) compounds.
Magnesium has 2, 8, 2: it gives away two electrons. One chlorine can take only one, so two chlorines are needed, like two friends sharing two rotis, one each. That is why the formula is MgCl₂.
To work out the properties of ionic compounds, keep table salt in your head:
| Property | Why |
|---|---|
| Hard, brittle solids | strong attraction between ions |
| High melting and boiling points (NaCl melts at 1074 K) | a lot of energy is needed to break that attraction |
| Dissolve in water, not in kerosene or petrol | water pulls the ions apart |
| Conduct electricity when dissolved or molten, not as a solid | the ions are free to move only then |
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