Compounds that carry charge
In a circuit, salt solution lights a bulb. Dry salt does not.
2, 8, 1 counts the electrons in each shell.
- Sodium: 2, 8, 1. Chlorine: 2, 8, 7.
- Sodium's lone outer electron moves to chlorine.
- Sodium becomes Na⁺. Chlorine becomes Cl⁻.
- Opposite charges attract: sodium chloride forms.
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The rule
A metal gives electrons to a non-metal. Both become charged ions, and opposite charges attract. The result is an ionic compound. Ionic compounds conduct when molten or dissolved, not when solid.
An ion is a charged atom. Sodium loses one electron and becomes Na⁺. Chlorine gains one and becomes Cl⁻. Electrons carry negative charge. So an atom that loses one becomes positive. An atom that gains one becomes negative.
| Property | Ionic compound |
|---|---|
| State at room temperature | Hard, brittle solid |
| Melting and boiling points | High |
| Dissolves in water | Usually yes |
| Conducts current | When molten or dissolved, not when solid |
Worked example
In a solid the ions are locked in a fixed pattern. They cannot move, so no current flows. A current needs moving charges. Melt the salt or dissolve it. The ions are free to move, and the bulb lights.
Sugar dissolves in water but its solution does not conduct. Sugar has no ions.