Gaining an electron
Chlorine does the opposite of sodium. It has a high electronegativity, meaning it pulls hard on electrons, and it needs only one more to fill its outer shell.
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With 17 protons and 17 electrons the chlorine atom is neutral, exactly as the sodium atom was. Taking in one electron gives it 2, 8, 8 and leaves an extra negative charge behind.
Cl + e− → Cl−
A chlorine atom becomes a chloride ion by accepting one electron.
| Cl atom | Cl− ion | |
|---|---|---|
| Protons | 17 (+17) | 17 (+17) |
| Electrons | 17 (−17) | 18 (−18) |
| Configuration | 2, 8, 7 | 2, 8, 8 |
| Overall charge | 0 | −1 |
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Anion
An ion carrying a negative charge is a negative ion, or anion. Note the name change: the atom is chlorine, the ion is chloride.
Ions made of several atoms
A charge does not have to sit on a single atom. Some groups of atoms stay bound together and carry a charge as a unit.
The ammonium ion carries +1 across all five of its atoms; the sulfate ion carries −2 across five; the nitrate ion −1 across four. Each behaves in a compound as a single charged particle.
What an ion is
An atom or a group of atoms carrying an electric charge is an ion. Neutral atoms that release electrons form positive ions; neutral atoms that accept electrons form negative ions.