Why atoms join: ions and the ionic bond
Sodium chloride is a solid and hydrogen chloride is a gas, though both contain chlorine. How the atoms are bonded decides the properties. Atoms tend to reach the stable electron arrangement of the nearest noble gas: a full outer level, with 2 electrons for helium and 8 for the others. In an ionic bond, a metal atom gives electrons to a nonmetal atom. A sodium atom (11 protons, 11 electrons) loses its one outer electron and becomes Na⁺ (11 protons, 10 electrons), a positive ion called a cation. A chlorine atom (17 protons, 17 electrons) gains that electron and becomes Cl⁻ (17 protons, 18 electrons), a negative ion called an anion. Opposite charges attract, and this attraction is the ionic bond. The compound is neutral overall, because the charges cancel. Sodium chloride, NaCl, is a compound of sodium ions and chloride ions packed in a crystal, not a molecule.
| Particle | Protons | Electrons | Charge |
|---|---|---|---|
| Na | 11 | 11 | 0 |
| Na⁺ | 11 | 10 | +1 |
| Cl | 17 | 17 | 0 |
| Cl⁻ | 17 | 18 | −1 |
Hand-over game. One child is a metal atom holding a ball that stands for its outer electron. Another child is a nonmetal atom who needs one more. The first hands over the ball. Stick a “+” label on the first child and a “−” label on the second. Now they stand together as a pair that attract. Repeat with a drawing: draw Na and Cl with the electron moving, then draw the ions.