Metallic bond
Positive ions in a sea of electrons
1. General properties: mass, radius and metallic property
2. Neighbours, very different
Sodium melts at 98 °C. Magnesium, the very next element, melts at 650 °C.
Pick the right answer
Predict: why does magnesium hold together so much more strongly?
- A. Each magnesium atom gives more electrons to the bond
- B. Magnesium atoms are much heavier
- C. Magnesium atoms share electron pairs, like Cl2
Explanation
Each magnesium atom releases 2 valence electrons into the sea, while each sodium atom releases 1. More free electrons hold the positive ions more strongly, so the metallic bond of magnesium is stronger and it melts at a higher temperature.
3. A sea of electrons
Metallic bond
The attraction between the positive metal ions and the free valence (outer) electrons around them.
Each atom releases its valence electrons. They move freely among the ions and hold them together.
4. More electrons, stronger bond
Choose the number of valence electrons each atom releases. Watch the sea and the melting point.
valence electrons per atom 1
Na melting point 98 °C
Na boiling point 883 °C
5. What the sea explains
The same free electrons explain two familiar metal properties.
- sea of free electrons → free electrons move through the metal
- free electrons move through the metal → conducts electricity
- sea of free electrons → layers of ions slide, still held by the sea
- layers of ions slide, still held by the sea → can be hammered into shape
Conducts electricity
The free electrons can move through the metal and carry the current.
Can be hammered
Layers of ions can slide over each other. The sea of electrons still holds them, so the metal bends instead of breaking.
6. A new pair
Pick the right answer
Potassium (1 valence electron) melts at 63 °C. Calcium (2 valence electrons) melts at 842 °C. Which explains the difference?
- A. Calcium releases more free electrons per atom, so its metallic bond is stronger
- B. Calcium atoms are much heavier
- C. Calcium atoms share electron pairs, like a non-metal
Explanation
Calcium releases 2 free electrons per atom and potassium 1, so the metallic bond of calcium is stronger and its melting point is higher. Their masses (40 and 39) are almost the same, and metal atoms do not share electron pairs.
Fill in the blanks
In a metal, the positive ions are held together by a sea of free _____ electrons.
Explanation
Each metal atom releases its valence (outer) electrons into the sea.
7. Summary
- Metallic bond: positive metal ions held together by a sea of free valence electrons.
- For neighbours such as Na, Mg and Al: more free electrons per atom → stronger bond → higher melting and boiling points.
- The free electrons conduct electricity; sliding layers make metals easy to shape.
- Next in Chapter 1: in transition metals, both 4s and 3d electrons can join this bond.
| Metal | Valence electrons per atom | Melting point | Boiling point |
|---|---|---|---|
| Na | 1 | 98 °C | 883 °C |
| Mg | 2 | 650 °C | 1090 °C |
| Al | 3 | 660 °C | 2519 °C |
Can you explain why magnesium melts at a much higher temperature than sodium?