Chemical Bonding
Senior 2 · Chemistry
Chemical Bonding
Lesson 3 of 4
18 min
A blacksmith (umucuzi) heats a bar of iron and hammers it into the blade of a hoe. The iron changes shape but does not shatter. A copper wire carries electricity to a bulb and can be bent round a corner a hundred times. An aluminium saucepan passes the heat of a charcoal stove straight through to the beans.
None of this could happen with ionic or simple molecular substances. Metals are held together by a third kind of bond. By the end of this lesson you will be able to describe metallic bonding and use it to explain the typical properties of metals.
In a piece of metal, the atoms are packed tightly together in a regular pattern. Each atom releases its outer electrons, which no longer belong to any one atom. These delocalised electrons are free to move through the whole structure, like a "sea" of electrons. What remains of each atom is a positive metal ion.
Metallic bonding is the electrostatic attraction between the positive metal ions and the sea of delocalised electrons around them. The attraction acts in all directions throughout the metal, so a metal is a giant metallic lattice.
| Property | Explanation |
|---|---|
| Good electrical conductors, both solid and molten | The delocalised electrons are free to move. When a voltage is applied, they drift through the metal and carry the current. |
| Good conductors of heat | The free electrons carry energy quickly from the hot part to the cold part; the closely packed, vibrating ions also pass energy on. |
| Malleable and ductile | The ions are arranged in layers. A force can make one layer slide over another, but the sea of electrons moves with them and keeps holding the ions together, so the metal changes shape instead of breaking. |
| Usually high melting points | The attraction between the ions and the electron sea is strong, and a lot of energy is needed to overcome it. |
| Shiny when freshly cut | The free electrons reflect light. |
Compare this with an ionic crystal. When a layer of ions in salt slides, positive ions end up facing positive ions and negative ions face negative ions. They repel, and the crystal shatters. In a metal there are no negative ions to line up, only the flexible electron sea.
Sodium melts at 98 °C but magnesium melts at 650 °C. Explain the difference using metallic bonding.
Solution. Each sodium atom gives 1 electron to the sea and becomes Na+. Each magnesium atom gives 2 electrons and becomes Mg2+. So magnesium has ions with a larger charge and twice as many delocalised electrons per ion. The attraction between the ions and the electron sea is stronger in magnesium, so more energy is needed to melt it.
In a pure metal, all the ions are the same size, so the layers slide easily: pure metals are fairly soft. In an alloy, atoms of a different size are mixed in. They disrupt the regular layers and make sliding harder, so the alloy is harder and stronger than the pure metal. Steel (iron with a little carbon) is used for building bars and tools for this reason.
Tip
Three bonds, three pictures. Ionic = transfer (metal + non-metal). Covalent = share (non-metal + non-metal). Metallic = pool (metal atoms only, electrons pooled in a sea).
Exam technique: for any metal property, name the delocalised electrons or the sliding layers of ions. "Metals conduct because they have free electrons" earns the mark; "metals conduct because they are metals" does not.
Try each item before you read the solution.
Both copper and sodium chloride contain charged particles. Why does solid copper conduct electricity when solid sodium chloride does not?
Solution: In copper, the delocalised electrons are free to move through the lattice and carry the current. In solid sodium chloride, the ions are held in fixed positions and cannot move, and there are no free electrons.
Explain why an iron hoe can be hammered into shape, but a lump of rock salt shatters when hit.
Solution: In iron, layers of positive ions slide over each other while the sea of delocalised electrons keeps holding them together, so the metal bends. In rock salt, sliding brings ions of the same charge face to face; they repel and the crystal splits.
Aluminium (Al3+) melts at 660 °C, magnesium at 650 °C and sodium at 98 °C. What pattern links these melting points to the charge on the ions?
Solution: The higher the charge on the metal ion, the more electrons each atom adds to the sea, and the stronger the metallic bonding. Sodium (Na+) has the weakest bonding and the lowest melting point; magnesium (Mg2+) and aluminium (Al3+) have much stronger bonding.
Why is steel, not pure iron, used for building bars?
Solution: Steel is an alloy. The carbon atoms are a different size from iron atoms and disrupt the layers of iron ions, so the layers cannot slide easily. Steel is therefore harder and stronger than pure iron.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
