- Covalent bonds
- Properties of metals · coming soon
The same salt, two different answers
Connect a battery, a small bulb and two metal rods into a circuit, and push the rods into a pile of dry salt crystals. The bulb stays dark: solid salt does not conduct electricity. Now dissolve the same salt in water and dip the rods in. The bulb lights. Melt the salt instead, at over 800 °C, and the bulb lights again.
The ions are the same in all three cases. Sodium chloride is made of Na⁺ and Cl⁻ ions whether it is solid, melted or dissolved. So why does it conduct only sometimes?
The last lecture promised that every property of an ionic compound follows from one picture: the giant lattice. This lecture works through them one by one: high melting points, conductivity, brittleness and dissolving. Each one is a short argument from the structure. Learn the arguments, not a list of facts.
Property 1: high melting points
In a giant ionic lattice, every ion is attracted to all its neighbours of opposite charge, in every direction. To melt the solid, the ions must be shaken loose from their places so that they can move past each other. That means overcoming a huge number of strong electrostatic attractions, which needs a lot of energy, and so a high temperature.
The last lecture's data show the pattern: sodium chloride melts at 801 °C, sodium fluoride at 996 °C, and magnesium oxide, with its doubly charged ions, at about 2800 °C. Compare ice, made of separate water molecules, which melts at 0 °C. Ionic compounds are almost all solids at room temperature, for this reason.
Property 2: conducting electricity, but only sometimes
An electric current is a flow of charge. For a substance to conduct, it must contain charged particles that are free to move. Charge alone is not enough. Movement is the point.
- Solid. The ions are held in fixed positions in the lattice. They vibrate, but they cannot travel. No charge flows. No conduction.
- Molten. Above the melting point, the lattice has broken down and the ions can move. Connect a battery, and the positive ions drift towards the negative electrode while the negative ions drift towards the positive electrode. Charge flows. Conducts.
- Dissolved in water. The lattice has come apart and the ions spread through the water. They are free to move in the same way. Conducts.
That is it. That is the whole idea. It is not the compound that conducts, but its free ions. Lock the ions in place, and conduction stops; free them, and it starts.
The negative electrode is called the cathode and the positive electrode the anode. When ions reach the electrodes, they can be changed into new substances. Using electricity to split a compound this way is called electrolysis, and it has its own lectures later.
Solid sodium chloride is packed with charged particles. So why isn't it at least a weak conductor?
Show the answer
Because a current needs charges that travel through the material, from one electrode towards the other. In the solid, every ion only vibrates about a fixed point. There is no overall movement of charge in any direction, so there is no current at all. Metals are different: they conduct as solids because they contain electrons that are free to move, which a later lecture explains.
This property made history. In 1807, Humphry Davy passed a current through melted caustic potash (potassium hydroxide) and saw globules of a new metal, potassium, form at the negative electrode. A few days later, he obtained sodium in the same way from caustic soda (sodium hydroxide). Both metals react so strongly that ordinary chemical methods had never freed them. Sodium is still made by electrolysis today: in the Downs cell, molten sodium chloride is electrolysed, with calcium chloride added to lower the melting point so that the cell can run at about 600 °C.
Property 3: hard but brittle
Ionic crystals are hard: it takes a large force to dent them. But hit a salt crystal sharply, and it does not bend. It shatters, often along flat faces. Why?
In the lattice, every ion has neighbours of the opposite charge. Now imagine a blow that pushes one layer of ions along by a little. Try it below.
Move the slider to slide the top layer of ions along. Watch which ions end up facing each other, and whether the two layers attract or repel.
A simple model: one row of ions above another, the rows one ion spacing apart. The force between the rows depends on how far the top row is shifted:
| Shift (ion spacings) | Facing ions | Between the layers |
|---|---|---|
| 0 | opposite charges | attraction (100%) |
| 0.25 | mostly opposite | attraction (76%) |
| 0.5 | halfway | none overall |
| 0.75 | mostly the same | repulsion (76%) |
| 1 | the same charges | repulsion (100%) |
Shift the layer by one ion, and attraction turns into an equally strong repulsion. The layers push apart, and the crystal splits.
Did you see the switch? Slide the layer by just one ion, and every positive ion faces a positive ion and every negative faces a negative. The attraction that held the layers together becomes repulsion, and the crystal splits along that plane. A metal behaves differently: its layers can slide over each other without bringing like charges together, so it bends instead of breaking. The lecture on the properties of metals explains why.
Property 4: dissolving in water, but not in everything
Many ionic compounds dissolve in water. Few dissolve in liquids such as hexane, a part of petrol. The difference lies in the solvent's molecules.
A water molecule is slightly charged at each end. The oxygen end carries a small negative charge, written δ−, and the hydrogen end a small positive charge, δ+. (The lecture on electronegativity explained why: oxygen pulls the shared electrons towards itself.) When salt meets water, water molecules gather round the ions at the edge of the crystal. Their δ− ends point at the Na⁺ ions, and their δ+ ends point at the Cl⁻ ions. These attractions between ions and the charged ends of water molecules are called ion–dipole attractions.
The energy released as water molecules surround each ion helps to pay for pulling the ions out of the lattice. One by one, the ions leave the crystal, each wrapped in a shell of water molecules. They are now hydrated ions, shown by the state symbol (aq):
Hexane molecules have no charged ends, so they cannot attract the ions, and nothing pays for breaking up the lattice. Salt stays undissolved in hexane. Chemists sum this up as "like dissolves like": charged and polar substances dissolve in polar solvents such as water.
But not every ionic compound dissolves, even in water. Whether it does depends on a contest: the lattice energy holding the ions together against the energy released by hydrating them. When the lattice is very strong, the lattice can win. Magnesium oxide, with its doubly charged ions, hardly dissolves at all, and neither do calcium carbonate (chalk and limestone) or silver chloride.
Worked examples
Explain why solid sodium chloride does not conduct electricity, but molten sodium chloride does.
A current needs charged particles that are free to move. In solid sodium chloride, the Na⁺ and Cl⁻ ions are held in fixed positions in the giant lattice, so they cannot move and no charge flows.
When it melts, above 801 °C, the lattice breaks down and the ions can move. With a voltage applied, Na⁺ ions move towards the negative electrode and Cl⁻ ions towards the positive electrode, so charge flows.
Notice the key words an answer needs: ions (not electrons), fixed in the solid, free to move in the liquid.
Write an equation, with state symbols, for calcium chloride dissolving in water. How many ions does one formula unit release?
Three ions: one Ca²⁺ and two Cl⁻, each surrounded by water molecules. Because it releases more ions, and one of them doubly charged, a calcium chloride solution usually conducts better than a sodium chloride solution containing the same number of formula units.
Pure sodium chloride melts at 801 °C. The Downs cell adds calcium chloride so that the mixture is molten at about 600 °C. Why does a lower operating temperature help?
Electrolysis needs the ions free to move, so the salt must be molten. Keeping a large cell molten at 801 °C costs a great deal of energy. A mixture of salts melts at a lower temperature than either pure salt, so adding calcium chloride lets the cell stay molten, and conducting, while using less energy to keep it hot.
Now decide which of these conduct.
For each one, decide whether it conducts electricity. Ask: are there charged particles that are free to move?
Each case and whether it conducts:
| Case | Conducts? |
|---|---|
| Solid sodium chloride | no: ions fixed |
| Molten sodium chloride | yes: free ions |
| Sodium chloride dissolved in water | yes: free ions |
| Solid magnesium oxide | no: ions fixed |
| Magnesium oxide heated above about 2800 °C | yes: molten, free ions |
| Calcium chloride dissolved in water | yes: free ions |
| Sugar dissolved in water | no: molecules, no ions |
| Sodium chloride stirred into hexane | no: it does not dissolve |
Three classic mistakes
In molten or dissolved ionic compounds, the current is carried by moving ions. Free electrons carry current in metals. Write "ions are free to move", never "electrons are free to move", for an ionic compound.
They are full of charges. The charges simply cannot move. The reason is fixed ions, not missing ones.
Many do, but not all. Magnesium oxide, calcium carbonate and silver chloride barely dissolve, because their lattices hold together more strongly than water can pull them apart.
Work each one on paper first. Open a hint only when you are stuck.
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Potassium bromide is an ionic compound. Predict whether it conducts electricity (a) as a solid, (b) when molten, (c) dissolved in water. Explain each.
Hint 1
Ask in each case: are there charged particles free to move?
Hint 2
The ions are K⁺ and Br⁻.
Full answer
(a) No: the K⁺ and Br⁻ ions are fixed in the lattice. (b) Yes: the lattice breaks down and the ions are free to move. (c) Yes: the ions leave the lattice and move freely in the water.
-
Write an equation, with state symbols, for magnesium chloride dissolving in water. How many ions are released per formula unit?
Hint 1
The ions are Mg²⁺ and Cl⁻. The formula is MgCl₂.
Hint 2
Follow Worked Example 2.
Full answer
. Three ions per formula unit. Check: the charges on the right add to , like the solid.
-
A white solid melts at 770 °C. As a solid it does not conduct, but when molten it does. Is it more likely to be ionic or made of small molecules? Explain using both observations.
Hint 1
What kind of substance has a high melting point?
Hint 2
What does conducting only when molten tell you about the particles?
Full answer
Ionic. The high melting point suggests strong forces throughout a giant structure, like a lattice. Conducting only when molten shows it contains charged particles that are fixed in the solid but free in the liquid: ions. A substance of small molecules would melt at a low temperature and would not conduct even when molten.
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A student hits a crystal of calcium fluoride, CaF₂, with a hammer, and it splits cleanly along a flat face. Another student hammers a piece of copper, and it flattens without breaking. Explain the calcium fluoride result in terms of its ions, and say what this suggests about how the copper's particles must be arranged differently.
Hint 1
What happens to the charges when one layer of ions slides past another?
Hint 2
If copper does not split, sliding its layers must not create strong repulsion.
Full answer
In calcium fluoride, the blow shifts one layer of ions against the next. Ions of the same charge come face to face (Ca²⁺ against Ca²⁺, F⁻ against F⁻), so the layers repel and the crystal splits along that plane. Copper's layers can slide without this repulsion, so its particles cannot be held together by alternating positive and negative ions in the same way. The lecture on metallic bonding shows the arrangement: positive ions in a sea of moving electrons, which hold the layers together however they slide.
Look back over this topic. One idea, electrons moving from a metal to a non-metal, led to ions. Ions led to a lattice. And the lattice explains everything an ionic compound does: why salt is a hard, high-melting solid, why it conducts only when molten or dissolved, why it shatters, and why it dissolves in water but not in petrol.
This is the pattern for all of chemical bonding: structure explains properties. The next topic starts with a different way of holding atoms together: sharing electrons instead of transferring them. You will see the same pattern give a completely different set of properties.
- Ionic compounds have high melting points because many strong electrostatic attractions act in every direction throughout the giant lattice.
- Ionic compounds conduct electricity only when molten or dissolved, because only then are their ions free to move.
- Ionic crystals are brittle because a shift of one layer brings ions of the same charge face to face, so the layers repel and split apart.
- Many ionic compounds dissolve in water because ion–dipole attractions to water molecules release energy that helps to break up the lattice.
- Not all ionic compounds dissolve, because a very strong lattice, such as that of magnesium oxide, can hold together better than water can pull it apart.