The ionic lattice

Leave a shallow dish of seawater in the sun until it dries. The salt left behind often grows as tiny, square-sided cubes. Who cut them? The tempting answer is that salt is made of little cube-shaped molecules, packing together like sugar cubes in a box. But there is no such thing as a molecule of salt. Sodium chloride has no separate pairs or molecules in it at all. Nobody cut those cubes. The particles inside stack themselves in one regular pattern, and a cube is the shape that pattern grows into. Those stacking particles are ions, and something has to hold billions of them in place.

Start where ionic bonding leaves off, the handover drawn step by step in the video on ionic bonding and dot-and-cross diagrams. Sodium has given an electron to chlorine, so there are sodium ions, one plus, and chloride ions, one minus. In a real reaction it is never one pair. Billions upon billions of ions form at once, and they do not stay in pairs. Every positive ion attracts the negative ions around it, and every negative ion attracts the positive ones. They pack together alternating, plus beside minus, in every direction. The result is a giant ionic lattice. A lattice is a regular repeating arrangement, and giant means it keeps repeating until the crystal ends. In sodium chloride, each sodium ion has six chloride ions touching it: above, below, left, right, in front and behind. Each chloride ion has six sodium ions in the same places. Say all three words, giant ionic lattice, every time. Giant lattice on its own also describes diamond, so it does not say which kind. Sodium chloride is the one ionic structure you need to know. Other ionic compounds have a lattice too, but you are not expected to know theirs. This is on both the Foundation and the Higher papers. Three descriptions of solid sodium chloride. The first: separate sodium chloride molecules. The second: a giant covalent lattice. The third: a giant ionic lattice. Which of those three describes the solid correctly? The giant ionic lattice. There are no separate molecules to find, and covalent means shared electrons, which this solid does not have.

The lattice is held together by strong electrostatic forces of attraction between oppositely charged ions. Electrostatic means the pull between electric charges. Pick any one sodium ion. It is pulled by the six chloride ions around it, in every direction at once, and not along one line. So there is no single bond pointing from one ion to one partner. The attraction surrounds each ion on all sides. Sodium ions repel other sodium ions, and there are plenty of those nearby too. So why does the lattice hold together instead of flying apart? Because every ion's nearest neighbours carry the opposite charge. The pull from those close neighbours beats the push from same-charged ions further off. When a question says explain the structure, that pull is the answer it wants. Name what holds the lattice together, not only what it looks like.

If there are no sodium chloride molecules, the formula cannot be counting one. So what is it counting? Here is a model of a small cube of the crystal. Count it and you find fourteen sodium ions and thirteen chloride ions. The numbers in the model are real, and they still do not give the formula. What is the formula of sodium chloride? Why is it not N a fourteen C l thirteen? N a C l. The model is only a small block of a lattice that keeps going, so its edges are cut off where the model stops. Across the whole crystal there is one sodium ion for every chloride ion. That one to one ratio is the formula, and a one is never written in a formula. That is what makes an ionic formula different from a molecule's. It is a ratio for a structure with no end, not a count of particles in one unit.

Answers about the lattice turn on a few exact words, and the next two student answers show where. A student writes: sodium chloride has a high melting point because there are strong electrostatic forces of attraction between the bonds, and a lot of energy is needed to break them. Why the melting point is high belongs to the next chapter. Here, almost every word sounds like a mark scheme. Which phrase in that answer is wrong, and what should it say? Between the bonds. Forces act between particles, and the particles in this solid are ions. It should say between oppositely charged ions. One examiner's report on a Higher paper says: "Fewer referred correctly to the particles that those forces acted between", and "some referred incorrectly to the forces being between bonds." In plain words, students named the forces and then attached them to the wrong thing. The habit is a fixed phrase: forces, then between oppositely charged ions. A second student stops at this: sodium atoms lose an electron and chlorine atoms gain one, so sodium ions and chloride ions are made. Everything in that answer is right, and it still stops too soon for a question about the solid. What is missing from the end of that answer? The lattice. The ions then pack into a giant ionic lattice, held by strong electrostatic forces of attraction between oppositely charged ions, acting in every direction. Another report puts it this way: "Although many students understood the formation of sodium and chloride ions, very few appeared to appreciate the consequence of this in the formation of an ionic lattice." Answers made the ions and never stacked them. What works instead is carrying every answer on, from the ions forming to the lattice they build. One report on a Higher paper notes that "recognition of ionic bonds or an ionic lattice was enough to score." If you can say giant ionic lattice, you have that wording now. Then stop, and keep molecules and atoms out of it. Sodium chloride contains neither; it is ions all the way through.

Time to check the lattice has stuck, one question at a time. What holds a giant ionic lattice together? Name it in full. Strong electrostatic forces of attraction between oppositely charged ions, acting in every direction. This one is a compound the video has not drawn: potassium chloride, K C l. Is potassium chloride made of molecules, and what does K C l tell you? No molecules. It is a giant ionic lattice of potassium and chloride ions, and K C l is the one to one ratio. Try a student who wrote two words where three were needed. A student describes sodium chloride as a giant lattice. Which word is missing? Ionic. Giant lattice alone would fit diamond as well, so it names no type. One more, and it is the question the video opened with. In the seawater dish, what grew the salt into cubes? Sodium ions and chloride ions stacking into a giant ionic lattice. The cube is the shape of that repeating pattern, and nobody cut it.

That finishes the ionic side of the chapter, from one electron changing hands to a whole crystal. If there is no need to see this twice, give it a thumbs up so it drops off your list. If it is not settled yet, save it, give it a day, and look again tomorrow.

Next in the chapter: Covalent bonding and dot-and-cross for small molecules. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh four, oh four.

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