Why metals conduct and bend, and why an alloy is harder

Why does mixing copper into gold make it harder, when copper on its own is a soft metal too? Pure gold is soft enough that a ring made of it bends and scratches in everyday wear. So jewellers mix in some copper or silver, and the ring keeps its shape. Two soft metals, and together they are harder than either one. The ring waits until the plain metal is understood, layer by layer.

A metal has a giant structure. Positive metal ions sit in regular layers, and their outer electrons have left them to form a sea of delocalised electrons that moves through the whole structure. The positive ions and the negative electrons attract each other strongly. That attraction, acting right through the structure, is metallic bonding. Students at Foundation and at Higher explain every property in this video from this one picture. Structure here means that picture of ions and electrons. It does not mean the electron shells drawn for one element on its own, which answers a different question. So open any answer about a metal's properties with the same words: positive metal ions in a sea of delocalised electrons.

First, the melting point. Melting means overcoming the attraction between the positive ions and the delocalised electrons, and in a giant structure there is a huge amount of it. That takes a lot of energy, so metals generally have high melting and boiling points. Conduction is next. Anything that conducts needs charged particles that are free to move. Which particles in a metal are free to move and so let it conduct electricity, and what do they do? The delocalised electrons. They are free to move through the whole structure, and they carry the charge. The same moving electrons carry thermal energy too, so metals conduct heat well. That is why a metal floor feels colder under bare feet than a wooden one at the same temperature: it carries heat away from your skin faster. Now a different kind of challenge. A molten ionic compound conducts too, and the reason is not the same. Can you explain both kinds of conduction in two sentences that share no noun? In a metal, delocalised electrons move through the structure and carry the charge. In a melted salt, freed ions drift through the liquid and carry the current. Graphite has delocalised electrons too, as the video on graphite explains, and that is the one way it behaves like a metal.

Metals can be bent and hammered into shape, and a metal that can is called malleable. The reason is the layers. In a pure metal, the positive ions are all the same size and sit in regular layers. A push can make one layer slide over the next. As the layers slide, the sea of electrons moves with them and keeps holding the ions, so the metal changes shape without breaking apart.

Now the ring. An alloy is a mixture of a metal with other elements, often other metals. In an alloy, atoms of a different size are mixed in among the metal's own atoms. Picture two diagrams. In the first, regular layers of identical circles. In the second, the same layers with a scattering of noticeably larger circles among them. Harder means the metal keeps its shape when it is pushed. Which of the two diagrams is the harder metal, the first or the second? The second. Around each larger atom, the rows buckle, so the layers no longer line up neatly. Written as a chain, it has three links. Atoms of a different size are mixed in. They distort the layers. Distorted layers cannot slide over each other so easily, so the alloy is harder. The middle link is the one an examiner's report found rarest: "Those who gained one mark usually referred to the difference in the sizes of the atoms and the second most common mark was for layers not being able to slide. Layers are distorted was less frequently given." Different sizes, then slide, with nothing in between, skips the cause. The fix is to say distorted every time, between the two. The noun matters as much as the chain. The things that differ in size are atoms, or positive ions. Not metals, and not particles. A report on an alloy question says why: "Some lost credit because of poor use of language; it is not 'metals' which are different sizes, but their atoms, or positive ions. A common error was to refer to different shapes of atoms. 'Particles' is also insufficient as it could refer to other types of particle than atoms." Different sizes, never different shapes, and atoms, never metals or particles. That habit keeps every answer about alloys on the right nouns. A student writes: the alloy is harder because the different metals bond more strongly together. That sentence goes wrong in two places at once. What are the two things wrong with the student's sentence about the alloy? The bonding is not stronger; it is the same metallic bonding, positive ions in a sea of electrons. And metals should be atoms. What changes is the arrangement: different-sized atoms distort the layers. That is the heart of it. An alloy changes geometry, not bonding. The report that named the wrong nouns found this too: "Common errors often involved an attempt to describe some sort of stronger bonding, sometimes covalent or ionic, in the alloy." It also says: "This question discriminated well, with over a half of students scoring 2 or 3 marks. There were many well-expressed, complete answers." The explanation is within reach; the slips are single words.

Before the ring comes back, four questions on the metal underneath it. Why can a pure metal be hammered into shape without breaking? Its ions are all the same size, in regular layers that can slide over each other, and the sea of electrons keeps holding them as they move. Now a new case: a metal gets a few smaller atoms of another element mixed in. Harder or softer, and why? Harder. Smaller atoms are still a different size, so they distort the layers, which then cannot slide as easily. A different question: why does a metal conduct electricity? Its delocalised electrons are free to move through the structure and carry the charge. One more, the ring: why does mixing copper into gold make it harder? Copper atoms are a different size from gold atoms. They distort the layers, so the layers cannot slide over each other easily, and the ring holds its shape.

Combined science students need nothing further in this chapter, so the thumbs-up matters here. If you nailed it, press it, and your list shows this topic finished. If not, keep it saved and unthumbed. Write the alloy chain out once, in three lines on paper, and it stays with you.

Next in the chapter: nanoparticles, size and surface area to volume ratio. That video and the one after it are for separate chemistry only. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh five, oh nine.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.