The three types of strong bond, and spotting each structure

Every pencil has a grey core that everyone calls the lead. What is that lead made of? The obvious answer is lead, the soft grey metal. The name says so, and the core even has a dull metal shine. There is no lead in a pencil at all. The core is graphite, a form of carbon, mixed with clay to make it firmer. People once mistook graphite for a kind of lead, and the name stuck. Carbon is a non-metal, and it is the element, not the name or the look, that decides how a substance is held together.

Every substance is held together by bonds. A bond is an attraction, a pull between opposite charges, and not a stick or a blob of glue sitting between particles. In GCSE chemistry there are only three kinds of strong bond: ionic, covalent and metallic. Ionic bonding happens between a metal and a non-metal. The metal atom hands its outer electrons over to the non-metal atom. Both atoms end up charged, and a charged atom is called an ion. The ionic bond is the attraction between those oppositely charged ions. Covalent bonding happens between non-metal atoms only. Two atoms share a pair of electrons, and that shared pair is pulled on by both atoms at once. Metallic bonding happens inside a metal. Each atom gives its outer electrons to one shared pool, and the bond is the attraction between the positive metal ions left behind and that pool. So each bond has its own verb. In ionic bonding, electrons are transferred. In covalent bonding, a pair of electrons is shared. In metallic bonding, electrons are pooled. Every part of this sorting is on the Foundation and the Higher papers alike. The elements in a substance decide its bond, and you read them off the periodic table the exam gives you. The table has a staircase line near its right-hand side. Metals sit to the left of it, non-metals to the right, and hydrogen counts as a non-metal too. Metal with non-metal gives ionic bonding. Non-metal with non-metal gives covalent bonding. A metal on its own, or metals mixed together, gives metallic bonding. Try that on potassium bromide, the white solid made when potassium and bromine react. Which bond holds potassium bromide together: ionic, covalent or metallic? Ionic. Potassium sits left of the line, so it is a metal, and bromine sits right of it, a non-metal. Metal with non-metal means electrons transferred.

A bond name is only half an answer. The other half is the structure, which means how the particles are arranged across the whole lump of substance. Three of the four structures are called giant. Giant means one arrangement repeats over and over, with no fixed number of particles, until the lump ends. A giant ionic structure is a huge regular arrangement of oppositely charged ions. Its bonding is ionic. A giant metallic structure is rows of positive metal ions sitting in that shared pool of electrons. Its bonding is metallic. Covalent bonding is the one that splits two ways. Most substances made only of non-metals exist as molecules, small separate groups of atoms joined by covalent bonds. That structure is called simple molecular. Water, oxygen and carbon dioxide are all simple molecular. A few non-metal substances are one giant network instead, every atom joined to its neighbours by covalent bonds, with no separate molecules at all. That structure is giant covalent. At GCSE the giant covalent ones are a short list: carbon, as diamond or graphite, and silicon dioxide, which is sand. Polymers, long covalent chains, get a video of their own in the next chapter. So every answer fills two boxes, always in the same order: the structure first, then the bonding. Take magnesium oxide. Magnesium is a metal and oxygen is a non-metal, so the bonding is ionic, and the structure that goes with it is giant ionic. Box one, giant ionic. Box two, ionic bonding. Both boxes are filled, and in that order. Now iodine, a grey-black solid made of iodine and nothing else. Iodine on its own: which structure does it have, and which bonding? Simple molecular, then covalent. Iodine is a non-metal that is not on the giant covalent list, so it forms small molecules of two atoms sharing a pair. The same two boxes work for the shiny grey metal that coats a galvanised bucket. That metal is zinc, on its own: which structure, and which bonding? Giant metallic, then metallic. A metal on its own means positive metal ions held in a pool of electrons, repeating through the lump. Ammonia is nitrogen and hydrogen, two non-metals, so its bonding box is already filled: covalent. So what goes in ammonia's structure box? Simple molecular. Ammonia is neither carbon nor silicon dioxide, so it exists as small separate molecules, each one N H three. One examiner's report on a Higher paper says: "Giant lattice was regularly seen, with no reference to the type of bonding." The same report adds: "Some recognised it was covalent bonding, but failed to classify it as a giant structure." In plain words, students filled one box and left the other empty, either way round. The fix is the habit you have just used: the structure, then the bonding, every time.

A question can also run backwards. It gives you measured properties of a mystery substance and asks for its structure. Each structure has a signature, a pattern of properties you can match against. Giant ionic melts high. It does not conduct electricity as a solid, but it does conduct when molten or dissolved in water. Simple molecular melts low, often below room temperature, and does not conduct in any state. Giant covalent melts very high and does not conduct. Giant metallic melts high, conducts as a solid, and can be hammered into shape without breaking. These are patterns to match, not explanations. Why each structure behaves like this is what the next chapter is for. And the properties belong to the structure, not to one particle. A single copper atom is not shiny and cannot bend; shine and bending only appear in the lump. Here are three unknown solids, P, Q and R, with their properties in a table. P melts high, does not conduct as a solid, conducts when molten and dissolves in water. Q melts low, never conducts and does not dissolve. R melts high, conducts as a solid and can be hammered flat. Fill both boxes for each of the three solids, P, Q and R. Pause it there and work it out for yourself. P is giant ionic, with ionic bonding: it conducts when molten but not as a solid. Q is simple molecular, with covalent bonding: low melting, no conducting. R is giant metallic, with metallic bonding. It conducts as a solid and it bends. P and R both melt at a high temperature. Why can that high melting point not tell P apart from R? Because giant ionic, giant covalent and giant metallic substances can all melt high. The row that separates P from R is conducting as a solid, and only R does. And when P conducts as a liquid, the charged particles carrying the current are ions, never electrons. One examiner's report on a Higher paper notes that "Lower ability candidates often thought that the bonding in substance B was metallic due to its very high melting and boiling points." A high melting point was read as a metal. What works instead is checking the conducts-as-a-solid row before you ever write metallic.

One more kind of wrong answer ticks two rows of the table at once. A student writes this: calcium chloride is giant covalent and ionic, because it is a giant lattice. Giant covalent and ionic: why can those two never both be true of one substance? Because covalent means a pair of electrons shared between non-metals, and ionic means electrons transferred from a metal. A giant covalent structure is held by covalent bonds, so its bonding cannot be ionic too. The student was reaching for a real answer underneath all that. Now write the student's answer properly: which structure, and which bonding? Giant ionic, then ionic. Calcium is a metal and chlorine is a non-metal. And giant lattice on its own fits diamond too, so it names nothing. One examiner's report on a Higher paper records: "There were many contradictions such as giant ionic and giant covalent." Students named two rows at once. The fix is to choose one row of the table, and only one, before you write anything.

Four questions to finish, and the last one goes back to the pencil. What two things does a complete answer name, and in what order? The structure comes first, and then the bonding that holds it. This one is a gas, and dissolved in water it makes an acid. Hydrogen chloride: which structure, and which bonding? Simple molecular, with covalent bonding. Hydrogen and chlorine are both non-metals, and neither is on the giant covalent list. Here's a different kind, given as properties rather than a name. A solid conducts as a solid and hammers flat. Which structure is it? Giant metallic, with metallic bonding. Conducting as a solid is the sign of a metal. One more, and this is where the video started. Pencil lead is graphite, carbon only. Which structure, and which bonding? Giant covalent, then covalent: carbon is a non-metal, and graphite is on the short list. It also conducts, which is the exception the video on graphite explains.

That is the sorting done, and the videos after this one in the chapter open each bond up in turn. If you nailed it, give this one a thumbs up so you know you can skip it next time. If not, save it; three substances in, the sorting starts to feel normal.

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

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.