Group 7: the halogens

Bleach smells of chlorine, and it can take the colour out of a shirt in about a minute. The brown liquid painted on skin before an operation is a solution of iodine, and it can stay there for hours without harm. Chlorine gas was used as a weapon in the First World War. Iodine is gentle enough to put on a cut. Yet both sit in the same column of the periodic table, with the same number of outer electrons. The column cannot be the whole story.

You are watching video five of nine, part of The periodic table. Group 1: the alkali metals is worth having behind you, because almost everything here runs the opposite way.

Group seven is the column second from the right, and its elements are called the halogens. The three you need are chlorine, bromine and iodine. Both Foundation and Higher students need all of it. Each has its own look at room temperature. Chlorine is a green gas. Bromine is an orange-brown liquid. Iodine is a grey-black solid that gives off a purple vapour when warmed. All three come as molecules of two atoms joined together: C l two, B r two and I two. In an equation, a halogen is never written as a single atom. Each atom has seven electrons in the outer shell, one short of a full shell. That one gap drives everything they do. With a metal, a halogen atom gains one electron and becomes an ion, a charged particle, with a charge of one minus. Chlorine becomes the chloride ion, C l minus. That gives two words to keep apart. A halogen is the element, a two-atom molecule with a colour you can see. A halide is the ion, or a compound containing it, like sodium chloride. Now, a potassium bromide solution: does it contain bromine molecules or bromide ions? Bromide ions. Potassium bromide is a salt, so the bromine in it is there as B r minus ions, dissolved and colourless. With non-metals, halogens share electrons instead, making molecules. Hydrogen and chlorine make hydrogen chloride, H C l, which is a gas. Hydrogen halides like that dissolve in water to give acidic solutions. What makes a solution acidic belongs to the video on acids and alkalis.

Chlorine, bromine and iodine react in similar ways, and the reason is short: each has seven electrons in its outer shell. One examiner's report on a Higher paper notes: "Just over half of students stated that Group 7 elements react in similar ways because they all have 7 electrons in their outer shell. Many omitted the word outer which was a requirement for the mark." Without the word outer, the sentence is wrong: a chlorine atom has seventeen electrons in all. Treat outer as part of the answer instead of decoration. Now the first trend. Going down the group, the melting and boiling points rise: chlorine is a gas, bromine a liquid, iodine a solid. Picture two C l two molecules side by side. Inside each one, a bond holds its two atoms together. Between the two molecules there is a much weaker force of attraction. When a halogen melts or boils, only the forces between molecules are overcome. The bonds inside the molecules do not break. Down the group the molecules get bigger, and bigger molecules have stronger forces between them. More energy is needed to pull them apart, so melting and boiling points rise. At room temperature, bromine is a liquid and iodine is a solid. Why is iodine the solid of the two, and what is pulled apart when it melts? Iodine molecules are bigger, so the forces between them are stronger. Melting pulls whole molecules away from each other, and the bond inside each I two stays whole. One examiner's report on a Higher paper found that "More than 60% gained 1 mark, usually for stating that the melting points increase going down Group 7." It goes on: "However students were unable to explain that this is because the molecules increase in size going down the group and the intermolecular forces increase". Intermolecular forces means forces between molecules. The habit is to finish the chain every time: bigger molecules, stronger forces between them, more energy to overcome them.

Reactivity goes the other way. It falls down the group: chlorine is the most reactive of the three and iodine the least. Reading that order from results is something students do well. One examiner's report on a Foundation paper says "Many candidates analysed the information and gave the correct order of reactivity of the group 7 elements." Set it beside Group one for a moment. An alkali metal reacts by losing an electron, so being further out helps it. A halogen reacts by gaining one, so being further out hurts. Here is the chain for Group seven. Going down, each atom has one more shell, so the outer shell sits further from the nucleus and is more shielded by the shells inside it. An incoming electron is attracted less strongly. It is gained less easily. And the halogen is less reactive. Now one to explain from start to finish, with nothing given away. Why is chlorine more reactive than bromine? Start at the shells, end at the electron gained. Chlorine has three shells and bromine has four, so chlorine's outer shell is closer to the nucleus and less shielded. Its nucleus attracts an incoming electron more strongly, so chlorine gains an electron more easily, which makes it more reactive. It becomes the chloride ion, C l minus. One examiner's report on a Higher paper says many candidates "scored 0 for describing the trend in reactivity", rather than explaining it. A description says which halogen is more reactive. An explanation says why, so the habit is never to stop before the word because. The same report lists common errors, among them "It is easier for chlorine to lose an electron (confusion here with group 1 elements)", and "Many said 'intermolecular forces' instead of force of attraction." The way round both is to keep the two trends apart. Halogens gain an electron, and reactivity is about the nucleus pulling on it, never about forces between molecules. A more reactive halogen can even push a less reactive one out of its compound, and that is the job of the video on halogen displacement reactions.

To close: fluorine first, then three more on what this video built. Fluorine sits above chlorine. Is its boiling point higher or lower than chlorine's? Lower. Fluorine molecules are smaller, so the forces between them are weaker and less energy pulls them apart. Now, when iodine melts, what is overcome? The forces between iodine molecules. The bond inside each I two does not break. Next, chloride or chlorine: which one is the green gas? Chlorine, the halogen. Chloride is the ion, and in solution it is colourless. Try this: does bromine gain an electron more or less easily than chlorine? Less easily. Its outer shell is further from the nucleus, so the pull on an incoming electron is weaker. The bleach and the iodine share a column and seven outer electrons. What separates them is how far down they sit: chlorine pulls in an electron hard, and iodine much less so.

Group 7: the halogens is number five of the nine in The periodic table. If there is no need to see this twice, thumbs up, and it is crossed off your list for good. Otherwise, save it; the next video, on displacement, leans on all of this, so it comes back round anyway.

Next in the chapter: Halogen displacement reactions.

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