Predicting an element's reactions from its position

Cut a fresh piece of rubidium, a metal most people will never see. Will the shiny face stay shiny? It is a metal, so you would expect it to, but the shine dulls almost at once. Its place in the periodic table, just below potassium, tells you that without ever seeing it. The periodic table is a tool for being right about elements you have never met. Rubidium's dulling face has a three-step answer, and it comes right after this.

Every prediction from the periodic table runs in three steps: classify the element, predict what it does, then place it in its group to judge how vigorously. Foundation and Higher students need all three. Step one, classify. Group one is the alkali metals, Group seven the halogens, Group zero the noble gases. Anywhere else, say metal or non-metal from the side it sits on. Step two, predict. Alkali metals lose one electron and react with water, oxygen and chlorine. Halogens gain one electron and react with metals. Noble gases have a full outer shell and barely react. Step three, place it. Down Group one, reactivity rises. Down Group seven, it falls. Say both directions together, every time. They are opposite for one reason. A Group one atom loses an electron, so being further from the nucleus makes that easier. A Group seven atom gains one, so being further out makes it harder. Rubidium, from the start, sits in Group one, Period five, just below potassium. Run all three steps on rubidium, and have a go before I show you. Classify: Group one, so rubidium is an alkali metal, with one electron in its outer shell. Predict: a freshly cut face is shiny and soon dulls. In water it fizzes, giving off hydrogen, and leaves rubidium hydroxide dissolved, an alkaline solution. Place it: below potassium, so even more vigorous than potassium. Its outer electron is further from the nucleus and more shielded, so it is lost more easily still. Now, which is most reactive: fluorine, chlorine or iodine? Fluorine, at the top of Group seven. Iodine, near the bottom, is the least reactive of those three. One examiner's report on a Foundation paper, about the most reactive halogen, says: "Iodine was the most common incorrect response and showed that, even when candidates did not know the answer, they still had some understanding of the existence of trends within the Periodic Table." That instinct, that the table has trends, is right. Iodine is the Group one direction used on Group seven. Tie the reason to each arrow instead: losing, further out is easier; gaining, further out is harder.

When a question says use the periodic table, it wants the position in your answer, not only the electrons. A full answer has two halves: where the element is, and what that means. For example: towards the bottom of Group one, so its outer electron is lost easily. This next element comes with no name at all, only a place on the table. Group six, Period four: will it react with sodium? Answer from its position. Yes. It sits on the right of the table, so it is a non-metal, and in Group six it gains electrons. Sodium, in Group one, loses one, so the two react. That element is selenium, and the product would be named sodium selenide. Working out its formula is a job for a later chapter. One examiner's report on a Higher paper says: "Those who failed to score usually did not answer in terms of the position in the periodic table, as required." It adds that "many just stated that tellurium has six electrons in its outer shell, without mentioning the position." Say where first, instead: on the right, towards the bottom, in Group six. Then say what that position means for the electrons. Here is a group this chapter has not opened. Calcium sits below magnesium in Group two. Which is more reactive, calcium or magnesium? Use the position, and say why. Calcium. It is further down Group two, so it has one more shell. Its outer electrons are further from the nucleus and more shielded, so they are lost more easily. One examiner's report on a Higher paper, comparing beryllium with magnesium, notes: "Trends in reactivity had to refer to the group (as the question stated to use the periodic table) and not the periodic table or reactivity series." That other ladder ranks metals from all over the table, and it is not printed on your paper. A group trend runs down one column of a table that is, so when a question says use the periodic table, the answer is a column.

One last move: marking an answer. A question names two Group one metals and asks which is more reactive, using the periodic table. A student writes a single line in reply: reactivity increases down the group. What is missing from that one-line answer? Three things: the two metals by name, where they sit, and why. It states the trend and explains nothing. One examiner's report on a Higher paper records answers that "gave a simple statement of the reactivity increasing down the group". Others "referred to different metals in the group rather than the two that were asked about." A full answer is a claim, then because, with both named elements in it. Potassium is more reactive than sodium because it is further down Group one, so its outer electron is further from the nucleus and lost more easily.

Three elements this video never touched, and one rule, to finish on. Caesium is below rubidium. More or less vigorous with water, and why? More vigorous. One more shell puts its outer electron further from the nucleus, so it is lost even more easily. Now classify radon, at the bottom of Group zero. What will it do? It is a noble gas with a full outer shell, so it will hardly react at all. Over to Group seven: astatine sits at the bottom. More or less reactive than iodine? Less reactive. Being further down, it pulls an incoming electron in less strongly, so it gains one less easily. One more: a question says use the periodic table. What must your answer name? The position: the group, and how far down it the element sits. Then what that means for its electrons. Rubidium, the metal you will never see, is no mystery after all: Group one, just below potassium, one electron very easily lost, so a freshly cut face dulls almost at once.

Number eight of the nine in The periodic table is this one, on predicting an element's reactions. If you fully understood it, thumbs up, and you can pass it by when you revise. If not, keep it saved: the table is in front of you in the exam, so every practice prediction you make with it is revision done.

Next in the chapter: The transition metals, which is for separate chemistry only.

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