Group 1: the alkali metals

Picture a piece of lithium the size of a pea, dropped into a bowl of water. It fizzes and skates about on the surface. The same sized piece of sodium does the same thing faster, and sometimes it catches fire. Potassium, same size, same bowl, same water, goes up at once and burns with a lilac flame. Nothing about the experiment changed. Three metals from one column, each one row below the last, and a fizz has turned into a fire.

You are on video four of nine in The periodic table, the first to open up a single group. It stands on reading the outer shell, from The periodic table: groups, periods and position.

Group one is the first column on the left, and its metals are called the alkali metals: lithium, sodium and potassium are the three you need. Each of their atoms has one electron in its outer shell, the shell furthest from the nucleus. Whichever tier you sit, Foundation or Higher, all of this is yours. Keep that phrase whole, one electron in the outer shell. Every atom has exactly one outer shell, so one outer shell on its own says nothing about the electrons. They are soft enough to cut with a knife, and light enough to float on water. A freshly cut face is shiny, but within seconds it goes dull. What has dulled that cut face: the shine wearing off, damp, or oxygen from the air? Oxygen from the air. It joins the metal to make an oxide, a new substance coating the surface. Because oxygen has joined it, the dull layer weighs more than the shiny metal it came from. That is also why these metals are kept under oil, away from the air. For lithium, the equation is four L i, solid, plus O two, gas, gives two L i two O, solid. That product is lithium oxide. One warning about the pictures: a single atom is not soft, and it is not shiny. Softness and shine belong to the lump of metal, not to one atom inside it.

When a question asks what you would see, it wants things you could point at: the colours, the movement, the products. Sodium and chlorine begin as a silvery metal and a green gas. The sodium burns, and a white solid forms, sodium chloride. In symbols, two N a, solid, plus C l two, gas, gives two N a C l, solid. In water, each of the three floats, fizzes as hydrogen gas is given off, moves about on the surface, and gets smaller until it is gone. Left in the water is a hydroxide, dissolved. That solution is an alkali, which is where the name alkali metal comes from, and what an alkali is belongs to the video on acids and alkalis. Here is a full one to try, from the last bowl in the hook. Potassium in water: give the word equation, the symbol equation, and what you see. In words: potassium plus water gives potassium hydroxide plus hydrogen. In symbols, start unbalanced: K plus H two O gives K O H plus H two. Count the hydrogens, two on the left and three on the right. Put a two in front of K O H, which makes four hydrogens on the right. A two in front of the water matches them, and a two in front of K matches the potassium. Two K, solid, plus two H two O, liquid, gives two K O H in solution, plus H two, gas. The state symbols tell the story: a solid goes in, a gas leaves, and the hydroxide stays dissolved. And what you would see: the potassium floats and moves about, it fizzes, it burns with a lilac flame, and it shrinks until it disappears. One examiner's report on a Higher paper, about sodium and chlorine, says: "Very few described what they would actually see." It goes on: "Few students stated chlorine was a green gas or sodium a silvery metal." Instead, many "described how sodium reacted with chlorine in terms of electron transfer." The fix: when a question says see, give the things you could point at first, the colours, the movement and the solids, and keep the reasons for a question that asks why.

Down the group the reactions get more vigorous: lithium fizzes, sodium fizzes faster, potassium burns. The reason is inside the atoms. Spotting that two of these metals share a group is something students do well. One examiner's report on a Foundation paper says "Most candidates gained the mark by stating that sodium and lithium were in the same group". The same report notes that saying they were both reactive metals, or "next to each other", "was insufficient." The group is the start of an answer, not the end of one. Every alkali metal reacts by losing its one outer electron. Losing it leaves a full shell underneath, which is why losing one is the easy move. One examiner's report on a Higher paper names the opposite idea: "A common misconception was the need to 'gain' one electron which was incorrect." The way to keep it straight is to picture the electron leaving, never arriving. Now the chain, one link at a time. Going down the group, each atom has one more shell than the one above. That puts the outer electron further from the nucleus, with more shells in between shielding it. The nucleus attracts it less strongly. The electron is lost more easily. And the metal is more reactive. Here are two structures to compare: two, one, against two, eight, eight, one. Lithium and potassium: what is shared, what differs, and which reacts faster, and why? The similarity: each has one electron in its outer shell. The difference: lithium has two shells and potassium has four. Potassium reacts more vigorously. Its outer electron is further from the nucleus and more shielded, so it is attracted less and lost more easily. One examiner's report on a Foundation paper, on sodium and potassium, notes: "Sometimes the wrong number of shells was given for the two elements: usually 2 shells for sodium and 3 for potassium." It adds: "Some stated that potassium has more outer shells than sodium." The habit that prevents both is counting the numbers in the structure: sodium has three shells, potassium four, and each has one outer shell. One last pair, sodium and potassium, with most of the chain already done. Potassium has one more shell, so the attraction on that electron is weaker. Now finish it: which electron does potassium lose more easily than sodium? Its outer electron, the single electron in the outer shell. Naming which electron is part of the answer, not a detail. One examiner's report on a Higher paper says that when answers fell short, "it was usually because they did not state that it was the outermost or outer shell electron that is lost." So say outer electron, every time. Below potassium sit rubidium and caesium. What they would do is a prediction, and the video on predicting an element's reactions from its position is where it gets made.

Four to finish. A couple you have met, and a couple you have not. Lithium reacts with water too. What is its balanced equation? Two L i, solid, plus two H two O, liquid, gives two L i O H in solution, plus H two, gas. Now fill the gap: more shells, outer electron further out and shielded, then what? Less attraction from the nucleus. Then it is lost more easily, and the metal is more reactive. Next, does an alkali metal atom gain or lose an electron when it reacts? It loses one, its single outer electron. One more: sodium burns in chlorine. What would you see form? A white solid, sodium chloride, made from a silvery metal and a green gas. That is why two rows down one column turned a fizz into a fire. Potassium's outer electron is the furthest from its nucleus of the three, so it leaves the most easily.

Group 1: the alkali metals sits fourth among the nine videos of The periodic table. When you own this one, a thumbs up takes it off your list, so revision time goes on the rest. If the chain still wobbles, keep it saved, because nobody gets the whole chain first time.

Next in the chapter: Group 7: the halogens.

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