Metals and non-metals

Picture a copper wire and a lump of sulfur side by side. The copper is shiny, bends round a corner without snapping, and carries electricity to everything in a house. The sulfur is dull yellow, crumbles between your fingers, and carries no current at all. On the periodic table they sit only a row and a few boxes apart, yet almost every property you could test comes out opposite. There is a dividing line on that table, and which side an element falls decides almost everything about it. But the table you are given has no line drawn on it.

This is video three of nine in The periodic table. It builds on reading an element's position, which is taught in The periodic table: groups, periods and position.

A metal is an element that reacts to form positive ions. That one sentence settles every awkward case, so it comes first. An ion is an atom that has lost or gained electrons, so it carries a charge. A metal atom loses the electrons in its outer shell, and is left positive. Foundation and Higher students need every part of this video. Magnesium is two, eight, two. It loses its two outer electrons and is left as two, eight, with a charge of two plus: the magnesium two plus ion. A non-metal does the opposite. Its atoms gain electrons to become negative ions, or share electrons with other atoms instead. Metals are usually shiny, usually conduct and usually have high melting points. Those are typical properties, and typical means usually, not always. Mercury is a liquid at room temperature, and it looks nothing like a lump of iron. What makes mercury a metal: its shine, its state, or the ions it forms? The ions it forms. Mercury reacts to form positive ions, so it is a metal, whether it is runny or not. Graphite, a form of carbon, goes the other way. It conducts electricity, but carbon does not form positive ions, so it is a non-metal. The definition decides what an element is. The properties only describe what it is usually like.

On the periodic table, metals sit on the left and towards the bottom, and most elements are metals. Non-metals sit on the right and towards the top. Group zero, the column on the far right, counts among the non-metals too. The table you are given has no dividing line drawn on it. Any shading you see here is ours, put on to show the region, then taken off again. Say both halves every time, right and top. The right alone would sweep in metals near the bottom, like tin and lead. Picture a student handed a table with no colouring at all, and told to shade in the non-metals. Pause and describe the region they should shade, and the group they must include. The top right of the table, with all of Group zero swept in down the far right edge. One examiner's report on a Foundation paper says: "Only one student in six could give a correct description of where the non-metals are found, despite there being a clear statement in the specification." It goes on: "Many referred to the right of the table, but few referred to the top." It notes that "some omitted Group 0", and that "the periodic table provided does not show a staircase." The way to earn it is to describe the region in words that need no drawing: the right and the top, with Group zero included. Now, with no shading at all: Group two, Period six. Metal or non-metal? A metal. Group two is over on the left, and Period six is near the bottom. That element is barium.

With the definition and the region in place, the typical properties come in pairs. Metals conduct electricity and heat, and non-metals usually do not. Metals bend and can be hammered into shape, and solid non-metals usually shatter. Metals are shiny and dense, with high melting points. Non-metals are usually dull and less dense, and melt at lower temperatures. Why metals conduct and bend is for the video on metallic bonding. Here the properties are facts to describe and compare. Here are two unknowns. Element A is shiny, conducts, and has to be heated strongly before it melts. Element B is a brittle yellow solid that does not conduct, and melts easily. Which of the two, element A or element B, is the metal? Element A. Every one of its properties is typical of a metal, and element B matches a non-metal on each. Now one difference between them, written as a sentence. Here are two attempts. First: element A conducts electricity. Second: element A conducts electricity, whereas element B does not. Which of those two attempts is a comparison, and why? The second. A comparison names both elements and says how they differ, and the word whereas joins the two halves. One examiner's report on a Foundation paper puts it this way: "Many answers were not comparative so could not gain credit." It adds that "Some gave two halves of the same comparison as two different differences", and that "Answers in terms of atomic structure, rather than properties of the element, did not gain credit." The habit that avoids all three: one sentence per difference, both elements named, one property each. Two differences means two different properties. Metals also share a reaction. They burn in oxygen to form oxides. In words, magnesium plus oxygen gives magnesium oxide. In symbols, magnesium is M g, a solid. Oxygen is O two, a gas, because oxygen comes as molecules of two atoms. Magnesium oxide is one of each. How many magnesium atoms use up one O two molecule? Two. That gives two M g solid, plus O two gas, makes two M g O solid, with two atoms of each element on both sides. The balancing two always goes in front of a formula. Writing M g O two instead would describe a different substance, so the small numbers inside a formula never change.

Before you go, four questions, and the definition settles most of them. Iodine crystals are shiny, but iodine forms negative ions. Which kind of element is it? A non-metal. Shine is only a typical property; it forms negative ions, not positive ones, and the definition decides. Now, where are the non-metals, in words that need no drawing? On the right and at the top, with Group zero included. Next, sodium conducts electricity. How would you compare it with chlorine? Sodium conducts electricity, whereas chlorine does not. Both named, one property, joined by whereas. One more: calcium burns to form C a O. How many calcium atoms balance one O two? Two. Two C a solid, plus O two gas, gives two C a O solid. The copper wire and the lump of sulfur were never a mystery: copper forms positive ions and sits among the metals, and sulfur sits up on the right with the non-metals.

Metals and non-metals is the third video of the nine in The periodic table. If that's in the bag, press the thumbs up, and next time you revise you will know you can skip it. Not there yet? Save it, and say the two words, right and top, out loud once, because that is how they stick.

Next in the chapter: Group 1: the alkali metals.

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