The transition metals (triple)

Rust is orange-brown, and it is a compound of iron. So how can iron also make a pale green compound? It is tempting to blame a dye or another metal mixed into the green one, but the only metal in either is iron. Iron is a transition metal, from the wide middle block of the periodic table. Before the colours comes a comparison: how unlike the Group one metals those middle-block metals are. Iron's two colours get their explanation once we can count its charges.

The transition metals are the wide block in the middle of the table, between Group two and Group three. Nothing earlier in this chapter has used them. It is for separate chemistry only, and for both tiers of that course, Foundation and Higher. The ones you need are chromium, manganese, iron, cobalt, nickel and copper, all in the first row of that block. First, a reminder about Group one: its metals are soft, light and violently reactive. Now compare their properties, one at a time, with both families in every sentence. Transition metals have higher melting points than Group one metals. Transition metals are denser than Group one metals, and harder and stronger as well. And transition metals are much less reactive than Group one metals, with oxygen, with water and with the halogens. Iron rusts over months; sodium tarnishes in seconds. Now, in a hot pan, which would melt first: iron or sodium? Sodium, long before the iron. Its melting point is below the temperature at which water boils. Here is an answer that sounds right and still falls short. 'Transition metals have high melting points.' What is missing from that? The other family. A comparison names both: transition metals have higher melting points than Group one metals. One examiner's report on a Foundation paper says: "Most students described a property of either a Group 1 element or a transition element without reference to the other; this did not gain credit as a comparison was needed." It goes on: "Of those students who did answer in comparative terms, most knew that Group 1 metals are softer than transition metals, but there was confusion about which had lower melting points." A report on a Higher paper goes further: "All possible answers were seen by examiners as the properties of Group 1 metals compared to transition metals remains a common area for errors for candidates." Say it twice and it is fixed. Group one metals have the lower melting points. Transition metals have the higher ones. One sentence per difference, both families named. The comparison gets skipped in a second way too. A student explains, in one line, why copper does not react with a dilute acid. 'Copper does not react because it is a transition metal.' Why is that no reason? It names a category and stops. It never says what being a transition metal means for how reactive copper is. One examiner's report on a Higher paper found "many that described copper as a transition metal but did not link that to reactivity in any way." Two words repair it: which means. Copper is a transition metal, which means it is far less reactive than a Group one metal. It is one of the least reactive of them, so it does not react with the acid.

Three properties are typical of transition metals and of almost nothing else. They form ions, charged particles, with different charges, their compounds are coloured, and they can be used as catalysts. Different charges first. A Group one metal always forms a one plus ion. Iron can form two ions: iron two plus, F e two plus, and iron three plus, F e three plus. That is where the numbers in brackets in a name come from. Iron two oxide, written with a roman numeral two, contains F e two plus. Iron three oxide contains F e three plus. Coloured compounds next, and the word compounds matters. Copper metal is reddish-brown, but a solution of copper sulfate is blue. The colour belongs to the compound, not the metal. And catalysts, substances that speed up a reaction without being used up. Iron is the catalyst used in making ammonia. Transition metals can be catalysts; that does not mean every one speeds up everything. Back to the start: orange-brown rust, and the pale green one, both made from iron. Which property explains one metal making two colours, and which two ions? Ions with different charges. Iron forms F e two plus and F e three plus, and each gives compounds of its own colour. The pale green compound contains iron two plus, and the orange-brown rust contains iron three plus. Two charges, two sets of compounds, two colours. Using a colour to identify which metal ion is present is a separate job, for the video on identifying metal ions.

Closing questions now: two on metals this video never worked through, two on what it did. Copper forms copper one oxide and copper two oxide. Which copper ion is in each? C u plus in copper one oxide, and C u two plus in copper two oxide. One metal, two charges. Now, iron is hard. How would you compare it with sodium? Iron is harder than sodium. Both are named, and the sentence is about one property. Here's another: what are the three typical properties of a transition metal? Ions with different charges, coloured compounds, and use as catalysts. One more: nickel is used to speed up a reaction. Which typical property is that? Use as a catalyst. Nickel speeds up the reaction without being used up itself. And that answers the rust from the start. Iron forms two ions, iron three plus in orange-brown rust and iron two plus in the pale green compound, so one metal gives two colours.

The transition metals is video nine of nine, the last in The periodic table. If you nailed it, thumbs up, and it leaves your list. If it has not landed, save it, and try explaining the comparison to someone else, because that is the quickest way to find the gap.

That completes our chapter on The periodic table. Next chapter: States of matter and separating mixtures.

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