The Periodic Table repeats itself. Once you know how structure, bonding and acid-base behaviour change across Period 3, you can identify an unknown element from its data and predict the chemistry of elements you have never studied.
The trends seen in Period 3 recur in every period. To place an unknown element, use several clues together: the formulae of its oxide and chloride give its oxidation number and so its group; successive ionisation energies jump after the last outer electron; a very high melting point and non-conducting behaviour point to a giant covalent structure; oxide and chloride behaviour with water (basic, amphoteric, acidic, hydrolysed) shows where it sits between metal and non-metal.
Use the trends simulator as a reference table while you work through each clue below.
| Clue | What it tells you |
|---|---|
| formula of the highest oxide or chloride (e.g. XO2, XCl4) | oxidation number +4, so Group 14 |
| big jump between the 3rd and 4th ionisation energies | three outer electrons, so Group 13 |
| very high melting point, no conduction, insoluble oxide that reacts with hot NaOH | giant covalent element with an acidic oxide: silicon-like |
| oxide reacts with both HCl and NaOH | amphoteric: near the metal/non-metal border, like Al |
| chloride fumes in moist air, solution pH 1 to 2 | covalent chloride hydrolysed: a non-metal |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Element X forms a chloride XCl4, a liquid that fumes in moist air. In which group is X?
The successive ionisation energies of element Y show a large jump between the 2nd and 3rd. In which group is Y?
An element has a melting point of 1410 °C, is a semiconductor and forms an oxide that is insoluble in water but reacts with hot concentrated NaOH. The element is most likely:
Down a group, the oxides of the elements generally become:
Apply the same reasoning to elements in other periods. The same pattern repeats, shifted down a row.
Do not base an identification on a single property. Melting point alone cannot separate a strong metal from a giant covalent solid; combine it with conductivity, oxide behaviour and ionisation energy data. And remember that the dips in first ionisation energy (at Groups 13 and 16) repeat in every period.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
An element Z forms an oxide ZO2 with a very high melting point that does not conduct when molten. Its chloride ZCl4 reacts with water to give an acidic solution and a white solid. What is Z?
An element's oxide reacts with both dilute hydrochloric acid and aqueous sodium hydroxide. Its chloride gives a solution of pH 3. The element is most likely:
Using Period 3 as a guide, which property would you predict for germanium (Group 14, Period 4)?
Which pair of first ionisation energy dips would be seen across Period 2?
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