Chemical periodicity of other elements

AS Level Chemistry · The Periodic Table: chemical periodicity. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

You do not need to learn the chemistry of every element. Elements in the same group have the same number of outer-shell electrons, so they form compounds with similar formulas and similar properties. You can predict an unfamiliar element from the ones you know in its group.

Two patterns do most of the work. Down a group the atoms get larger, so metals become more reactive and Group 17 elements become less reactive. Across a period the elements change from metals with basic oxides and ionic chlorides to non-metals with acidic oxides and covalent chlorides.

To identify an unknown element, read each property as a clue to its group, then to its position in the group.

Rules to remember

Common mistake

Students often apply the wrong trend, for example saying that reactivity increases down every group. Metals become more reactive down a group, but the Group 17 non-metals become less reactive.

Worked example

Element Q forms an oxide QO2 that has a very high melting point and is insoluble in water. Its chloride, QCl4, is a volatile liquid that fumes in moist air. In which group is Q?

  1. The formulas QO2 and QCl4 show an oxidation number of +4, so Q has four outer-shell electrons.
  2. A volatile chloride that fumes in moist air is covalent and is being hydrolysed to HCl, like SiCl4.
  3. A high-melting, insoluble oxide suggests a giant covalent structure, like SiO2.

Answer: Q is in Group 14 and behaves like silicon.

Practice questions

Try each one, then open the answer.

1. Germanium is in Group 14, directly below silicon. Which is the most likely behaviour of germanium(IV) chloride, GeCl4?

  1. A
    it is an ionic solid that dissolves to give a neutral solution
  2. B
    it is a covalent liquid that dissolves in water without reacting, giving pH 7
  3. C
    it is a covalent liquid that is hydrolysed by water to give an acidic solution
  4. D
    it is an ionic solid that reacts with water to give an alkaline solution
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Answer: C. By analogy with SiCl4, GeCl4 is a simple covalent molecule (a liquid at room temperature) that is hydrolysed by water, forming the hydrated oxide and HCl, so the solution is strongly acidic.

2. Element Y burns brightly in oxygen to form a basic oxide YO, and its chloride YCl2 is an ionic solid with a high melting point. In which group is Y?

  1. A
    Group 1
  2. B
    Group 14
  3. C
    Group 2
  4. D
    Group 17
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Answer: C. The 2+ ion shown by YO and YCl2, combined with ionic compounds and a basic oxide, identifies a reactive metal with two outer electrons: Group 2.

3. Element X is a gas at room temperature. Its hydride, HX, is a gas that fumes in moist air, and aqueous NaX gives a white precipitate with aqueous silver nitrate that dissolves in dilute aqueous ammonia. What is X?

  1. A
    fluorine
  2. B
    bromine
  3. C
    chlorine
  4. D
    nitrogen
Show answer

Answer: C. A white silver halide precipitate that dissolves in dilute ammonia is AgCl. Bromine is a liquid and gives cream AgBr, and silver fluoride is soluble, so fluoride gives no precipitate.

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← Periodicity of chemical properties of the elements in Period 3Similarities and trends in the properties of the Group 2 metals, magnesium to barium →All rules on one pageStuck? Ask a question