Arrangement of elements

O Level / IGCSE Chemistry · The Periodic Table. 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

The Periodic Table lists the elements in order of increasing proton number. The rows are periods and the columns are groups.

The period number equals the number of electron shells in use. The group number equals the number of outer-shell electrons (Group VIII has a full outer shell). Elements in the same group have the same number of outer electrons, which is why they have similar chemical properties.

Across a period the elements change from metals on the left to non-metals on the right. So the position of an element lets you predict the charge on its ion, the type of oxide it forms and how it compares with its neighbours in the group.

Rules to remember

Common mistake

Students mix up groups and periods. Groups are the vertical columns (outer electrons); periods are the horizontal rows (number of shells).

Worked example

Element X has proton number 13. Where is X in the Periodic Table, what ion does it form and what is the formula of its chloride?

  1. 13 electrons arranged in shells: 2,8,3.
  2. Three occupied shells → Period 3. Three outer electrons → Group III.
  3. A Group III atom loses its 3 outer electrons to form X3+; chloride ions are Cl−, so three are needed to balance the charge.

Answer: X is in Group III, Period 3. It is a metal that forms X3+ ions, and its chloride is XCl3.

Practice questions

Try each one, then open the answer.

1. An element Y has the electronic configuration 2,8,1. Which element is most likely to have similar chemical properties to Y?

  1. A
    an element with configuration 2,8,2
  2. B
    an element with configuration 2,8,3
  3. C
    an element with configuration 2,8,8
  4. D
    an element with configuration 2,8,8,1
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Answer: D. Elements in the same group have the same number of outer-shell electrons, and it is the outer electrons that decide chemical properties. 2,8,8,1 has one outer electron, like 2,8,1, so both are in Group I.

2. Going across Period 3 from sodium to chlorine, the elements change from

  1. A
    gases to solids
  2. B
    reactive to unreactive
  3. C
    non-metals to metals
  4. D
    metals to non-metals
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Answer: D. Metallic character decreases across a period: Na, Mg, Al are metals, Si is a metalloid, P, S, Cl are non-metals.

3. An atom of an element in Group VII and Period 2 of the Periodic Table has how many electrons in total?

  1. A
    2
  2. B
    7
  3. C
    9
  4. D
    17
Show answer

Answer: C. Period 2 means two occupied shells and Group VII means seven outer electrons: the configuration is 2,7, which is 9 electrons (fluorine).

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