AQA GCSE Chemistry

Revision Notes

1.2.1 Arranging the Elements

How the Elements are Ordered

  • There are over 100 chemical elements which have been isolated and identified
  • Elements are arranged on the periodic table in order of increasing atomic number
    • Each element has one proton more than the element preceding it
    • This is done so that elements end up in columns with other elements which have similar properties
  • The table is arranged in vertical columns called groups and in rows called periods
    • Period: These are the horizontal rows that show the number of shells of electrons an atom has and are numbered from 1 – 7
    • E.g. elements in period 2 have two electron shells, elements in period 3 have three electron shells
  • Group: These are the vertical columns that show how many outer electrons each atom has and are numbered from 1 – 7, with a final group called group 0 (instead of group 8)
    • E.g. group 4 elements have atoms with 4 electrons in the outermost shell, group 6 elements have atoms with 6 electrons in the outermost shell and so on

The Periodic Table of the Elements

Exam Tip

The atomic number is unique to each element and could be considered as an element’s “fingerprint”.
The number of electrons changes during chemical reactions, but the atomic number does not change.

Predicting Reactions

  • The group number of an element which is given on the periodic table indicates the number of electrons in the outer shell (valence electrons)
    • This rule holds true for all elements except helium; although is in group 0, it has only one shell, the first and innermost shell, which holds only 2 electrons
  • We can use the group number to predict how elements will react as the number of valence shell electrons in an element influences how the element reacts.
  • Therefore, elements in the same group react similarly
    • By observing the reaction of one element from a group, you can predict how the other elements in that group will react
    • By reacting two or more elements from the same group and observing what happens in those reactions you can make predictions about reactivity and establish trends in reactivity in that group
  • For example, lithium, sodium and potassium are in group 1 and can all react with elements in group 7 to form an ionic compound
  • The group 1 metals become more reactive as you move down the group while the group 7 metals show a decrease in reactivity moving down the group

Exam Tip

The word “periodic” is used in the name of the periodic table as similar properties appear in elements placed at regular intervals throughout the table.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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