Arranging the Elements (AQA GCSE Chemistry: Combined Science)

• 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

The Periodic Table of the Elements

• 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 halides show a decrease in reactivity moving down the group