Groups and Periods in the Periodic Table — AQA GCSE Chemistry

The periodic table is organised so that an element's position tells you about its electronic structure and properties.

Periods and groups

• Periods are the horizontal rows. The period number tells you the number of occupied electron shells.
• Groups are the vertical columns. Elements in the same group have the same number of electrons in their outer shell, which gives them similar chemical properties.

The group number (for groups 1–7) equals the number of outer-shell electrons. Group 0 elements have a full outer shell.

Why group members react similarly

Chemical reactions involve the outer-shell electrons. Because all members of a group have the same number of outer electrons, they react in similar ways and form ions/compounds with similar formulae.

Metals and non-metals

• Metals (left and centre) lose outer electrons to form positive ions.
• Non-metals (right) tend to gain or share electrons.
• The reactivity trends differ between groups (e.g. Group 1 gets more reactive down the group; Group 7 gets less reactive down the group).

Exam tips

• Period = number of shells; group = number of outer electrons.
• Same group → same outer electrons → similar properties.
• Metals lose electrons (positive ions); non-metals gain/share.
• Use an element's position to predict its electronic structure and reactivity.