Organisation of the Periodic Table
Atomic number increases through the Periodic Table. Arranging the elements into rows and columns brings together atoms with related electron configurations and properties.
1
Periods
A period is a horizontal row.
- Elements within one period have the same number of occupied principal energy levels.
- The period number corresponds to the outer principal energy level containing electrons.
2
Groups
A group is a vertical column.
- Group members generally have related electronic configurations in their outer shells.
- The electrons in the outer shell are called valence electrons.
- Related outer-shell configurations help account for similarities in properties within a group.
Hydrogen and helium
Hydrogen and helium have unusual electron configurations, so their positions need special consideration.
- Helium is placed in Group 0 because its physical and chemical behaviour resembles that of the other Group 0 elements.
- Hydrogen does not match the behaviour of another group closely enough, so it is kept separate rather than being grouped normally with other elements.