Periodicity = repeating patterns
As you go across the period , the ionic radii decrease
As you go down the period, the ionic radii increase
Electronegativy and First Ionisation Energy also change as you go across, and go down the period.
(there are always exceptions to every trend, but the trend holds true for majority of the atoms in the table, hence 'periodicity')
You can also talk about the change in bonding and intermolecular forces as you move across the period.