2.1 understand the terms group and period

Groups
A group is a column in the periodic table. All elements in a group have similar chemical properties (because they have the same number of electrons on their outer shell), however, the properties of each element (such as reactivity) often gradually change as you go down a group (as the atomic number increases).

Periods
A period is a row in the periodic table. Properties of elements in the same period are not similar and quite often change.