In a periodic table, chemical elements are arranged in groups and periods according to their atomic number, electron configuration, and their chemical properties.
Elements in the same period have the same number of electron shells and tend to have similar properties. Thus, it is relatively easy to predict the chemical properties of an element if one knows the properties of the elements around it. A new row (period) is started when a new electron shell has its first electron.
Elements in the same group have the same electron configuration in their valence shell.
Figure 1: Periodic Table