How can elements be classified based on electron configurations?
The amount of electrons in the last shell allow you to classify elements.
Elements can be determined by the amount of the electrons in their last shell (known as their outer shell). The amount of electrons in their outer shell determines their group, and hence their electronic properties.
The alkaline metals all have 1 electron in their outer shells. They all react vigorously with water.
If you know the last number in the electronic configuration, you can work out the group it belongs to and therefore you can determine the properties.