Final answer:
The Lanthanides and Actinides are placed below other elements for reasons related to their atomic properties, not to save space. Alkali metals in Group 1 are indeed very reactive, while Noble gases in Group 18 are nonreactive. Halogen gases in Group 17 are also highly reactive.
Step-by-step explanation:
The reason the Lanthanides and Actinides are located below the other elements is not to keep the table from becoming too wide for a sheet of paper. The elements in the periodic table are arranged based on their atomic numbers and chemical properties. The lanthanides and actinides are placed at the bottom to maintain the overall structure and organization of the table.
Alkali metals in Group 1 are indeed very reactive. They have a single valence electron, which makes them highly reactive and easily lose that electron to form positive ions.
The Noble gases in group 18 are nonreactive. They have a full outer electron shell and are stable, which makes them not readily participate in chemical reactions.
The Halogen gases in group 17 are indeed very reactive. They have seven valence electrons, which makes them highly reactive and easily gain one electron to complete their outer electron shell.