Elements in the same group have the same number of outer electrons, so they react in similar ways. If you know how one element in a group reacts, you can predict the others. Every Group 1 metal reacts with water to make a metal hydroxide and hydrogen. For example, caesium is below potassium, so it will react in the same way, but much more violently.
Reactivity follows the trends down a group. Group 1 metals and Group 2 metals get more reactive going down. Group 7 non-metals get less reactive going down, so fluorine is more reactive than chlorine and it would displace chlorine from a chloride. Group 0 elements are unreactive, so you predict that a noble gas such as krypton will not react.
Metals lose electrons to form positive ions, and non-metals gain electrons to form negative ions. When a Group 1 metal reacts with a halogen it forms an ionic compound called a salt. Sodium reacts with chlorine to make sodium chloride, NaCl. Potassium reacts with bromine in the same way to make potassium bromide, KBr.