An element's position in the Periodic Table lets us predict how it reacts. Elements in the same group have similar chemical properties because their atoms have the same number of outer shell electrons. Going down a group of metals, reactivity increases. Going down Group 7, the reactivity of the non-metals decreases, so non-metal reactivity increases going up the group.
The group number also predicts the ion formed. Metals lose electrons to form positive ions: Group 1 forms 1+ ions and Group 2 forms 2+ ions. Non-metals gain electrons to form negative ions: Group 7 forms 1- ions and Group 6 forms 2- ions. When a metal reacts with a non-metal, electrons are transferred and an ionic compound forms.
Here are some predictions. Calcium is in Group 2 below magnesium, so it is more reactive than magnesium and forms a 2+ ion. Reactivity increases down Group 2, so strontium, below calcium, is predicted to be more reactive still and to form a 2+ ion. Rubidium is in Group 1 below potassium, so it reacts with water to form rubidium hydroxide and hydrogen gas, more vigorously than potassium. Bromine is above iodine in Group 7, so bromine is more reactive. If bromine water is added to sodium iodide solution, bromine displaces iodine and the solution turns brown.