Elements in the same group share the number of electrons in their outermost shell, so they react alike — which is what lets you predict the behaviour of an element you have never handled from the one above it.
Chemistry is almost entirely a negotiation over outer-shell electrons. Two elements with the same outer count make the same bargains, so sodium behaves like lithium and iodine like chlorine, whatever else differs between them.
The likeness weakens as you go down a group and the outer shell sits further from the nucleus — lithium is a poor guide to caesium, and fluorine is odd enough that chemists treat it separately. The first row is worse still: hydrogen sits in group 1 without being an alkali metal, and helium sits in group 18 with two outer electrons rather than eight. And the transition metals break the rule sideways, resembling their neighbours across a period as much as those above them.
Estimate with the rule, then check it against the calculator that models it properly.
Open Periodic Table →Elements in the same group share the number of electrons in their outermost shell, so they react alike — which is what lets you predict the behaviour of an element you have never handled from the one above it. Chemistry is almost entirely a negotiation over outer-shell electrons.