If you have not already done so, download Avogadro for free and watch the tutorials if necessary.
Call up the Periodic Table on an Avogadro page (click on the panel and open the drop down Element) then select all the elements of a group of your choice and look at the corresponding atoms. What do you notice? Do the same for a period of your choice. What do you notice? Describe in a few lines which periodic property it is.