Kids do not always learn the same way, and so it makes it hard for them to understand things the same way. With my seventh graders, who study physics, one of the hardest things to teach is the Periodic Table. Try cutting up a worksheet with the periodic table printed on it by dividing the periodic table into sections. Then, describe the elements by section, and assign small groups to each section. Finally, have the students present this to the class. I have found that this works quite well. The Periodic Table is an important part of Physics and needs to be understood. Please reply to me if there is any further questions.