Science experiments are often casual and fun. Maybe you mixed baking soda and vinegar in a pop bottle in the backyard just to see what would happen. Or you rubbed a balloon against your head to learn about static electricity. At other times, though, your science experiments will be more formal....

We know that elements are types of atoms, made up of varying numbers of protons, neutrons, and electrons. Scientists have discovered or made about 120 elements. The periodic table is a little like a cookbook. It...