Since there is no given order in which to discover the elements, you need to use a list with all items for reference. Probably the easiest way would be to watch my video with a list of all 580 elements on YouTube. Another way to do that: First, go to the Little Alchemy settings by clicking on the "burger" icon (3 horizontal bars) at the center bottom of the screen and then selecting the second tab ("Settings") at the top. Disable the "Hide final elements from the library" option, so you can see all elements you found thus far in the list on the right side. Then compare them one by one with the elements list on https://www.gambledude.com/ until you find the element you're missing.