The first part of the title, Elements, isn't as intriguing, but very clearly relates to your story up to now.
The second part of your title, The Lost Medallion, instantly grabs my attention. It's a title that makes me ask questions about your story (what is the medallion? Why is it important? How was it lost? Etc.), which is good. That builds intrigue. However, it does not have a clear relation to the story. There are sixteen story chapters in your book currently, yet I have not heard anything about a medallion.
My last problem with the title is very minor; why are there two parts to your title? Is it meant to be part of a series? If so, which part is the series title? Elements would be my guess, but you could stand to be a little more clear about this in your blurb (which I will go into more detail about later).
Tips:
There are a few simple guidelines for making the first part of your title more intriguing. (Keep in mind that this is not a game-changer or of vital importance unless you intend for Elements to be the name of the series.) Use 'The' before Elements to give it a sense of importance. Elements on its own is cool, but The Elements sends off a vibe of boldness to potential readers. For similar reasons, you might want to be more specific. Your second part is about the lost medallion, not just any medallion. Similarly, your series title should be specific rather than vague. Plenty of good authors do this too. Since your story is inspired by Rick Riordan's stories, I'll use Percy Jackson as an example a lot. Percy Jackson's second series is called The Heroes of Olympus. (See how it specifies which heroes we're talking about?)
Now not all authors go this route. You could also name your series after the main character, like in Percy Jackson's first series or in Harry Potter, or some other important name in your series, like how Fablehaven is the name of the 'sanctuary' the story takes place in. Now, single word titles aren't as good, but you can use them if they are the title of the first book and you add a 'the' before and something series-y after (trilogy, series, chronicles, etc.), like The Divergent Series.
