Nice concept! The rules are kind of confusing if you don't watch the YouTube video though... The graphics need some work, and a "Cancel" button for when you click an element and you don't want to place it would be nice.
The elements in your "inventory" could be organized like a deck of cards at the bottom of the screen like Yu-Gi-Oh or those poker games. Or, it could be somewhat like Minecraft's hotbar. (Basically, what I wanna say is that where they're currently at 1. Does not suggest that they're interactable 2. Does not tell you how they're used/what they're there for and 3. Block stuff, but I'm too dumb to properly word that and my brain doesn't function properly at 1:30 am)
There can also be a game menu, so that you aren't directly tossed into the game. On that note, the other waves don't start unless you click a button, so why make Wave 1 an exception?
Instead of making all the combinations available all at once and confusing to the uninitiated (like me), maybe make them collectable over a series of levels? So that they're like skills that a player can choose, then familiarize themselves over the course of a game?
Maybe make an element glow or otherwise indicate that it can be combined?
On a separate note, the game window isn't centered... and as @Iedah pointed out below, the first free tower (Hydrogen) doesn't appear unless you go fullscreen
Wracking my brain, I suddenly remembered another elements game I've played recently (I think it's similar. Don't sue me, sue sleep deprivation.). It's called Fusioneer, if you frequent r/Webgames you might've seen it: http://dreammodule.com/fusioneer
Did you know? Slitherio and Agario can both be found in r/Webgames... ah, the nostalgia...