I have been enabled, prepare for long as fuck rambling
Grian is Hydrogen! The first element for the first winner and game creator. It's a very light gas, fitting for a guy so often depicted with wings, and it is also Extremely Flammable. And explosive. But essential for life! And it's also most of the sun. For the Watcher-isms, hydrogen in its plasma state glows purple.
Scott is Xenon. I imagine reactivity here as corresponding strongly to aggression, and to a lesser degree energy. Scott is one of the calmest and least aggressive players, he very rarely starts shit (although he will absolutely finish it- and xenon isn't completely noble. it's reactive enough to form a few compounds!). Unreactive is his entire playstyle. So of course he's a noble gas. As for why xenon specifically- all noble gases can glow like neon lights do, in different colours, and xenon happens to be blue! More interestingly, xenon gas has anesthetic properties. I like that a lot for the way everyone always just seems to forget about killing him until he's one of the final greens.
Pearl is Mercury! Also a toxic heavy metal, known for connections to insanity ("mad hatter" comes from mercury compounds being used to make hats, so hatters tended to get mercury poisoning and went mad). I just really like the liquid form for her. And pure elemental mercury is actually fairly inert, and not quite as toxic as you may expect- which fits for Pearl's reputation, and the way people think of Scarlet Pearl before the much more chill way she usually is.
Martyn is Chlorine. I don't see him as aggressive in a radioactive way- radioactivity, to me, is strongly connected with death. Martyn is just a little shit. He's reactive. He's toxic. He's green. He's really good at forming salt. But at the same time, chlorine is an excellent disinfectant and essential for life, reflecting his softer side.
Scar is Carbon. Will bond with just about anything, especially itself, and loves to build huge molecules. I think the element of life works very well for our animal loving, terraforming Mr. Goodtimes. Carbon is also known for the many different forms it takes- it's a very versatile element, and Scar can play many different roles. Also, the specific forms it has are also very Scar. Coal for his arson tendencies, graphite for his artistry, and diamonds. Because diamonds.
Joel is Cesium. This one is mostly because it is the most reactive, explosive metal you can get. Cesium is the heaviest alkali metal, and will spontaneously catch fire in air and explode on contact with water. And... yeah. He's like that. He's a very aggressive player. Other interesting cesium facts are that it has a golden colour, unlike most grey/silvery metals, and it's got an extremely low melting point- around 30°C.
And Cleo is Tungsten. A heavy, durable, and extremely stubborn metal. It's hard to kill someone that's already dead, after all. It's also not malleable in the slightest. Cleo is not an easily manipulated person. If that's not enough, tungsten also has the highest melting point of all elements, so she can set as many fires as they want and not get burned. Not particularly reactive, but does definitely form strong bonds.
I have the non-winners assigned as well, but this reblog is getting long, so I'll save those for another time.