I'm feeling very sciencey today, probably because on today the 20th of July, the words "that's one step for man, one giant leap for mankind" were uttered (with mistake) by Neil Armstrong. So in this vein of science, this epistle is loosely related.
Roentgenium or Rg is the Nelson's Number of chemical elements. Being 111 it used to have two far nicer names - Unununium and eka-gold. Due to its presence in Group 11 it is a transition metal and so probably would appear as a heavy, solid, shiny metal. One cannot speculate on whether it would be colored like gold or not.
Ununoctium or Uuo as element 118 must have some weird properties. It sits firmly in the Noble Gas column but it's too heavy to be a gas, so there is a distinct possibility that it could in fact be a noble metal. There is another problem, namely the problem of gravity. With 118 protons in the nucleus, at what point will the positive charge be enough for them all to repel each other thus overtaking gravity?
Illudium is a chemical element that exists on Mars. We know for a fact that it has highly explosive properties and that its chief known use is in the manufacture of Intergalatic Explosive Space Modulators. At least one Intergalatic Explosive Space Modulator has been trained upon the Earth because it obstructs the view of Venus from Mars.
Another element that bears mention is Phosdex, more commonly known as the "shaving cream atom" and only found on Planet X.
Common hydrogen has one proton, one electron, and no neutrons. Deuterium and tritium have, respectively, one and two neutrons per atom, and are used for hydrogen bombs. Quadium, following logic, must have three neutrons, and is, in the story, capable of blasting an entire continent off the face of the Earth, at least according to The Mouse That Roared.
and lastly... Davidtron.
A question was posed in my high school Physics class when someone on the same desk as me postulated that all unobserved matter in the universe could be explained via Davidtron.
Davidtron is an unobservable chihuahua shaped particle that only exists when it is unseen. If you look at a wall for instance, everything on the other side of the wall must be made of Davidtron. Thus because I can't see you as you're reading this, you are therefore made of Davidtron by my observation. If a tree falls in the woods and there is no-one around to hear or see it, then it must be made of Davidtron.