In its natural state, uranium is actually only mildly radioactive. You might not want to make and wear jewelry out of this incredibly dense metal, but overall, natural uranium is nothing to lose sleep over. Now, if a good portion of the isotopes of that grayish chunk of uranium you are playing with happen to be uranium-235, then you might want to set it down and back away, because that there is fissile material. U-235 is the same type of uranium used in the nuclear bomb dropped over Hiroshima, in fact, and yes, it can occur naturally. There is even evidence in the early geology of the earth, back when things were a bit hotter (meaning back when this place was a seething ball of lava) of naturally occurring nuclear explosions.
You know that rather stinky scent your stove or grill gives off when the gas if flowing but the burner is not yet lit? That’s methane, kids! But guess what, that’s not what methane smells like, not naturally, anyway. Your thoughtful natural gas provider adds in compounds to give this wildly flammable gas an easily detected odor, because in its natural state, methane is colorless and odorless. Methane is highly abundant in nature, thus its ever more common use as a fuel source. Chances are good that there is plenty of methane flowing into your home right now, in fact. And ideally it’s the stinky kind from the gas company and not leaking out of the ground undetected…
Here’s a tip: don’t bring any cesium to your next water balloon fight or beach party. Why, you ask? That’s because this alkali metal is highly explosive when it comes into contact with H20. That’s right, when cesium touches water, it explodes. Oh, and don’t bring any cesium to your non-aquatic-themed gathering, either. Cesium can also auto-ignite and explode when it comes into contact with… air. In its purest, refined state, cesium is a silvery metal usually existing in liquid form (much like mercury) as it melts at a few degrees above room temperature.
Why must the most common element in the universe also be so very explosive? Hydrogen, in many of its innumerable forms, such as when present in molecules of water or hydrogen peroxide, is stable and really quite lovely, allowing life as we know it to persist. In its purest form, though, or in one of its hundreds of unstable compounds, is a ticking time bomb with a hundred different fuses just waiting to set it off. Look no further than the 1837 Hindenburg disaster wherein a zeppelin filled with pure, lighter-than-air hydrogen caught fire and erupted into a flaming ball of scorched humanity.
The good news is that you will never see enough francium in one place to cause a problem. At any given time, less than an ounce of this wildly reactive element exists anywhere on the whole planet. Francium is so volatile that it has a half-life of only about twenty minutes! That means that as soon as a few isotopes of this strange alkali metal have been created due to the decay of other elements, such as uranium, or if it has been synthesized in a laboratory setting, francium immediately begins to vaporize, destroying itself in a hot, violent fit.