All matter exists in one of four different states or phases: solid, liquid, plasma, or gas. Gases each have unique physical and chemical properties. Some are colored and others invisible. Some are harmless and others toxic. Some are heavier and others lighter than air. As this video demonstrates, some gases are inert (or "boring") and others violently reactive. Let's take a closer look at two of these properties.

Whether a gas is heavier or lighter than air depends on its density in relation to that of air. Lighter-than-air gases have less molecular mass than the average molecular mass of air (28.97). Hydrogen and helium are the lightest gases. They have a molecular mass of 2 and 4, respectively. Even though helium is twice as heavy as hydrogen, it is still far lighter than air. And because helium is not combustible, it is used more frequently for filling party balloons, hot-air balloons, and airships.

So what makes one gas non-reactive and another reactive? In chemistry, a reaction happens when two or more molecules interact and form a chemical bond between atoms. This in turn forms a new chemical compound. The types of reactions that occur between substances are determined by atomic structure and, more specifically, by electron configurations.

The reason that atoms react with each other is to reach a state in which their outermost electron shells, or valence shells, are full. Hydrogen and helium only have electrons in their first electron shells. The capacity of this shell is two. Thus helium, which has two electrons, already has a full valence shell and falls into the group of inert elements called noble gases. But hydrogen only has one electron, and thus readily reacts with oxygen.

If you put pure hydrogen gas (H₂) and pure oxygen gas in a room, the atoms will bond to form water, albeit very slowly. But add a spark—or in the case of the video, a flame—and those gases create an explosive reaction. When heat released in a reaction cannot escape, as with a contained space like the inside of a balloon, the rate of the reaction increases rapidly, and an explosion may result.

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