The question of why hydrogen is explosive is a very good. There are a number of examples of this, from the smallest of scales (experiments in class where a flame is held to H₂ gas resulting in a really loud pop) to the largest (think of the Hindenburg Disaster).

Hydrogen is stable in the molecular form as H₂, but that bond is not all that strong. Because of that, hydrogen can reduce its energy by bonding with a lot of other atoms, many of which are present in air (like oxygen).

When hydrogen bonds with the other atoms, this requires an energy input to break the initial bond between the hydrogen atoms (like a flame). When the bonds form between the other atoms and the now separated hydrogens, that reaction proceeds very quickly and a lot of energy is released, resulting in an explosion! Ultimately, the reason hydrogen is so explosive is that it is very happy bonding with other atoms that are very present in air, meaning that if enough energy can be provided to break the bonds between hydrogen atoms, there will be a sudden and significant release of energy as new bonds form.

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It's not really explosive, so much as it just burns. Hydrogen combines with oxygen in the air to make water vapor, which releases a tremendous amount of heat that can result in an explosion if it's in a confined space.

I am a scientist in this science forum and here are some answers from my colleagues:

1. Hydrogen is very explosive, when there is something to react with. Hydrogen react with oxygen vigorously to produce energy that is explosives. Hydrogen will not explode if it doesn’t contact with element that reacts vigorously with it like nitrogen, although they react to form ammonia but they to not react with each other easily. Air contains about 28% of oxygen so hydrogen reacts with oxygen in the air to explode. But hydrogen rarely exist in the air because it is very reactive it reacts with the oxygen in the air to form water
2H₂ + O₂ → 2H₂O.

2. Actually, hydrogen is not explosive at all. Pure hydrogen in a container will not engage in any any kind of reaction, and certainly not an explosive one. Hydrogen is very flammable, however. Like other flammable substances, hydrogen is completely non-reactive (in regard to combustion,)unless it is in contact with oxygen (or another oxidizer.)

What people really mean when describing hydrogen as “explosive” is that it forms very reactive mixtures with oxygen. This means that it forms reactive mixtures with air as well, since air is about 21% oxygen. It is these mixtures that are called “explosive.” It is common to call rapidly burning gas mixtures “explosive.” (I personally have a strong dissenting opinion about this. I believe that the word explosive is used too loosely.) In everyday speech, it very common to call highly flammable materials “explosive,” but this is very wrong from a technical viewpoint.

There is no doubt that hydrogen has a strong reputation for being “explosive.” Two possible reasons for this are; One, hydrogen forms flammable mixtures with air in an unusually wide range of concentrations. Secondly, air-hydrogen mixtures have some of the greatest flame speeds among flammable gas mixtures (these speeds are far lower than could be accurately called “explosive,” though).