Here's why:

* Hydrogen is highly reactive and readily combines with oxygen to form water, releasing a large amount of energy in the process. This is why it's used as a fuel.

* Chlorine is a highly reactive gas but not flammable in the traditional sense. It can support combustion, meaning it can help other things burn, but it doesn't burn itself.

* Oxygen is a key component of combustion, but it doesn't burn on its own.

* Neon and nitrogen are both inert gases, meaning they are not flammable and don't readily react with other substances.