Which element do we breathe in to stay alive?

Oxygen, about a fifth of the air. Almost every cell uses it to burn sugar, and the reason it works is that oxygen is extremely reactive - it pulls electrons hard, and the energy released in the process is what your body runs on.

That reactivity is also why it is dangerous. Oxygen corrodes, oxidises and burns, and the same chemistry that powers you damages you: a fraction of it becomes reactive fragments that attack DNA and proteins, and mopping those up is a continuous internal maintenance job. Breathing pure oxygen at pressure is toxic and causes convulsions, which is why deep divers breathe mixtures with less of it, not more.

There was almost none of it in the air for the first half of Earth's history. Cyanobacteria began producing it around two and a half billion years ago and it accumulated into what is sometimes called the Great Oxidation Event - a catastrophe for almost everything alive at the time, since to them it was poison. Every animal is descended from the survivors of the worst pollution incident in the planet's history.