Here's why:

* Atomic Mass: Hydrogen has the lowest atomic mass of all elements. This means that individual hydrogen atoms are very light.

* Molecular Structure: Hydrogen exists as a diatomic molecule (H2), meaning two hydrogen atoms bond together.

* Low Molecular Weight: The combined low atomic mass of hydrogen atoms results in a very low molecular weight for the H2 molecule.

These factors contribute to hydrogen having the lowest density among all gases under standard conditions.