No, it is not theoretically possible to have a gas that is lighter than hydrogen under normal conditions. Hydrogen is the lightest element and has the lowest atomic mass of any element, so any gas composed of lighter elements or isotopes would not exist naturally.
The atomic mass of hydrogen is approximately 1 atomic mass unit (amu), which is the lowest possible value for an element. This is because atomic mass is determined by the number of protons and neutrons in an atom's nucleus, and hydrogen has the fewest of both. Any hypothetical element with fewer protons or neutrons would not be stable and would rapidly decay into hydrogen.
Moreover, hydrogen gas (H2) is the lightest gas at room temperature and atmospheric pressure because its molecular mass is the lowest among all gases. The hydrogen molecule consists of two hydrogen atoms, each with an atomic mass of approximately 1 amu, resulting in a molecular mass of 2 amu. No other naturally occurring gas has a lower molecular mass.
However, it is worth noting that in extreme conditions, such as in laboratories or specialized experiments, exotic particles like positronium or muonium might be created, which have lower masses than hydrogen. But these are not naturally occurring gases and exist only under specific circumstances.