Here's why:

* Molar Mass: Carbon dioxide (CO2) has a molar mass of 44 g/mol, while helium (He) has a molar mass of 4 g/mol. This means that a single molecule of carbon dioxide is significantly heavier than a single atom of helium.

* Density: The density of a gas is related to its molar mass. Since carbon dioxide has a higher molar mass, it will have a higher density than helium under the same conditions of temperature and pressure.

Therefore, carbon dioxide is denser and heavier than helium. This is why helium balloons float, while carbon dioxide would sink in air.