Hydrogen gas (H2) molecules contain two hydrogen atoms and hydrogen is the lightest of all the known elements. A molecule of carbon dioxide contains one carbon atom and two oxygen atoms, each of which is heavier than a hydrogen molecule. The difference in the weights of hydrogen and carbon dioxide can be demonstrated by filling two balloons, one with hydrogen gas and one with carbon dioxide gas. The hydrogen-filled balloon will float up into the sky when released while the balloon filled with carbon dioxide will fall to your feet.
