Hydrogen, helium, ammonia, methane, neon, coal gas and pure nitrogen are all lighter than air. A pocket of air can be made lighter than the surrounding air by heating it.
According to the Underwater Robotics Research Group, gases that are lighter than air have densities that are less than 1.2 kilograms per cubic meter or 1.2 grams per liter. Hydrogen is the lightest gas, with a density of 0.08999 kilograms per cubic meter, although it is also highly flammable. Helium, the second-lightest gas, with a density of 0.1785 kilograms per cubic meter, is commonly used for lifting objects like balloons. Ammonia, methane and coal gas, which is a blend of hydrogen and hydrocarbons, were occasionally used to fill balloons. Pure nitrogen is only 3 percent lighter than air, which consists of 78 percent nitrogen.