Hydrogen and Helium are the first two elements that are found in the periodic table of elements. Therefore, they are the smallest and lightest atoms on earth. Both of them are gaseous substances. Due to the characteristic features of hydrogen and helium, there are many applications of these gases in industry. Due to the very light weight of these gases, they are used to fill air balloons. The main difference between helium and hydrogen is that helium atom exists as a monoatomic gas in the atmosphere whereas hydrogen exists as a diatomic gas in the atmosphere.