Which of the following is the lightest substance?

Hydrogen is universally recognized as the lightest substance and the simplest element in the periodic table. Its atomic number is 1, which means it has just one proton in its nucleus and one electron revolving around it. The standard atomic weight of Hydrogen is 1.008 atomic mass units (u), making it lighter than all other elements and substances.

This property makes Hydrogen unique in both Chemistry and Physics. It is a colorless, odorless, and tasteless gas at room temperature. Being the lightest element, it easily escapes Earth’s gravitational pull, which is why very little free hydrogen is found in our atmosphere. However, it is the most abundant element in the universe, accounting for about 75% of all elemental mass, mainly in stars and interstellar space.

Hydrogen plays a vital role in chemical reactions. It forms water (H₂O) when combined with oxygen and contributes to many organic compounds that sustain life. It also exhibits isotopes such as protium, deuterium, and tritium, which are essential in nuclear reactions and scientific research.

Compared with the other options:

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  Uranium is one of the heaviest naturally occurring elements, far heavier than Hydrogen.

  
  


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  Oxygen has an atomic mass of 16 u, making it much heavier.

  
  


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  Nitrogen has an atomic mass of 14 u, still far heavier than Hydrogen.

  
  

Because of its lightness, Hydrogen is used in balloons, airships, and as a rocket fuel in liquid form. Moreover, it is also gaining attention as a clean energy carrier in the modern era, especially in hydrogen fuel cells that produce electricity without harmful emissions.

In conclusion, Hydrogen is the lightest substance known in nature and plays a central role in Chemistry, Astronomy, and clean energy technology.