If you remember the periodic table well, you will easily guess the right answer to this question, as hydrogen holds the first position of the table due to its features. It has the element symbol H and atomic number 1. Hydrogen's Atomic Weight: 1.00794. This makes hydrogen the lightest element. Hydrogen is a colorless and odorless gas with the lowest density of all the gases. It is so light, the pure element isn't bound by Earth's gravity. So, there is very little hydrogen gas left in the atmosphere.

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