Question

Hydrogen is by far the most abundant element cosmically. In interstellar space, it exists mainly as${\mathbf{H}}_{\mathbf{2}}$In contrast, on Earth, it exists very rarely ${\mathbf{H}}_{\mathbf{2}}$asand is ninth in abundance in the crust. Why is hydrogen so abundant in the universe? Why is hydrogen so rare as a diatomic gas in Earth’s atmosphere?

Short answer

Hydrogen leaves the Earth's atmosphere due to its low density, making it more plentiful in space and extremely rare as a diatomic gas in the atmosphere.

Step-by-step solution

Atmosphere

The gases that surround a star or planetary body are held in place by gravity and are referred to as the atmosphere. If gravity is high and the temperature of the atmosphere is low, a body is more likely to keep an atmosphere over time.

Explanation

Low-mass atoms like hydrogen and helium were created during the Big Bang, the beginning of life as we know it. We now know that hydrogen makes up roughly $73$ percent of the visible universe.

Since ${\mathrm{H}}_2$ is lighter than air and has a lower density, it "escapes" the atmosphere, which is why it isn't as common on Earth as it is in space.

It's still one of the abundant elements on the planet in compound state. It is found in so many vital molecules, such as water ${\mathrm{H}}_2\mathrm{O}$carbohydrates, amino acids, and so on.

Therefore, hydrogen has low density which makes it escape the atmosphere, that makes it more common in space and extremely rare as a diatomic gas in the atmosphere.