Question

Hydrogen is by far the most abundant element cosmically. In interstellar space, it exists mainly as \(\mathrm{H}_{2}\). In contrast, on Earth, it exists very rarely as \(\mathrm{H}_{2}\) and 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 is abundant in the universe due to its simplicity and formation during the Big Bang. \(\text{H}_{2}\) is scarce in Earth's atmosphere because it escapes Earth's gravity and reacts with oxygen.

Step-by-step solution

Understand Cosmic Abundance

Hydrogen is the simplest and lightest element in the universe, consisting of one proton and one electron. During the Big Bang, conditions favored the formation of hydrogen, leading to its abundance in the cosmos.

Explain Interstellar Hydrogen

In interstellar space, hydrogen mainly exists as \(\text{H}_{2}\), due to the low temperatures and densities. The hydrogen atoms can bond to form diatomic molecules (\text{H}_{2}), especially in molecular clouds.

Abundance in the Earth's Crust

On Earth, hydrogen is the ninth most abundant element in the crust, but it is predominantly found in chemical compounds such as water (H2O), hydrocarbons, and other minerals rather than as diatomic hydrogen gas.

Rarity of Diatomic Hydrogen in Atmosphere

Hydrogen gas \(\text{H}_{2}\) is rare in Earth's atmosphere because it is very light and escapes Earth's gravitational pull into space. Additionally, Earth's atmosphere is rich in oxygen, leading hydrogen to form water via oxidation.

Key concepts

Cosmic Abundance

Hydrogen's cosmic abundance is primarily due to events from the Big Bang. Hydrogen, being the simplest element with just one electron and one proton, was abundantly formed in those early conditions. Since there's so much hydrogen, it fuels stars, and through nuclear fusion in stars, it produces helium and other heavier elements. Moreover, hydrogen makes up around 75% of the universe's elemental mass. This universality in abundance is fundamental for the formation of galaxies, stars, and planetary systems. Without hydrogen, the universe wouldn't look anything like it does today.

Interstellar Hydrogen

Interstellar hydrogen mostly exists as \(\text{H}_2\). In the vast emptiness of space, particularly within molecular clouds, the conditions such as low temperatures and low densities help hydrogen atoms to bond, forming diatomic molecules. These regions are crucial for star formation since the dense clouds can eventually collapse under gravity to create new stars. Molecules like \(\text{H}_2\) are protected from destructive ultraviolet radiation by dust and gas, promoting the persistence of hydrogen in this form across vast cosmic distances. Additionally, interstellar space serves as a reservoir, continuously replenishing hydrogen needed for galactic processes.

Diatomic Hydrogen Rare Earth Atmosphere

Here on Earth, \(\text{H}_2\) as a gas is very rare. Earth's gravity isn't strong enough to hold onto such a light element; hydrogen can escape into space. Besides, on Earth, hydrogen tends not to stay as a free diatomic gas because it reacts readily with other elements. The presence of oxygen means that hydrogen often forms water (\text{H}_2\text{O}) through oxidation. Therefore, Earth's atmosphere is richer in heavier molecules like nitrogen (\text{N}_2\text{O}) and oxygen (\text{O}_2), making diatomic hydrogen very uncommon.

Chemical Compounds of Hydrogen

On Earth, hydrogen is frequently found in chemical compounds rather than in its diatomic form. The most common compound is water (\text{H}_2\text{O}), vital for all known forms of life. Hydrogen also appears in hydrocarbons, which are key components of fossil fuels and organic material. These compounds are essential for energy storage and various biological functions. In minerals, hydrogen can be part of complex silicates and other minerals essential for the crust's structure. Thus, while \(\text{H}_2\) might be rare as a free gas, hydrogen's elemental presence in diverse compounds is critically significant to Earth's chemical makeup.