Question

Although He is the second most abundant element in the universe, it is very rare on the earth. Why?

Short answer

Helium is rare on Earth due to its lightweight nature, its inability to be retained in minerals and rocks, its tendency to escape Earth's gravitational pull, and its limited production as a byproduct of radioactive decay within the Earth's crust. These factors, combined with its inert properties, result in helium's scarcity on our planet, despite its abundance in the universe.

Step-by-step solution

Understand the properties of helium

Helium is an inert gas with an atomic number of 2. It is a light element, and because it does not readily bond with other elements, it remains mostly in its atomic form. Helium is primarily formed through nuclear fusion processes in stars, like our Sun. This is a key property of helium to remember as we explore its rarity on Earth.

Examine helium's origin on Earth

Helium can be found on Earth in trace amounts, mainly formed as a byproduct of nuclear decay from heavier radioactive elements, like uranium and thorium, found inside the Earth's crust. As these elements decay, they release helium atoms, which eventually make their way to the surface and become part of the Earth's atmosphere. This process is slow, and the amount of helium produced is very small compared to its abundance in the universe.

Discuss helium's escape from Earth's atmosphere

Due to its lightweight nature and relatively high energy, helium atoms can easily escape Earth's gravitational pull when they reach the upper layers of the atmosphere. Once they escape, helium atoms drift off into space, further decreasing the amount of helium available on Earth.

The role of helium in Earth's geological processes

Helium is poorly retained in minerals and rocks; therefore, the geological processes, like the formation of mountains or the movement of tectonic plates, do not incorporate helium into Earth's crust. This property of helium also contributes to its low abundance on Earth.

Conclude the explanation of helium's rarity on Earth

Factors that cause helium to be rare on Earth include its lightweight nature, its poorly-retained property in minerals and rocks, its escape from Earth's gravitational pull, and its limited production through the decay of radioactive elements within the Earth's crust. Despite being the second most abundant element in the universe, these unique properties and processes result in helium's scarcity on our planet.