Problem 69 - Chemistry An Atoms First Approach

Question

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

Step-by-Step Solution

Answer

Helium is rare on Earth due to its light mass, non-reactive nature, and limited sources. Its low mass allows it to easily escape Earth's gravity and makes containment in the atmosphere difficult. Being non-reactive, helium doesn't form compounds in Earth's crust. It can primarily be extracted from natural gas reservoirs as a byproduct of radioactive decay, but these sources are limited.

Step 1: Understand the background of helium

Helium is a noble gas with the atomic number 2. It is a light, non-toxic, and inert element that does not easily react with other elements. In the universe, helium is primarily formed during nuclear fusion reactions in stars, a process called nucleosynthesis.

Step 2: Compare helium's abundance in the universe and on Earth

Helium is the second most abundant element in the universe, making up approximately 25% of its mass. However, helium only makes up 0.00052% of the Earth's atmosphere by volume and is even scarcer in the Earth's crust.

Step 3: Identify helium's properties that lead to its rarity

The reason for helium's rarity on Earth is due to its light mass and non-reactive nature. Due to its lightness, helium has a low boiling point and can easily escape Earth's gravity. In addition, the Earth's atmosphere cannot easily contain helium as it rises to the upper layers and eventually escapes into space.

Step 4: Discuss other sources of helium on Earth and their limitations

On Earth, helium is primarily extracted from natural gas reservoirs as a byproduct of the radioactive decay of heavier elements. However, these sources are limited, leading to a scarcity of helium on our planet.

Step 5: Conclusion

Despite being the second most abundant element in the universe, helium is rare on Earth due to its low mass, non-reactive nature, and limited sources available. Its light mass allows it to easily escape Earth's gravity and makes it difficult to be contained in the atmosphere, while its non-reactive property minimizes its presence in compounds found in the Earth's crust.