Question

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

Short answer

In conclusion, helium is rare on Earth due to its properties and behavior that cause it to easily escape our planet's atmosphere, its limited production through radioactive decay, and its increasing industrial use and consumption. Despite being the second most abundant element in the universe, these factors make helium concentrations on Earth far lower than those found in the cosmos.

Step-by-step solution

Understand helium abundance in the universe

Helium is the second most abundant element in the universe, after hydrogen. It accounts for roughly 24% of the total elemental mass of the universe. The abundance of helium is mainly due to the nuclear fusion processes occurring within stars. During the fusion process, hydrogen atoms combine to form helium atoms, which release large amounts of energy. This energy is what powers stars and causes them to shine.

Discuss helium's characteristics

Helium is a noble gas and has several unique properties. It is the lightest among the noble gases and has the lowest boiling and melting points of any element. Helium is chemically inert, which means it does not react with other elements or compounds under ordinary conditions. One of helium's key properties is that it is a monatomic gas and does not form any stable molecules with other elements.

Explain why helium is rare on Earth

There are several reasons why helium is rare on Earth, even though it is the second most abundant element in the universe: 1. Earth's atmosphere: Helium is lighter than the other gases that make up the Earth's atmosphere. As a result, helium atoms can easily escape the Earth's atmosphere and dissipate into outer space. 2. Production on Earth: Unlike in stars, the conditions necessary to produce helium through nuclear fusion do not exist within Earth. The primary source of helium on Earth comes from the radioactive decay of heavy elements found underground. However, this process produces helium at a relatively slow rate compared to its escape into space. 3. Industrial uses and consumption: Helium has various applications, such as in cryogenics, high-energy particle research, and medical imaging. Due to these uses and its scarcity, the extraction and consumption of helium have increased over time, further reducing its availability.

Summarize the answer

In conclusion, the rarity of helium on Earth can be attributed to its properties and behavior, which cause it to escape our planet's atmosphere easily, its limited production through radioactive decay, and its increasing industrial use and consumption. Despite being the second most abundant element in the universe, these factors make helium concentrations on Earth far lower than those found in the cosmos.

Key concepts

Element Abundance

Helium is incredibly abundant in the universe, making up roughly 24% of its elemental mass. This high percentage places helium as the second most abundant element, just after hydrogen. Its abundance is primarily a result of its formation through nuclear fusion in stars. When hydrogen atoms collide and fuse in the intense heat of stars, they form helium. This process releases a significant amount of energy, contributing to a star's luminosity.
However, despite its abundance in the universe, helium is quite rare on Earth. This discrepancy arises due to helium's lightweight nature that allows it to escape into outer space. Unlike heavier gases, helium doesn't collect near the planet's surface or remain in the atmosphere for long periods.
Interestingly, the rarity of helium on Earth highlights the unique conditions of our planet's atmosphere and geological processes, which do not extensively produce helium as stars do.

Noble Gases

Helium belongs to a group of elements known as noble gases. This group also includes neon, argon, krypton, xenon, and radon. Noble gases are characterized by their lack of reactivity with other elements.
Helium is the lightest of all noble gases and holds the lowest boiling and melting points amongst the elements. Because it is chemically inert, helium does not form compounds with other elements under normal conditions. This characteristic makes it particularly stable and safe to use in various applications, such as in filling balloons or in cryogenics.

• Inert Nature: Noble gases do not react easily with other substances due to their full valence electron shells.
• Isolation Challenge: The non-reactivity of noble gases means they do not readily form compounds, making them harder to isolate and extract.

Nuclear Fusion

Helium's cosmic abundance is a direct result of nuclear fusion processes taking place in stars. Nuclear fusion is the process where lighter atomic nuclei combine to form heavier nuclei, releasing energy in the process. This is the same energy that powers our sun and other stars.
In stars, nuclear fusion begins with hydrogen atoms fusing to create helium. As the atoms collide under high temperatures and pressures, they overcome repulsive forces, merge, and release a burst of energy. This chain reaction not only powers the stars but also contributes to the synthesis of new elements in the universe.
Fusion is different from nuclear fission, which occurs when a heavy nucleus splits into lighter nuclei. Fusion has the potential to become a more sustainable and powerful energy source. However, replicating the intense conditions required for fusion on Earth presents significant scientific challenges.

Radioactive Decay

On Earth, a limited amount of helium is produced through the radioactive decay of certain elements deep underground. Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting radiation. In this process, heavy elements such as uranium and thorium break down over time, eventually forming helium as a byproduct.
The helium produced this way accumulates in natural gas fields, trapped between layers of rock. When natural gas is extracted, helium can be isolated and collected to be used for various purposes, such as in medical imaging and as a cooling agent.
Despite this natural production method, the rate of helium generation on Earth is relatively slow, further contributing to its scarcity compared to its cosmic presence. This slowness is linked to the extended half-lives of radioactive materials and the slow movement of geological processes.

Earth's Atmosphere

Earth's atmosphere is composed predominantly of nitrogen and oxygen, with trace amounts of other gases including carbon dioxide, argon, and helium. Helium's presence in our atmosphere is minuscule compared to its universal abundance.
Its rarity here is primarily due to its low atomic weight, which allows helium atoms to rise and escape into outer space. Since helium is lighter than the heavier gases that make up most of the atmosphere, it doesn't stay on Earth for long before it escapes.

• Gravity's Role: Earth's gravity is insufficient to retain the lighter helium atoms, making it difficult for the planet to hold onto this gas over long timescales.
• Surface Production: Unlike other elements, there is no considerable surface production of helium on Earth that could contribute to maintaining its presence in the atmosphere.

This constant loss, combined with minimal production through radioactive decay, explains why helium is scarce in Earth's atmosphere despite being plentiful in the vast universe.