Hydrogen and helium are gases, existing in a gaseous state under normal atmospheric conditions, often referred to as standard temperature and pressure (STP). These two elements, found at the top of the periodic table, are the lightest elements in the universe. Their properties as gases are directly linked to their simple atomic structures and the weak forces of attraction between their atoms or molecules. These characteristics determine their unique behaviors and practical uses across science and industry.
The Atomic Reasons for Their Gaseous State
The reason hydrogen and helium remain gases stems from their minimal atomic mass and the nature of their intermolecular forces. All matter is held together by attractive forces, but the forces acting between individual atoms or molecules of hydrogen and helium are exceptionally weak. These attractions are primarily London dispersion forces, which arise from temporary fluctuations in electron distribution.
Helium atoms are monatomic with only two electrons, resulting in extremely weak dispersion forces. Molecular hydrogen (H₂), though diatomic, also possesses a very small mass and few electrons involved in intermolecular interactions. Since these attractive forces are slight, very little energy is required to overcome them and keep the atoms or molecules separated.
To force these atoms or molecules to condense into a liquid, the temperature must be lowered significantly, reducing their kinetic energy. Helium has the lowest boiling point of any element, liquefying at about 4.2 Kelvin (–268.95 °C) at standard pressure. Hydrogen liquefies at a slightly higher temperature, around 20.28 Kelvin (–252.87 °C). This requirement for temperatures approaching absolute zero ensures they remain gaseous in virtually every natural environment on Earth.
Distinct Properties of Hydrogen and Helium Gases
Once established as gases, hydrogen and helium exhibit profoundly different chemical and physical properties. Hydrogen gas (H₂) exists as a diatomic molecule, meaning two hydrogen atoms are chemically bonded together. It possesses a single valence electron per atom, making it highly reactive and combustible, reacting vigorously with oxygen to form water.
In contrast, helium (He) is a noble gas, existing as a single, monatomic atom with a full outer electron shell, which makes it chemically inert. This complete electron shell means helium does not readily form chemical bonds, rendering it completely non-flammable and stable. The two gases are also the lightest known, with hydrogen gas having a density of approximately 0.09 grams per liter (g/L) at STP, making it the least dense substance.
Helium is the second lightest, with a density of about 0.179 g/L at STP. Both gases are substantially lighter than air, which has a density of about 1.28 g/L. Although hydrogen is lighter and provides more lift, helium’s lack of chemical reactivity gives it an advantage where safety is a concern. The difference between hydrogen’s combustion and helium’s inertness is their most important distinction.
Common Applications Stemming from Their Gaseous Nature
The unique properties of these gases lead to a variety of practical applications. Hydrogen’s reactivity and low mass are harnessed as a powerful energy carrier, particularly in fuel cells that convert the gas into electricity through a chemical reaction with oxygen. This high energy density makes it an effective rocket propellant and a reducing agent in industrial processes, such as producing “green steel”.
Helium’s applications are largely based on its inertness and extremely low boiling point. Its low density provides buoyancy for weather balloons and airships. The gas is frequently used for leak detection in high-pressure systems because its small, non-reactive atoms can diffuse quickly through tiny openings.
Helium’s exceptionally low temperature of liquefaction makes it indispensable for cryogenics. Liquid helium is used to cool the superconducting magnets in Magnetic Resonance Imaging (MRI) scanners and advanced scientific research equipment. Its inert nature also makes it useful for creating protective atmospheres in processes like welding and semiconductor manufacturing, preventing contamination from reactive gases like oxygen.