Chemistry of Helium (He) - Practice Questions

Multiple Choice Questions (MCQs)

1. Which of the following noble gases has the lowest boiling point?

A) Neon B) Argon C) Helium D) Krypton

Solution: C) Helium

Explanation: Helium has the smallest atomic size and the lowest atomic mass among all noble gases. Consequently, the London dispersion forces (a type of van der Waals force) between helium atoms are the weakest. Overcoming these very weak intermolecular forces requires minimal energy, resulting in Helium having the lowest boiling point (approximately 4.2 K or -269 °C) of any known substance.

2. Helium is preferred over hydrogen for filling balloons because:

A) Helium is lighter than hydrogen. B) Helium has a higher lifting power. C) Helium is non-combustible. D) Helium is cheaper than hydrogen.

Solution: C) Helium is non-combustible.

Explanation: While hydrogen (H₂) is indeed lighter than helium (He) and offers marginally greater lifting power, its highly combustible nature makes it extremely dangerous for use in balloons, airships, and other applications where leakage could occur. Helium, being an inert noble gas, is non-flammable and chemically unreactive, providing a much safer alternative despite being slightly heavier and generally more expensive.

3. The exceptional inertness of Helium is primarily attributed to its:

A) Small atomic size. B) High ionization enthalpy. C) Completely filled valence shell. D) All of the above.

Solution: D) All of the above.

Explanation: Helium’s inertness is a cumulative effect of several factors:

• Completely filled valence shell (1s²): This stable duplet configuration makes it electronically satisfied, requiring no tendency to gain, lose, or share electrons.
• High ionization enthalpy: Due to its small size and stable electron configuration, a significant amount of energy is required to remove an electron from a helium atom.
• Virtually zero electron gain enthalpy: Helium has no tendency to accept an additional electron.
• Small atomic size: This contributes to its high ionization enthalpy and the compact, stable nature of its electron cloud.

Assertion-Reason Questions

1.

Assertion (A): Helium is a monoatomic gas. Reason (R): Helium has a completely filled valence electron shell, making it chemically unreactive. A) Both A and R are true, and R is the correct explanation of A. B) Both A and R are true, but R is not the correct explanation of A. C) A is true, but R is false. D) A is false, but R is true. E) Both A and R are false.

Solution: A) Both A and R are true, and R is the correct explanation of A.

Explanation: Helium exists as individual atoms (monoatomic) because its completely filled 1s² valence electron shell provides exceptional electronic stability. This stability means helium atoms have no inclination to form covalent bonds with other atoms to achieve a stable configuration. The chemical unreactivity mentioned in the reason is precisely why it remains monoatomic.

2.

Assertion (A): A mixture of Helium and Oxygen is used by deep-sea divers. Reason (R): Helium is more soluble in blood than nitrogen. A) Both A and R are true, and R is the correct explanation of A. B) Both A and R are true, but R is not the correct explanation of A. C) A is true, but R is false. D) A is false, but R is true. E) Both A and R are false.

Solution: C) A is true, but R is false.

Explanation: Deep-sea divers use a mixture of helium and oxygen (often with a small amount of nitrogen) to prevent “the bends” (decompression sickness). At high pressures underwater, nitrogen from ordinary air dissolves significantly in the blood and body tissues. Upon rapid ascent, the decrease in pressure causes dissolved nitrogen to come out of solution as bubbles, leading to pain, tissue damage, and potentially fatal conditions. Helium is chosen because it is significantly less soluble in blood than nitrogen, thereby minimizing the formation of bubbles during decompression. Therefore, Assertion (A) is true, but Reason (R) is false.

Short Answer Questions

1. Explain why Helium has a very low tendency to form chemical bonds.

Model Answer: Helium possesses an electronic configuration of 1s². This represents a completely filled valence electron shell (a stable duplet). According to the octet rule (or duplet rule for the first period), atoms tend to achieve such a stable configuration. Because helium already has this exceptionally stable electron arrangement, it has:

1. Very high ionization enthalpy: It requires a large amount of energy to remove an electron.
2. Practically zero electron gain enthalpy: It has no tendency to accept an additional electron.
3. No vacant orbitals of suitable energy to accommodate electrons or participate in bonding. Consequently, helium atoms have no need to gain, lose, or share electrons, which are the fundamental mechanisms for chemical bond formation. This intrinsic electronic stability renders helium chemically unreactive and gives it a very low tendency to form chemical bonds under normal conditions.

2. State two important uses of Helium and provide a scientific reason for each.

Model Answer:

1. Cryogenic Agent: Liquid helium has the lowest boiling point of any element (4.2 K or -269 °C). This property makes it an indispensable cryogenic agent for achieving and maintaining extremely low temperatures.
   • Reason: Its exceptionally low boiling point makes it ideal for cooling superconducting magnets used in Magnetic Resonance Imaging (MRI) scanners, Nuclear Magnetic Resonance (NMR) spectrometers, and particle accelerators, where temperatures close to absolute zero are required to achieve superconductivity.
2. Filling Balloons and Airships: Helium is the second lightest gas after hydrogen and is non-flammable.
   • Reason: Its inertness (non-combustibility) makes it much safer than hydrogen for lifting applications like weather balloons, meteorological balloons, and airships. Although hydrogen is lighter, its extreme flammability poses severe safety risks (as tragically demonstrated by the Hindenburg disaster), making non-reactive helium the preferred choice despite its higher cost and slightly greater density.

High-Order Thinking Skills (HOTS) Question

Despite being significantly lighter and cheaper to produce, hydrogen gas is rarely used for filling weather balloons or airships today, whereas helium is predominantly used. Provide a detailed chemical explanation for this preference.

Detailed Chemical Explanation:

The primary reason for the overwhelming preference for helium over hydrogen in lifting applications like weather balloons and airships, despite hydrogen’s lighter mass and lower production cost, lies in their fundamental chemical properties, specifically their reactivity and flammability.

1. Flammability and Reactivity:

   • Hydrogen (H₂): Hydrogen is a highly flammable gas. It reacts vigorously and exothermically with oxygen in the air upon ignition, forming water vapor: 2H₂(g) + O₂(g) → 2H₂O(g) + Energy (Heat and Light) This reaction can lead to explosive fires, posing an extreme safety hazard. The infamous Hindenburg airship disaster in 1937, though its exact cause is debated, dramatically illustrated the catastrophic consequences of using flammable hydrogen in large-volume aerial vessels.
   • Helium (He): Helium is a noble gas, belonging to Group 18 of the periodic table. Its electronic configuration (1s²) is a completely filled valence shell (duplet), which makes it exceptionally stable and chemically inert. Helium does not react with oxygen, nitrogen, or any other atmospheric component under normal conditions. It is non-combustible and non-flammable.

2. Safety Considerations:

   • In applications where a large volume of gas is contained and there’s a risk of leakage or rupture (e.g., a balloon being struck by lightning, damaged during landing, or simply leaking over time), hydrogen’s flammability presents an unacceptable danger to life and property. Any spark or static discharge could ignite the hydrogen, leading to a rapid and destructive fire or explosion.
   • Helium’s inertness, on the other hand, guarantees safety. Even if a helium-filled balloon ruptures or leaks, there is no risk of combustion or explosion, making it a far safer choice for operations involving human interaction or proximity to populated areas.

3. Lifting Power vs. Risk:

   • While hydrogen (molar mass ≈ 2 g/mol) is indeed lighter than helium (molar mass ≈ 4 g/mol) and thus provides about 8% more lifting power per unit volume, this marginal increase in lift is thoroughly overshadowed by the severe safety risks associated with its flammability. The chemical stability and non-flammable nature of helium are paramount safety advantages that make it the preferred choice, even at a higher cost and slightly reduced lifting efficiency compared to hydrogen.

In essence, the chemical inertness of helium, stemming from its stable electronic configuration, is the decisive factor that outweighs the economic and density advantages of hydrogen, making helium the standard for safe aerostatic applications.