Helium (He)

Helium has a completely filled 1s shell with electronic configuration \(1s^2\). This closed-shell structure gives it a very high first ionization energy \(\approx 24.6\,\text{eV}\) and zero tendency to gain or lose electrons.

As a result, \(\mathrm{He}\) forms no stable compounds under ordinary conditions and is placed in Group 18 (noble gases).

Helium atoms experience extremely weak London dispersion forces due to their small, spherically symmetric electron cloud. Thus, the temperature at which thermal energy overcomes intermolecular attraction is very low.

Consequently, helium’s normal boiling point is only \(T_b \approx 4.22\,\mathrm{K}\). At 1 atm, helium does not solidify even at \(0\,\mathrm{K}\); it requires applying pressure (about tens of atmospheres) to form a solid.

Isotopes:

• \(^4\!\mathrm{He}: 2 protons + 2 neutrons (boson). It is the common isotope and shows superfluidity below the lambda point.
• \(^3\!\mathrm{He}: 2 protons + 1 neutron (fermion). It becomes superfluid only at much lower temperatures via Cooper pairing of \(^3\!\mathrm{He}\) atoms.

Spin statistics (boson vs fermion) lead to different quantum behaviors at ultra-low temperatures.

Below the lambda temperature \(T_\lambda \approx 2.17\,\mathrm{K}\), liquid \(^4\!\mathrm{He}\) enters the superfluid phase (He II) with remarkable properties:

• Effectively zero viscosity (can flow through extremely fine pores).
• Thermal conductivity far greater than ordinary liquids.
• Fountain effect and thin film creep (it climbs container walls).

These arise from macroscopic quantum behavior described by Bose–Einstein statistics.

Helium gas is less dense than air. The buoyant force \(F_b\) on a balloon is approximately \(F_b = (\rho_{\text{air}} - \rho_{\text{He}})\,V\,g\), where \(\rho\) is density, \(V\) is volume, and \(g\) is gravitational acceleration. Because \(\rho_{\text{air}} > \rho_{\text{He}}\), the net upward force makes the balloon rise.

Why higher pitch? Sound travels faster in helium than in air due to its lower molar mass. This shifts the resonant frequencies in your vocal tract, making the timbre brighter and seemingly higher.

Safety: It can be dangerous. Helium displaces oxygen and may cause dizziness, hypoxia, or fainting. Never inhale from pressurized cylinders; avoid repeated or prolonged inhalation.

Key applications include:

• Cryogenics: Liquid helium (\(\approx 4.2\,\mathrm{K}\)) cools superconducting magnets in MRI, NMR, and particle physics.
• Inert atmosphere: Shielding gas in welding; leak detection due to small atomic size.
• Pressurizing & purging: In aerospace and rocketry.
• Breathing mixes: Heliox for deep diving to reduce nitrogen narcosis.

An alpha particle is essentially a helium-4 nucleus: \(\alpha = {}^{4}\!\mathrm{He}^{2+}\) (2 protons, 2 neutrons, no electrons). Many radioactive decays emit \(\alpha\)-particles, which can capture electrons to become neutral \(^4\!\mathrm{He}\).

In stellar cores (like the Sun), hydrogen nuclei fuse to form helium via the proton–proton (pp) chain:

\(4\,\mathrm{p} \;\rightarrow\; {}^{4}\!\mathrm{He} + 2\,e^{+} + 2\,\nu_{e} + \gamma + \approx 26.7\,\mathrm{MeV}\)

This nuclear fusion releases the energy that powers stars.

Yes. Helium on Earth is produced mainly by radioactive decay in crustal rocks (\(\alpha\)-emission) and accumulates in natural gas reservoirs. Once released, helium can escape Earth’s gravity over time due to its low mass.

Conservation matters: Recycling in labs and MRI facilities and careful handling reduce waste and help manage the limited terrestrial supply.