Helium is a colorless, odorless, and non-toxic gas, classified as a noble element because its atoms naturally possess a full outer shell of electrons. Helium does not react with other elements or produce light under normal conditions. The gas only glows when subjected to a powerful energy source, typically a high voltage electric current inside a sealed glass tube. This electrical energy creates a plasma, an excited state where the helium atoms are energized, causing them to emit visible light.
The Observed Color of Energized Helium
The light produced by pure helium in a low-pressure discharge tube is a distinctive and warm hue. When viewed directly, the combined light appears as a pale pink, salmon, or peach-orange color. This coloration is not a single, pure color but the visual blending of several distinct wavelengths emitted simultaneously by the excited atoms. The pink-orange designation is commonly used in laboratory settings.
The Atomic Mechanism for Light Emission
The mechanism that generates this glow involves the interaction of electrical energy and atomic structure. When a high voltage is applied across the gas, free electrons within the tube are accelerated, forming an electrical current. These fast-moving electrons collide with the neutral helium atoms, transferring energy to them. This energy transfer causes the helium atom’s own electrons to jump from their stable, low-energy orbitals to higher, more energetic orbitals, a state known as excitation.
The excited state is unstable, and the electron naturally seeks to return to a lower, more stable energy level. To shed the excess energy, the electron drops back down to a lower orbital, releasing the energy difference in the form of a photon, which is a particle of light. The specific amount of energy released corresponds exactly to the difference between the two energy levels, determining the wavelength, and thus the color, of the emitted photon. This continuous cycle creates the steady, visible glow of the helium plasma.
Components of the Helium Emission Spectrum
The pink-orange color perceived by the human eye is a composite of many individual, discrete colors that make up the element’s emission spectrum. Every element possesses a unique spectral “fingerprint,” which is a distinct set of wavelengths it emits when energized. For helium, this spectrum includes strong lines that span across the visible spectrum, from deep red to violet.
A particularly intense line in the helium spectrum is found in the yellow region, specifically at a wavelength of 587.6 nanometers. Other significant contributors are found in the red, green, and blue-violet areas of the visible light range. The dominance of the strong red and yellow spectral lines biases the overall combined light toward the warm, pink-orange appearance.
Factors Influencing the Visual Appearance
Several external variables can alter the visual appearance of the helium glow. The purity of the helium gas is a significant factor, as even trace amounts of other gases like nitrogen or neon will introduce their own distinct spectral lines into the emission. Since the color is a blend of all emitted wavelengths, the presence of an impurity can shift the overall perceived color, making the light appear slightly more red or violet.
The pressure of the gas within the discharge tube also plays a role in the resulting color. At higher pressures, the color near the electrodes can sometimes shift toward a violet hue. This change is due to complex atomic interactions, including the formation and emission from transient diatomic helium molecules (\(He_2\)), which produce a different set of spectral bands than the single helium atoms. The specific electrical current and voltage applied can also influence the intensity distribution of the spectral lines and the final color.