Helium produces more emission lines than hydrogen because its two electrons create a far more complex set of energy levels. Hydrogen, with just one electron, has a relatively simple energy structure where levels depend almost entirely on one number: how far the electron is from the nucleus. Helium’s second electron introduces electron-electron interactions, spin-state splitting, and shielding effects that multiply the number of distinct energy levels and, therefore, the number of possible transitions that produce spectral lines.
Why Hydrogen’s Spectrum Is Simple
In a hydrogen atom, the single electron orbits a single proton. The energy of each level depends only on the principal quantum number (n = 1, 2, 3, and so on). This means that within a given energy level, subshells like 2s and 2p have the same energy. Physicists call this “degeneracy,” and it’s a defining feature of one-electron systems.
Because so many subshells share the same energy, the number of distinct transitions between levels is limited. Hydrogen’s visible spectrum, the famous Balmer series, consists of just four lines that the eye can see. Its full spectrum across all wavelengths includes more, but the overall pattern is orderly and sparse compared to multi-electron atoms.
How a Second Electron Changes Everything
Helium has two electrons and a nucleus with a charge of +2. Neither electron experiences the full pull of that nuclear charge because each one partially shields the other from the nucleus. This shielding effect depends on the shape of the orbital the electron occupies. An electron in an s orbital (spherical, close to the nucleus) penetrates closer to the core and feels a stronger effective nuclear charge than an electron in a p orbital (lobed, extending farther out).
This breaks the degeneracy that hydrogen enjoys. In helium, the 2s and 2p orbitals no longer have the same energy. The same is true for 3s, 3p, and 3d, and so on up the ladder. Each subshell now sits at a slightly different energy, creating more distinct rungs on the energy ladder. More rungs means more possible jumps between them, and each jump can produce a spectral line at a unique wavelength.
Singlet and Triplet States Double the Structure
The most dramatic source of extra emission lines comes from how the two electrons’ spins interact. In helium, one electron typically stays in the ground-level 1s orbital while the other gets excited to a higher level. The spins of these two electrons can align in two fundamentally different ways.
When the spins point in opposite directions, the total spin is zero. This is called a singlet state, and helium in this configuration is known as parahelium. When the spins point in the same direction, the total spin is one. This is a triplet state, and helium in this configuration is called orthohelium. The triplet label comes from the fact that there are three possible orientations for this combined spin.
Here’s what matters for the spectrum: singlet and triplet versions of the same orbital configuration have different energies. In the triplet state, the requirement that the overall wavefunction obey the Pauli exclusion principle forces the two electrons to stay farther apart on average. This reduces their mutual repulsion and lowers the energy compared to the corresponding singlet state. The result is that for every excited configuration in helium, there are two distinct energy levels instead of one.
As McMaster University’s description of the helium spectrum puts it, there are two series of lines observed for helium for every single series of lines observed for hydrogen. Early spectroscopists actually thought they were looking at two different elements before realizing that parahelium and orthohelium were just two faces of the same atom.
Selection Rules and Forbidden Transitions
Not every possible jump between energy levels actually produces an observable line. Quantum mechanics imposes selection rules that govern which transitions are “allowed” (strong and easily observed) and which are “forbidden” (extremely weak or absent). For helium, the primary allowed transitions are electric dipole transitions, which require specific changes in the electron’s orbital angular momentum.
One important rule in helium is that transitions between singlet and triplet states are strongly suppressed. Because helium closely follows a coupling scheme where orbital and spin angular momentum are handled separately, jumping between the two spin systems is nearly forbidden. These “intercombination lines” do exist but are extremely weak. This is why parahelium and orthohelium behave almost like independent sets of energy levels, each producing its own series of spectral lines.
Even with this restriction, the sheer number of allowed transitions within each spin system is large. The combination of split subshells and two independent spin systems gives helium a far richer emission spectrum than hydrogen’s.
Putting It All Together
Three effects stack on top of each other to give helium more emission lines than hydrogen:
- Lifted degeneracy: Electron shielding makes s, p, d, and f orbitals within the same shell sit at different energies, creating more distinct levels to transition between.
- Spin-state splitting: Every excited configuration splits into a singlet and a triplet level, roughly doubling the number of available energy states.
- Two nearly independent spectral systems: Parahelium and orthohelium each generate their own series of emission lines with minimal crossover between them.
Hydrogen avoids all of this complexity. With one electron, there’s no shielding, no electron-electron repulsion, and no spin pairing to worry about. Its energy levels are determined by a single quantum number, and its spectrum is clean and predictable. The moment you add a second electron, as helium does, the physics becomes fundamentally richer, and the spectrum reflects that richness line by line.