Why do hydrogen and helium have different emission spectra? I thought that emission spectra were the result of electrons 'jumping' from one energy level to the next - since hydrogen and helium both have electrons on n=1, l=0 and only there, why are their emission spectra different?
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5$\begingroup$ Hydrogen has a nuclear charge of +1, but helium has a nuclear charge of +2. So although each has one electron the spectra are not the same. $\endgroup$– MaxWCommented Jan 27, 2017 at 3:13
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2 Answers
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We have solved the Schrödinger equation for hydrogen-like atoms such as $\ce{H}$, $\ce{He+}$, $\ce{Li^2+}$, $\ce{Be^3+}$, and $\ce{B^4+}$. The energy levels are neatly arranged:
$$E_n = - \frac {E_0} {n^2}$$
where:
- $E_n$ is the energy of the $n$-th energy level
- $n$ is a positive integer ($1$, $2$, $\dots$)
- $E_0 = 13.6~\mathrm{eV}$
This is possible because we only need to consider two particles: the nucleus which has a positive charge and the electron which has a negative charge. We only need to consider the interaction between those two particles. Also, the nucleus is not moving, which simplifies some calculations
However, for higher atoms such as helium, where there are more than one electron, the Schrödinger equation contains two terms for the two nucleus-electron attractions and a term for the electron-electron repulsion. The term for the electron-electron repulsion makes things difficult because both electrons can be moved. That term makes the equation impossible to solve analytically.
For hydrogen, the energy of an energy level is determined solely by its principal quantum number. That means, the $2s$ orbital has the same energy as the $2p$ orbital (ignoring the virtual particle effect).
This is not true in higher atoms such as helium. The energy levels of helium can be seen here:
(The energy levels of hydrogen is on the right of the diagram for comparison.)
Here, the azimuthal quantum number has an effect on the energy of the energy levels, which makes for more possible differences between energy levels, which makes for more lines as well as different on the emission spectrum.
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1$\begingroup$ Note that for 'Hydrogenic' atoms such as $\ce{Li^{2+}}$ the energy equation should be modified slightly to $E_n=-Z^2R/n^2$ where Z is the atomic number and R the Rydberg (which you label as $E_0$ and is $R_{\infty}=13.6057 \pu{eV} = 109737 \pu{cm^{-1}}$ ) but varies slightly as it depends on reduced mass (for nucleus of mass M, $R=R_{\infty}(1+m_e/M)$. The $Z^2$ terms is why the energy is so much lower for helium than hydrogen $\endgroup$ Commented Jan 27, 2017 at 8:42
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Electronic energy level for $\ce{H}$ and $\ce{He}$ are different because in $\ce{He}$ nucleus there is two proton and for $\ce{H}$ it has single proton. As their energy are different they require different amount of energy to excite them, so different spectra comes out with different wavelength.
