8.5 Atomic spectra and x-rays  (Page 6/12)

Atomic and molecular spectra are said to be “discrete,” because only certain spectral lines are observed. In contrast, spectra from a white light source (consisting of many photon frequencies) are continuous because a continuous “rainbow” of colors is observed.

UV light consists of relatively high frequency (short wavelength) photons. So the energy of the absorbed photon and the energy transition ( $\text{Δ}E$ ) in the atom is relatively large. In comparison, visible light consists of relatively lower-frequency photons. Therefore, the energy transition in the atom and the energy of the emitted photon is relatively small.

For macroscopic systems, the quantum numbers are very large, so the energy difference ( $\text{Δ}E$ ) between adjacent energy levels (orbits) is very small. The energy released in transitions between these closely space energy levels is much too small to be detected.

For ${\text{He}}^{+}$ , one electron “orbits” a nucleus with two protons and two neutrons ( $Z=2$ ). Ionization energy refers to the energy required to remove the electron from the atom. The energy needed to remove the electron in the ground state of ${\text{He}}^{+}$ ion to infinity is negative the value of the ground state energy, written:
$E=\mathrm{-54.4}\text{eV}.$
Thus, the energy to ionize the electron is $\text{+54.4}\text{eV.}$
Similarly, the energy needed to remove an electron in the first excited state of ${\text{Li}}^{\text{2+}}$ ion to infinity is negative the value of the first excited state energy, written:
$E=\mathrm{-30.6}\text{eV}.$
The energy to ionize the electron is 30.6 eV.