5-1 Looking Deeper into Astronomy Atomic Energy-Level Diagrams

oday’s view of the atom owes much to the Bohr or in a collision between atoms; they can make transitions to model, but it is different in certain ways. The modern lower energy levels by emission of a photon. picture is based on quantum mechanics, a branch of On the energy-level diagram for hydrogen, the n =∞level physics dealing with photons and subatomic particles that has an energy of 13.6 eV. If the electron is initially in the Twas developed during the 1920s. As a result of this work, ground state and the atom absorbs a photon of any energy physicists no longer picture electrons as moving in specific greater than 13.6 eV, the electron will be removed orbits about the nucleus. Instead, electrons are now said to completely from the atom and the atom will be ionized. The occupy certain energy levels in the atom. gaseous nebula NGC 2363 shown in Figure 5-16 on page An extremely useful way of displaying the structure of an 105 of Universe has hot stars in its neighborhood that atom is with an energy-level diagram, such as that shown for produce copious amounts of ultraviolet photons with hydrogen in Figure a. The lowest energy level, called the energies greater than 13.6 eV. These photons cause some ground state, corresponds to the n = 1 Bohr orbit. Higher of the hydrogen atoms in the nebula to become ionized. energy levels, called excited states, correspond to successively When the electrons recombine with the nuclei, they cascade larger Bohr orbits. down the energy levels to the ground state and emit visible An electron can jump from the ground state up to the light in the process. n = 2 level only if the atom absorbs a Lyman-alpha photon The atoms of heavier elements have more complex with a wavelength of 122 nm. The energy of a photon is energy-level diagrams. Figure b shows the energy-level determined by the relationship E = hν=hc/λ and is diagram for sodium, along with the wavelengths of photons commonly expressed in electron volts (eV). As explained in absorbed or emitted in some of sodium’s major electron Section 5-5 of Universe, an electron volt is a tiny amount of transitions. At visible wavelengths, the sodium spectrum is energy (1 eV = 1.602 × 10–19 J). The Lyman-alpha photon dominated by two strong lines, called the sodium D lines, at has an energy of 10.19 eV, so the energy level of n = 2 is 588.99 and 589.59 nm. These two lines are strong because shown in the figure as having an energy 10.19 eV above the they correspond to two transitions that are the primary energy of the ground state (which is conventionally assigned avenue through which electrons cascade from high orbits a value of 0 eV). Similarly, the n = 3 level is 12.07 eV above down to the ground state. Astronomers find energy-level the ground state, and so forth. Electrons can make diagrams useful in understanding the lines they observe in transitions to a higher energy level by absorption of a photon the spectra of stars and nebulae. 5.14 eV 5.0 eV 9 8 8 7 8 7 7 6 7 6 6 5 5 6 5 5 4 4 9 514.91 2 5 . 4.0 eV 8 9 4 4 4 1267.761845.95 n = ∞ 13.6 eV 3 2208.4 2205.7 n = 4 12.73 eV 4 615.43 285.30 330.29 497.86 3.0 eV 1140.421138.24 n = 3 12.07 eV Paschen series 568.82 819.48 568.27 818.33 Absorption n = 2 10.19 eV 2.0 eV 33 Balmer series

330.23

285.28

1.0 eV

342.71 D589.59 Emission Ionization

D588.99

0 eV 3

Ground state n = 1 0 eV Lyman a series b