Atomic structure: Bohr model

The Thomas-Fermi (TF)[15] and Hartree-Fock (HF) schemes model atomic structure empirically, by assuming spectroscopically determined electron configurations. On the other hand, the correct form of the periodic table emerges from elementary number theory, which suggests an alternative interpretation of the Bohr radii. 

In the early part of the twentieth century, then, a simple model of atomic structure became accepted, now known as the Rutherford nuclear model of the atom, or, subsequently, the Bohr-Rutherford model. This supposed that most of the mass of the atom is concentrated in the nucleus, which consists of protons (positively charged particles) and neutrons (electrically neutral particles, of approximately the same mass). The number of protons in the nucleus is called the atomic number, which essentially defines the nature of... 

As a final note, closer inspection of the emission lines from Na shows that most emission lines are not, in fact, single lines, but are closely spaced doublets or triplets - for example, the strong yellow line discussed above at 589.3 nm is composed of two separate lines at 589.0 and 589.6 nm. This is termed fine structure, and is not predictable from the Bohr model of the atom. It is addressed in the Bohr-Sommerfield model, and is the result of a quantum mechanical interaction, known as spin-orbit coupling, further discussion of which is not necessary for this volume. 

In this chapter, you learned about the electronic structure of the atom in terms of the older Bohr model and the newer quantum mechanical model. You learned about the wave properties of matter, and how to describe each individual electron in terms of its four quantum numbers. You then learned how to write the electron configuration of an atom and some exceptions to the general rules. 

An estimate of die size of the proton and an understanding of the structure of the hydrogen atom resulted from two major developments in atomic physics the Rudierford scattering experiment (1911) and the Bohr model of die atom (1913). Rutherford showed that the nucleus is vanishingly small compared to the size of an atom. The radius of a proton is on the order of 10-13 centimeter as compared with atomic radii of 10-3 centimeter, Thus, the size of a hydrogen atom is determined by the radius of the electron orbits, but the mass is essentially that of the proton,... 

Models of nuclei have grown in sophistication as new discoveries about subatomic particles have been made. One of the simplest was suggested by Niels Bohr, the Danish scientist who contributed a great deal to our understanding of atomic structure. Bohr compared the nucleus to a drop of liquid. His liquid drop model proposes that nucleons are packed together like the molecules in a liquid. Nucleons at the surface of the... 

With the particlelike nature of energy and the wavelike nature of matter now established, let s return to the problem of atomic structure. Several models of atomic structure were proposed in the late nineteenth and early twentieth centuries. A model proposed in 1914 by the Danish physicist Niels Bohr (1885-1962), for example, described the hydrogen atom as a nucleus with an electron circling around it, much as a planet orbits the sun. Furthermore, said Bohr, only certain specific orbits corresponding to certain specific energy levels for the electron are available. The Bohr model was extremely important historically because of its conclusion that electrons have only specific energy levels available to them, but the model fails for atoms with more than one electron. 

In the Bohr model of atomic structure, electrons are constrained to orbit a nucleus at specific distances, given by the equation... 

Because hydrogen has more than two energy levels, it actually emits electromagnetic radiation at more than one frequency. Bohr s formulation accounted for all of hydrogen s observed emissions. Bohr published his new atomic structure in 1913. According to Albert Einstein, the Bohr model of the atom was one of the greatest discoveries. ... 

Bohr s importance can be attributed first and foremost to his model of the hydrogen atom. In a series of three papers, now called the Trilogy, Bohr laid the foundation for a quantum theory of atomic structure. The Trilogy was published in Phiksophical Magazine in 1913 and these three papers established Bohr s reputation. ... 

Figure 4.4 A. diagram of the Bohr model of the hydrogen atom as shown by Arnold Sommerfeld in Atomic Structure and Spectral Lines. The radii of the pictured orbits n = 2 and w = 3 are four times and nine times larger than the radius = 1. Orbits forw = 4, 5, 6, and so on are even larger. Spectral lines originate when the atom passes from one energy state, w = 3, to others, n = 2 and n = 1, as pictured.

The electrons are negatively charged particles. The mass of an electron is about 2000 times smaller than that of an proton or neutron at 0.00055 amu. Electrons circle so fast that it cannot be determined where electrons are at any point in time, rather, we talk about the probability of finding an electron at a point in space relative to a nucleus at any point in time. The image depicts the old Bohr model of the atom, in which the electrons inhabit discrete "orbitals" around the nucleus much like planets orbit the sun. This model is outdated. Current models of the atomic structure hold that electrons occupy fuzzy clouds around the nucleus of specific shapes, some spherical, some dumbbell shaped, some with even more complex shapes. Even though the simpler Bohr model of atomic structure has been superseded, we still refer to these electron clouds as "orbitals". The number of electrons and the nature of the orbitals they occupy basically determines the chemical properties and reactivity of all atoms and molecules. 

The Bohr model was an immensely important contribution to the understanding of atomic structure. The idea that electrons exist in specific energy states and that transitions between states involve quanta of energy provided the linkage between atomic structure and atomic spectra. However, some limitations of this model quickly became apparent. Although it explained the hydrogen spectrum, it provided only a crude approximation of the spectra for more complex atoms. Subsequent development of more sophisticated experimental techniques demonstrated that there are problems with the Bohr theory even in the case of hydrogen. 

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