Early Models of the Atom

One of the early models of the atom was the plum pudding model, in which the electrons were pictured as embedded in a positively charged spherical cloud, much as raisins are distributed in an old-fashioned plum pudding. 

Chemistr) is often at the forefront of technological change, whethcT the ancient quest to make gold, early models of the atom, or solar cells, bio-based fuels, green chemistr), and the drive (or ( nvironmental sustainability, this reference breaks down the essentials. The Handy Clieniisiry Ansiver Book covers nearly 1,()()() common chemistr) questions, sucii as ... 

Early models of the atom had electrons spinning around the nucleus in a random fashion. But as scientists learned more about the atom, they found that this representation probably wasn t accurate. Today, two models of atomic structure are used the Bohr model and the quantum mechanical model. The Bohr model is simple and relatively easy to understand the quantum mechanical model is based on mathematics and is more difficult to understand. Both, though, are helpful in understanding the atom, so I explain each in the following sections (without resorting to a lot of math). 

Thomson model of the atom. In this early model of the atom, negative particles (electrons) were thought to be embedded In a positively charged sphere. It Is sometimes called the plum pudding model. 

In the last 200 years, vast amounts of data have been accumulated to support atomic theory. When atoms were originally suggested by the early Greeks, no physical evidence existed to support their ideas. Early chemists did a variety of experiments, which culminated in Dalton s model of the atom. Because of the limitations of Dalton s model, modifications were proposed first by Thomson and then by Rutherford, which eventually led to our modern concept of the nuclear atom. These early models of the atom work reasonably well—in fact, we continue to use them to visualize a variety of chemical concepts. There remain questions that these models cannot answer, including an explanation of how atomic structure relates to the periodic table. In this chapter, we will present our modern model of the atom we will see how it varies from and improves upon the earlier atomic models. 

One of the early models of the atom proposed that atoms were wispy balls of positive charge with the electrons evenly distributed throughout. What would you expect to observe if you conducted Rutherford s experiment and the atom had this structure ... 

For nearly half a century, Mendeleev s periodic table remained an empirical compilation of the relationship of the elements. Only after the first atomic model was developed by the physicists of the early twentieth century, which took form in Bohr s model, was it possible to reconcile the involved general concepts with the specificity of the chemical elements. Bohr indeed expanded Rutherford s model of the atom, which tried to connect the chemical specificity of the elements grouped in Mendeleev s table with the behavior of electrons spinning around the nucleus. Bohr hit upon the idea that Mendeleev s periodicity could... 

In an early model of the hydrogen atom proposed by Niels Bohr, the electron traveled in a circular orbit of radius uncertainty principle rules out this model. 

The first steps toward the understanding of the nature of the chemical bond could not be taken until the composition and structure of atoms had been elucidated. The model of the atom that emerged from the early work of Thomson, Rutherford, Moseley, and Bohr was of... 

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... 

In the early development of the atomic model scientists initially thought that, they could define the sub-atomic particles by the laws of classical physics—that is, they were tiny bits of matter. However, they later discovered that this particle view of the atom could not explain many of the observations that scientists were making. About this time, a model (the quantum mechanical model) that attributed the properties of both matter and waves to particles began to gain favor. This model described the behavior of electrons in terms of waves (electromagnetic radiation). 

Nineteenth-century atomic theory could not explain why the aurora exhibits a limited range of specific wavelengths, rather than the full visible spectrum. Early in the twentieth century, however, scientists developed a revolutionary new model of the atom. This model, and the theory that supports it, helped to account for many puzzling phenomena that the existing atomic theory had failed to explain. Among these phenomena are the characteristic colours of the northern lights. 

You have seen how scientists in the late nineteenth and early twentieth century developed and modified the atomic model. Changes in this model resulted from both experimental evidence and new ideas about the nature of matter and energy. By 1913, chemists and physicists had a working model that pointed tantalizingly in a promising direction. During the third decade of the twentieth century, the promise was fulfilled. In the next section, you will learn how physicists extended the ideas of Planck, Einstein, and Bohr to develop an entirely new branch of physics, and a new model of the atom. 

Several steps were needed to determine the structure of the core particle to higher resolution (Fig. Id). The X-ray phases of the low-resolution models were insufficient to extend the structure to higher resolution, since the resolution of the early models of the NCP was severely limited by disorder in the crystals. The disorder was presumed to derive from both the random sequences of the DNA and from heterogeneity of the histone proteins caused by variability in post-translational modification of the native proteins. One strategy for developing an atomic position model of the NCP was to develop a high-resolution structure of the histone core. This structure could then be used with molecular replacement techniques to determine the histone core within the NCP and subsequently identify the DNA in difference Fourier electron density maps. 

Each element emits its own characteristic spectral pattern, which can be used to identify the element just as a fingerprint can be used to identify a person. As we discuss in this chapter, scientists of the early 1900s saw these spectral patterns as clues to the internal structure and dynamics of atoms. By studying spectral patterns and by conducting experiments, these scientists were able to develop models of the atom. Through these models, which continue to be refined even today, chemists gain a powerful understanding of how atoms behave. 

Bohr frequency condition The relation between the change in energy of an atom or molecule and the frequency of radiation emitted or absorbed AE = hv. Bohr radius a0 In an early model of the hydrogen atom, the radius of the lowest energy orbit now a specific combination of fundamental constants (aG =... 

According to an early theory about the atom, the atom looks like a mini solar system. The nucleus of the atom would be the Sun and the electrons are the orbiting planets. This model of the atom is called the Bohr model. It is named for the Danish physicist, Niels Bohr, who proposed electron shells in 1913. The Bohr model is very useful for understanding how atoms work, but it does not answer some questions about the behavior of all atoms. 

By the early twentieth century, chemists and physicists recognized that the atoms of which chemical elements are composed are themselves made up of electrons and protons, of electrically negative and positive subatomic particles that were the universal constituents of all chemical elements. Sir Joseph Thomson had discovered the electron in 1897. Ernest Rutherford postulated the existence of a positive nucleus in atoms in 1911, and he used this in developing his planetary model of the atom, with a positive center and orbiting electrons. He discovered the proton in 1919, in experiments on the disintegration of atomic nuclei. Much later, in 1932, the British physicist James Chadwick (1891— 1974) discovered a third subatomic particle, the electrically neutral neutron. 

There were two major problems with these early, many-elec-tron models of the atom. First, atoms are electrically neutral. What provides the positive charge required to neutralize the negative charge of the electron As there was no evidence on which to base a definitive response, physicists at first largely finessed this question. The second problem was the inherent instability of the many-electron models. Since atoms are stable, any tenable model of the atom must account for its stability. An atom made up of... 

The nuclear model of the atom, as envisioned by Rutherford and Bohr, had much in common with the solar system. In each there is a massive core that exerts a controlling influence over less massive satellites orbiting around the central core. In both the solar system and the atom, the force between the central core and the orbiting satellites decreases as the square of their separation. In the case of the solar system, it was Johannes Kepler, early in the seventeenth century, who first allowed hard data—data he knew to be accurate—to sit in judgment on his speculations about the orbits of the Sun s planets. 

From an electrostatic point of view, it is amazing that positively charged protons can be packed so closely together. Yet many nuclei do not spontaneously decompose, so they must be stable. In the early twentieth century when Rutherford postulated the nuclear model of the atom, scientists were puzzled by such a situation. Physicists have since detected many very short-lived subatomic particles (in addition to protons, neutrons, and electrons) as products of nuclear reactions. Well over 100 have been identified. A discussion of these many particles is beyond the scope of a chemistry text. Furthermore their functions are not entirely understood, but it is now thought that they help to overcome the proton-proton repulsions and to bind nuclear particles (nucleons) together. The attractive forces among nucleons appear to be important over only extremely small distances, about 10 cm. 

The oscillator strength was originally defined as unity for an electron oscillating harmonically in three dimensions, an early model of an atom. It can be determined experimentally by integration of an absorption band (Equation 2.20), where e is the molar absorption coefficient. 

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