Model of the Atom

Dalton had no way to study the properties or structure of individual atoms. His model is often referred to as the little hard sphere model of the atom. In Dalton s mind, atoms had no internal structure but were essentially like tiny ball bearings or marbles. What distinguished atoms of one element from atoms of a different element was relative mass. Consequently, from that time on it was common to list the known elements in order of relative mass. 

Lists always began with hydrogen, the lightest element, which was assigned a mass of 1. 

From there, lithium would be next (since helium was not discovered until much later) with a mass of 7, beryllium with a mass of 9, boron with a mass of 11, carbon with a mass of 12, and so forth through uranium, the heaviest naturally occurring element with a mass of 238. 

Dalton s little hard sphere model of the atom may seem primitive by today s standards, but it was an essential step in the evolution of chemical knowledge. Dalton s model persisted for almost one hundred years before anyone could think of any way to improve upon it. What is especially remarkable is that Dalton s theory was not completely accepted by the scientific community. Until 1900, there remained prominent physicists and chemists who continued to deny the existence of atoms. Actually, probably the most unsatisfying thing about Dalton s model is that it offered no explanation for the differences in chemical and physical properties that were observed among the elements. Even Dmitri Mendeleev, who, in 1869, developed the modern periodic table of the elements, could offer no explanation for the regular, or periodic, trends in the elements that were displayed in his periodic table. For that explanation, we must turn the clock forward to the events of the 1890s. 

John Dalton reintroduced the notion of atoms in the first decade of the nineteenth century. For the next one hundred years, scientists hotly debated whether or not atoms even existed. It was not until the first decade of the twentieth century that atomic theory finally was fully accepted. 

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Despite its success in reproducing the hydrogen atom spectmm, the Bolir model of the atom rapidly encountered difficulties. Advances in the resolution obtained in spectroscopic experiments had shown that the spectral features of the hydrogen atom are actually composed of several closely spaced lines these are not accounted for by quantum jumps between Bolir s allowed orbits. However, by modifying the Bolir model to... 

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... 

On the base of the developed mathematical models was developed regression model of the atomizer efficiency via main design pai ameters such as linear dimensions and operation temperatures. 

The first detailed model of the atom, proposed by J. J. Thomson in 1898, was based upon the expectation that the atom was a sphere of positive electricity in which electrons were embedded like plums in a pudding. This picture of the atom was not particularly satisfying because it was not useful in predicting or explaining the chemical properties of the atom. Finally, in 1911, a series of experiments performed in the McGill University laboratory of Ernest Rutherford showed that Thomson s picture of the atom had to be abandoned. 

The first observation made with this apparatus was that apparently all the alpha particles passed through the foil undeflected. Let us see if this result is consistent with the model of the atom proposed by Thomson. You will recall that Thomson s picture of the atom assumed that the positive charge is distributed evenly throughout the entire volume of the atom with the negative electrons embedded in il. Since the electrons weigh so little, the positive part accounts for nearly all of the mass of the atom. Thus the Thomson model pictures the atom as a body of uniform density. 

Using this kind of model of the atom, we can account for the alpha particles that are deflected through both... 

Electrons. If the discovery of isotopes threatened ro undermine the periodic system, the discovery of the electron explained many of the periodic properties on which the table was based. J. J. Thomson attempted to explain the periodic system by postulating rings of electrons embedded in the positive charge that made up his phim pudding model of the atom. Thomson s model was quickly superseded by more sophisticated and elaborate mod-... 

Scientists commonly interpret a theory in terms of a model, a simplified version of the object of study. Like hypotheses, theories and models must be subjected to experiment and revised if experimental results do not support them. For example, our current model of the atom has gone through many formulations and progressive revisions, starting from Dalton s vision of an atom as an uncut-table solid sphere to our current much more detailed model, which is described in Chapter 1. One of the main goals of this text is to show you how to build models, turn them into a testable form, and then refine them in the light of additional evidence. 

The Greeks asked what would happen if they continued to cut matter into ever smaller pieces. Is there a point at which they would have to stop because the pieces no longer had the same properties as the whole or could they go on cutting forever We now know that there is a point at which we have to stop. That is, matter consists of almost unimaginably tiny particles. The smallest particle of an element that can exist is called an atom. The story of the development of the modern model of the atom is an excellent illustration of how scientific models are developed. 

We develop the modern model of an atom in Chapter 1. At this stage, all we need to know is that according to the current nuclear model of the atom, an atom consists of a small positively charged nucleus, which is responsible for almost all its mass, surrounded by negatively charged electrons (denoted e ). Compared with the size of the nucleus (about 10 14 m in diameter), the space occupied by the electrons is enormous... 

In the nuclear model of the atom, the positive charge and almost all of the mass is concentrated in the tiny nucleus, and the surrounding negatively charged electrons take up most of the space. The atomic number is the number of protons in the nucleus. 

What Do We Need to Know Already We need to be familiar with the nuclear model of the atom and the general layout of the periodic table (Fundamentals Section B). We also need the concepts of kinetic and potential energy (Section A). 

This chapter builds an understanding of atomic structure in four steps. First, we review the experiments that led to our current nuclear model of the atom and see how spectroscopy reveals information about the arrangement of electrons around the nucleus. Then we describe the experiments that led to the replacement of classical mechanics by quantum mechanics, introduce some of its central features, and illustrate them by considering a very simple system. Next, we apply those ideas to the simplest atom of all, the hydrogen atom. Finally, we extend these concepts to the atoms of all the elements of the periodic table and see the origin of the periodicity of the elements. 

FIGURE 1.6 Rutherford s model of the atom explains why most u particles pass almost straight through the platinum foil, whereas a very few—those scoring a direct hit on the nucleus—undergo verv large deflections. Most of the atom is nearly empty space thinly populated by the atom s electrons. The nuclei are much smaller relative to their atoms than shown here. 

We are now ready to build a quantum mechanical model of a hydrogen atom. Our task is to combine our knowledge that an electron has wavelike properties and is described by a wavefunction with the nuclear model of the atom, and explain the ladder of energy levels suggested by spectroscopy. 

Describe the experiments that led to the formulation of the nuclear model of the atom (Section 1.1). 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

Schematic drawing of an atom, showing a central, positive nucleus surrounded by a cloud of electrons. This model of the atom is consistent with the results of Rutherford s scattering experiments.

How large are orbitals Experiments that measure atomic radii provide information about the size of an orbital. In addition, theoretical models of the atom predict how the electron density of a particular orbital changes with distance from the nucleus, r. When these sources of information are combined, they reveal several regular features about orbital size. 

Figure 1.2 The plum pudding model of the atom consisted of electrons scattered in a sphere of positive charge.

He became interested in a new theory of chemical bonds based on Niels Bohr s 1913 model of the atom, which Thomas Midgley also used to discover tetraethyl lead and CFCs (Chapter 6). Scientists already knew that atoms could form molecules by transferring electrons, but the new theory suggested that chemical bonds could also be formed when atoms share electrons. 

Sir Ernest Rutherford (1871-1937 Nobel Prize for chemistry 1908, which as a physicist he puzzled over) was a brilliant experimentalist endowed with an equal genius of being able to interpret the results. He recognized three types of radiation (alpha, beta, and gamma). He used scattering experiments with alpha radiation, which consists of helium nuclei, to prove that the atom is almost empty. The diameter of the atomic nucleus is about 10 000 times smaller than the atom itself. Furthermore, he proved that atoms are not indivisible and that in addition to protons, there must also be neutrons present in their nucleus. With Niels Bohr he developed the core-shell model of the atom. 

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... 

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