Modern Atomic Theory and the Periodic Table

CHAPTER 10 MODERN ATOMIC THEORY AND THE PERIODIC TABLE... 

This section started with the discovery of Soddy and Fajans on radioactive decay around 1910 and the relationship of radioactive decay to the periodic table. At this point in the history, we understand the periodic table and we understand the role of isotopes in the periodic table. We have not yet understood the structure of the modern Table, i.e. first row two elements, second row eight elements, etc. That understanding can be based on Bohr theory of the hydrogen atom originally developed in 1911 and is summarized in Bohr s famous article in Zeitschrift fur Physik (Bohr 1922). 

When Mendeleev invented the periodic table, he was well-acquainted with Dalton s atomic theory. He knew nothing, however, about subatomic particles, and especially the electron, which is the foundation for the modern periodic table s distinctive shape. Because the original periodic table developed out of experimental observations, chemists did not need an understanding of atomic structure to develop it. (As you will see in section 3.3, however, the periodic table easily accommodates details about atomic structure. In fact, you will learn that the modern periodic table s distinctive design is a natural consequence of atomic structure.)... 

Section 3.1 takes up the experimental laws on which Dalton based his atomic theory, and Section 3.2 discusses that theory itself. Some modern extensions of the theory, including subatomic particles and isotopes, are presented in Section 3.3. The concept of the masses of atoms of the individual elements is presented in Section 3.4, and the development of the periodic table is traced in Section 3.5. A much more sophisticated theory of the atom will be presented in Chapter 4. 

In the early days of atomic theory, it was often assumed that only an empirical understanding could be achieved beyond the first few rows of the periodic table, where the simple formulation of the aufbau principle breaks down. The modern view is that shell and subshell filling can be accounted for within the central field approximation, provided the centrifugal barrier effects are included. 

The development of the periodic table brought chemistry into the modern world. Prior to having an organized table of the elements, there were hundreds of theories about why atoms reacted in different ways. The periodic table erased this confusion and made it possible for anyone to understand the differences between elements, based on where they sit on the periodic table. 

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. 

But it is the second theory of modern physics that has exerted by far the more important influence in attempts to understand the periodic system theoretically. Quantum theory was actually born in the year 1900, some 14 years before the discovery of atomic number. It was first applied to atoms by Niels Bohr, who pursued the notion that the similarities between the elements in any group of the periodic table could be explained by their having equal numbers of outer-shell electrons. " The very notion of a particular number of electrons in an electron shell is an essentially quantumUke concept. Electrons are assumed to possess only certain quanta, or packets, of energy, and depending on how many such quanta they possess, they lie in one or another shell around the nucleus of the atom. 

We begin this chapter with a brief survey of early chemical discoveries, culminating in Dalton s atomic theory. This is followed by a description of the physical evidence leading to the modern picture of the nuclear atom, in which protons and neutrons are combined into a nucleus with electrons in space surrounding the nucleus. We will also introduce the periodic table as the primary means of organizing elements into groups with similar properties. Finally, we will introduce the... 

As you know, Dalton s atomic theory no longer applies in its original form, and Mendeleev s periodic table has undergone many changes. For example, scientists later discovered that atoms are not the most basic unit of matter because they are divisible. As well, the modern periodic table lists the elements in order of their atomic number, not their atomic mass. Of course, it also includes elements that had not been discovered in Mendeleev s time. Even so, in modified form, both of these inventions are still studied and used today in every chemistry course around the world. 

That Mendeleev was able to predict the properties of new elements helped convince many scientists of the accuracy of Dalton s atomic hypothesis. This in turn helped promote Dalton s proposed atomic nature of matter from a hypothesis to a more widely accepted theory. Mendeleev s chart, which ultimately led to our modern periodic table with its horizontal periods and vertical groups, also helped lay the groundwork for our understanding of atomic behavior and is recognized as one of the most important achievements of modern science. 

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. 

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