Modern Models of the Atom

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. 

In this chapter, you will learn about the developments that led to the modern model of the atom. You will also learn about the model itself and how it relates to periodic trends and the periodic table. 

In the modern model of the atom, based on wave mechanics, the conception of electronic orbits in the old model is replaced by the idea of the probability of the occurrence of an electron at a given point. The conclusions, however, which can be drawn from the older model remain the same in the newer conception, and it is important to remember that in this new model the essential points of Bohr s theory have not been discarded, but merely interpreted differently and very greatly refined. 

The existence of atoms was confirmed and the modern model of the atom developed from physical measurements that determined the properties of the electron and the nucleus and demonstrated the existence of isotopes. 

Relate historic experiments to the development of the modern model of the atom. Illustrate the modern model of an atom. Interpret the information available in an element block of the periodic table. 

How did Bohr s view of energy levels differ from the way energy levels are depicted in the modern model of the atom ... 

The idea of electrons existing in definite energy states was fine, but another way had to be devised to describe the location of the electron about the nucleus. The solution to this problem produced the modern model of the atom, often called the quantum mechanical model. In this new model of the hydrogen atom, electrons do not travel in circular orbits but exist in orbitals with three-dimensional shapes that are inconsistent with circular paths. The modern model of the atom treats the electron not as a particle with a definite mass and velocity, but as a wave with the properties of waves. The mathematics of the quantum mechanical model are much more complex, but the results are a great improvement over the Bohr model and are in better agreement with what we know about nature. In the quantum mechanical model of the atom, the location of an electron about the nucleus is described in terms of probability, not paths, and these volumes where the probability of finding the electron is high are called orbitals. 

Before the modern model of the atom evolved, the concept of an element had been purely speculative. One of the definitions of an element belongs to Aristotle, one of the greatest philosophers of antiquity, who wrote Elements are simple substances of which the universe is composed and one of which cannot be separated into the other. Aristotle held that there is one primary matter and four fundamental qualities heat and coldness, dryness and wetness. Their combinations are material elements fire, water, air, and earth. According to Aristotle, all bodies are composed of these elements. Aristotle s teaching was the theoretical foundation of al-... 

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. 

The Bohr model was important because, while not the correct model, it led to the modern model of the atom, the model that is widely accepted today as the true model of atomic structure. 

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... 

Our modern model describes the atom as an electrically neutral sphere with a tiny nucleus in the center containing positively charged protons and neutral neutrons. The negatively charged electrons are moving in complex paths outside the nucleus in energy levels at different distances from the nucleus. These subatomic particles have very little mass expressed in grams so we often use the unit of an atomic mass unit (amu or simply u). An amu is 1/12 the mass of a carbon atom that contains six protons and six neutrons. Table 2.1 summarizes the properties of the three subatomic particles. 

The modern, quantum mechanical model of the atom has broadened your understanding of the elements, the composition of their atoms, and their chemical and physical behaviour in the world around you. 

Over the centuries, many other concepts were proposed to explain the nature of matter— many of them extensions of the Greek concept of an ultimately indivisible and indestructible elementary bit of matter. But it was not until J. J. Thomson proposed his model of the atom, consisting of a sphere with an agglomeration of particles with negative electric charges somehow positioned randomly inside a very small ball of matter, that the modern structure of the atom began to take shape. 

Quantum mechanical model The modern, more accurate model of the atom in which the chance of finding an electron in a certain position at any given time are calculated and plotted. The model resembles a fuzzy cloud. 

The modern concept of the atom corresponds to Schrodinger s model. 

A little-known paper of fundamental importance to modern atomic theory was published by Hantaro Nagaoka in 1904 [10]. Apart from oblique citation, it was soon buried and forgotten. With hindsight it deserved better than that. It contained the seminal ideas underlying the nuclear model of the atom, the standing-wave nature of orbital electrons and radiationless stationary states. It was so far ahead of contemporary thinking that later imitators either failed to appreciate its significance, or pretended to be unaware of it. 

By 1935, the current model of the atom had evolved. This model explains electron behavior by interpreting the emission spectra of aU the elements. It pictures energy levels as regions of space where there is a high probability of finding electrons. Before going on to the modern atomic theory, take another look at what you already know about atoms and electrons. 

Describe the nuclear model of the atom, including the general location of the protons, neutrons, and electrons, the relative size of the nucleus compared to the size of the atom, and the modern description of the electron. 

Scientists have known for a long time that it is incorrect to think of electrons as tiny particles orbiting the nucleus like planets around the sun. Nevertheless, nonscientists have become used to picturing them in this way. In some circumstances, this solar system model of the atom may be useful, but you should know that the electron is much more unusual than that model suggests. The electron is extremely tiny, and modern physics tells us that strange things happen in the realm of the very, very small. 

The initial enthusiasm for the Bohr model of the atom began to fade soon after its introduction. New discoveries in modern physics indicated that Bohr s ideas needed to be modified. The Bohr model of the hydrogen atom pinpointed both the electron s energy and its distance from the nucleus at exactly the same time. However, the Uncertainty Principle of Heisenberg required that if the energy of the electron is known exactly there must be uncertainty concerning its location. Both could not be known exactly at the same time. 

Ernest Rutherford proposed the modern nuclear model of the atom. 

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