ATOMS, MOLECULES, AND IONS

Iron atoms arranged in circle on copper metal surface by scanning tunneling microscope probe. 

Sodium metal and chlorine gas are particular forms of matter. In burning, they undergo a chemical change—a chemical reaction—in which these forms of matter change to a form of matter with different chemical and physical properties. How do we explain the differences in properties of different forms of matter And how do we explain chemical reactions such as the burning of sodium metal in chlorine gas This chapter and the next take an introductory look at these basic questions in chemistry. In later chapters we will develop the concepts introduced here. 

8 Naming Simple Compounds Chemical Reactions Equations 

Ionic Compounds Molecular Compounds Acids and Bases Hydrates 

Development of the Atomic Theory The search for the fundamental units of matter began in ancient times. The modem version of atomic theory was laid out by John Dalton, who postulated that elements are compost of extremely small particles, called atoms, and that aU atoms of a given element are identical, but they are (hlTerent from atoms of all other elements. 

The Structure of the Atom Through experimentation in the nineteenth and early twentieth centuries, scientists have learned that an atom is composed of three elementary particles proton, electron, and neutron. The proton has a positive charge, the electron has a negative charge, and the neutron has no charge. Protons and neutrons are located in a small r ion at the center of the atom, called the nucleus, and electrons are spread out about the nucleus at some distance from it. 

Ways to Identify Atoms Atomic number is the number of protons in a nucleus atoms of different elements have different atomic nmnbers. Isotopes are atoms of the same element having different numbers of neutrons. Mass number is the sum of the number of protons and neutrons in an atom. Because an atom is electrically neutral, the number of its protons is equal to the number of its electrons. 

The Periodic Table Elements can be grouped together according to their chemical and physical properties in a chart called the periodic table. The periodic table enables us to classify elements (as metals, metalloids, and nonmetals) and correlate their properties in a systematic way. It is the most useful source of chemical information. 

Colored images of the radioactive emission of radium (Ra). The models show the nuclei of radium and the radioactive decay products—radon (Rn) and an alpha particle, which has two protons and two neutrons. Study of radioactivity helped to advance scientists knowledge about atomic structure. 

Since ancient times humans have pondered the nature of matter. Our modern ideas of the structure of matter began to take shape in the early nineteenth century with Dalton s atomic theory. We now know that all matter is made of atoms, molecules, and ions. All of chemistry is concerned in one way or another with these species. 

Dalton s work marked the beginning of the modem era of chemistry. The hypotheses about the nature of matter on which Dalton s atomic theory is based can be summarized as follows  

Elements are composed of extremely small particles called atoms. 

AU atoms of a given element are identical, having the same size, mass, and chemical properties. The atoms of one element are different from the atoms of aU other elements. 

Atomic Number, Mass Numbed and Isotopes The Periodic Table The Atomic Mass Scale and Average Atomic Mass Molecules and Molecular Compounds 

molecules, and ions make up the substances we encounter every day. Some of these substances are important components of a balanced diet. An estimated 25 percent of the world s population suffers from iron deficiency, the most common nutritional deficiency in the world. Iron is necessary for the production of hemoglobin, the component in red blood cells responsible for the transport of oxygen. An inadequate supply of iron and the resulting shortage of hemoglobin can cause iron deficiency anemia (IDA). Some of the symptoms of IDA are fatigue, weakness, pale color, poor appetite, headache, and light-headedness. 

Although IDA can be caused by loss of blood or by poor absorption of iron, the most common cause is insufficient iron in the diet. Dietary iron comes from such sources as meat, eggs, leafy green vegetables, dried beans, and dried fruits. Some breakfast cereals, such as Total, are fortified with iron in the form of iron metal, also known as elemental or reduced iron. The absorption of dietary iron can be enhanced by the intake of vitamin C (ascorbic acid). When the diet fails to provide enough iron, a nutritional supplement may be necessary to prevent a deficiency. 

Many supplements provide iron in the form of a compound called ferrous sulfate. 

Elemental iron, ascorbic acid, and the iron in ferrous sulfate are examples of some of the atoms, molecules, and ions that are essential for human health. 

Early in this chapter (Section 2.4), we will introduce a classification system for elements known as the periodic table. It will prove useful in this chapter and throughout the remainder of the text. 

Iron absorption can be diminisbed by certain disordets, such as Ciohn disease, and by some medications. 

and some vegetables and cereals are good sources of dietary iron. When diet alone does not provide an adequate supply, iron supplements can be taken. 

Although the materials in our world vary greatly in their properties, everything is formed from only about 100 elements and, therefore, from only about 100 chemically different kinds of atoms. In a sense, these different atoms are like the 26 letters of the English alphabet that join in different combinations to form the immense number of words in our language. But what rules govern the ways in which atoms combine How do the properties of a substance relate to the kinds of atoms it contains Indeed, what is an atom like, and what makes the atoms of one element different from those of another  

In this chapter we introduce the basic structure of atoms and discuss the formation of molecules and ions, thereby providing a foundation for exploring chemistry more deeply in later chapters. 

1 THE ATOMIC THEORY OF MATTER We begin with a brief history of the notion of atoms—the smallest pieces of matter. 

2 THE DISCOVERY OF ATOMIC STRUCTURE We then look at some key experiments that led to the discovery of electrons and to the nuclear model of the atom. 

We explore the modern theory of atomic structure, including the ideas of atomic numbers, mass numbers, and isotopes. 

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Microwave discharges at pressures below 1 Pa witli low collision frequencies can be generated in tlie presence of a magnetic field B where tlie electrons rotate witli tlie electron cyclotron frequency. In a magnetic field of 875 G tlie rotational motion of tlie electrons is in resonance witli tlie microwaves of 2.45 GHz. In such low-pressure electron cyclotron resonance plasma sources collisions between tlie atoms, molecules and ions are reduced and the fonnation of unwanted particles in tlie plasma volume ( dusty plasma ) is largely avoided. 

Extra energy can be added to atoms, molecules, and ions, causing them to become energetically excited. For atoms, molecules, and ions, the extra energy can make them move faster (an increase in kinetic or translational energy). 

Schutte, C. J. H. (1968) The Wave Mechanics of Atoms, Molecules and Ions, Arnold, London. 

Why Do We Need to Know This Material In the first three chapters, we investigated the nature of atoms, molecules, and ions. Bulk matter is composed of immense numbers of these particles and its properties emerge from the behavior of the constituent particles. Gases are the simplest state of matter, and so the connections between the properties of individual molecules and those of bulk matter are relatively easy to identify. In later chapters, these concepts will be used to study thermodynamics, equilibrium, and the rates of chemical reactions. 

The three representations that are referred to in this study are (1) macroscopic representations that describe the bulk observable properties of matter, for example, heat energy, pH and colour changes, and the formation of gases and precipitates, (2) submicroscopic (or molecular) representations that provide explanations at the particulate level in which matter is described as being composed of atoms, molecules and ions, and (3) symbolic (or iconic) representations that involve the use of chemical symbols, formulas and equations, as well as molecular structure drawings, models and computer simulations that symbolise matter (Andersson, 1986 Boo, 1998 Johnstone, 1991, 1993 Nakhleh Krajcik, 1994 Treagust Chittleborough, 2001). 

The quantum theory of the previous chapter may well appear to be of limited relevance to chemistry. As a matter of fact, nothing that pertains to either chemical reactivity or interaction has emerged. Only background material has been developed and the quantum behaviour of real chemical systems remains to be explored. If quantum theory is to elucidate chemical effects it should go beyond an analysis of atomic hydrogen. It should deal with all types of atom, molecules and ions, explain their interaction with each other and predict the course of chemical reactions as a function of environmental factors. It is not the same as providing the classical models of chemistry with a quantum-mechanical gloss a theme not without some common-sense appeal, but destined to obscure the non-classical features of molecular systems. 

Kameta, K. Kouchi, N. Hatano, Y. In Landolt-Bomstein, New series volume I/17C, Photon and Electron Interactions with Atoms, Molecules and Ions — Photon- and electron-interactions with molecules Ionization and dissociation, Itikawa, Y., Ed. Springer-Verlag Berlin, 2003 4-1-4-61, Chapter 4. 

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