Why do chemical bonds form

The answer to this question would ideally be a simple, easily understood theory that would not only explain why atoms bind together to form molecules, but would also predict the three-dimensional structures of the resulting compounds as well as the energies and other properties of the bonds themselves. Unfortunately, no one theory exists that accomplishes these goals in a satisfactory way for all of the many categories of compounds that are known. Moreover, it seems likely that if such a theory does ever come into being, it will be far from simple. 

Given the extraordinary variety of ways in which atoms combine into aggregates, it should come as no surprise that a number of useful bonding models have been developed. Most of them apply only to certain classes of compounds, or attempt to explain only a restricted range of phenomena. In this section we will provide brief descriptions of some of the bonding models the more important of these will be treated in much more detail in later parts of this chapter. 

Intense speculation about chemical affinity began in the 18th century. Some likened the tendency of one atom to close with another as an expression of a human-like kind of affection. Others attributed bonding to magnetic-like forces (left) or to varying numbers of hooks on different kinds of atoms (right). The latter constituted a primitive (and extremely limited) model of combining power or valence. 

By classical, we mean models that do not take into account the quantum behavior of small particles, notably the electron. These models generally assume that electrons and ions behave as point charges which attract and repel according to the laws of electro- 

It turns out that this is not true generally, but a model built on this assumption does a fairly good job of explaining a rather small but important class of compounds that are called ionic solids. The most well known example of such a compound is sodium chloride, which consists of two interpenetrating lattices of Na+ and CE ions arranged in such as way that every ion of one type is surrounded (in three dimensional space) by six ions of opposite charge. 

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Why do chemical bonds form What basic forces are involved in bonding ... 

Why do atoms bond tof thcr, and how can bonds bo described elt ctrom-cally The why question is relatively easy to answer Atonts bond toother because the compound that results is more stable (has less enerny) than tho separate atoms. Just as water flows downhill, energy is released and flows out of the chemical system when a diemical txmd is formed. Con versely, energy is absorbed and must be put into the system when a chem ical bond is broken. The how < uestion is more diflteult. To answer it, we need to know more about the properties of atoms. 

The Rutherford model of the atom is consistent with the evidence presented so far, but it has some serious limitations. It does not answer such important questions as Why do different elements have such different chemical and physical properties Why does chemical bonding occur at all IVhy does each element form compounds with characteristic formulas How can atoms of different elements give off or absorb light only of characteristic colors (as was known long before 1900) ... 

Why Do We Need to Know This Material The existence of compounds is central to the science of chemistry and by seeing how bonds form between atoms, we come to see how chemists design new materials. Research into artificial blood, new pharmaceuticals, agricultural chemicals, and the polymers used in materials such as compact discs, cellular phones, and synthetic fibers is based on an understanding of how atoms link together. 

Why is carbon special, and why do chemists still treat organic chemistry as a separate branch of science The answers to these questions involve the ability of carbon atoms to bond together, forming long chains and rings. Of all the elements, only carbon is able to form such an immense array of compounds, from methane, with one carbon atom, to deoxyribonucleic acid (DNA), with tens of billions of carbon atoms. More than 19 million organic compounds have been made, and thousands of new ones are made each week in chemical laboratories throughout the world. 

Gold, like all metals, is shiny, malleable, ductile, and a good conductor of electricity and heat. Unlike most metals and other elements, however, gold is found in nature in its pure form, as an element. Most elements are chemically combined in the form of compounds. Why is this so Why do atoms of some elements join together as compounds, while others do not In this chapter, you will use the periodic trends you examined in Chapter 2 to help you answer these questions. You will learn about the bonds that hold elements together in compounds. At the same time, you will learn how to write chemical formulas and how to name compounds. 

Everything that counts in chemistry is related to the electronic structure of atoms and molecules. The formation of molecules from atoms, their behavior and reactivity all depend on the electronic structure. What is the role of symmetry in all this In various aspects of the electronic structure, symmetry can tell us a good deal why certain bonds can form and others cannot, why certain electronic transitions are allowed and others are not, and why certain chemical reactions occur and others do not. Our discussion of these points is based primarily on some monographs listed in References [2-8],... 

Elimination reactions One way to change an alkane into a chemically reactive substance is to form a second covalent bond between two carbon atoms, producing an alkene. The main industrial source of alkenes is the cracking of petroleum. The process of cracking, shown in Figure 23-17, breaks large alkanes into smaller alkanes, alkenes, and aromatic compounds. Why do you suppose the term cracking was applied to this process ... 

What determines the stability of a chemical bond Why is the Hi molecule more stable than a pair of separated hydrogen atoms in the gas phase at normal temperatures and pressures But what is the meaning of more stable And how do bonds form spontaneously once the atoms are close enough together ... 

Before a reaction can start, molecules of the reactants have to bump into each other, or collide. This makes sense because to form new chemical bonds, atoms have to be close together. But, not just any collision will do. The collision must be strong enough. This means the reactants must smash into each other with a certain amount of energy. Anything less, and the reaction win not occur. Why is this true ... 

Physicist consider different aspects and properties of molecular species to those chemists do. This explains why the chemical aspects of light noble gas elements, in particular their reactive behavior, have not been the subject so far of a review article. Chemists ask whether He, Ne or Ar can form stable covalent bonds with other elements they are interested in the resulting electronic struc-... 

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