Development of Chemical Bonding Theory

Problem 1.2 i How many electrons does each of the following elements have in its outermost electron shell  

By the mid-1800s, the new science of chemistry was developing rapidly and chemists had begun to probe the forces holding compounds together. In 1858, August Kekule and Archibald Couper independently proposed that, in all its compounds, carbon is tetravalent—it always forms four bonds when it joins other elements to form stable compounds. Furthermore, said Kekule, carbon atoms can bond to one another to form extended chains of linked atoms. 

Shortly after the tetravalent nature of carbon was proposed, extensions to the Kekule-Couper theory were made w hen the possibility of multiple bonding between atoms was suggested. Emil Eiienmeyer proposed a carbon-carbon triple bond for acetylene, and Alexander Crum Brown proposed a carbon-carbon double bond for ethylene. In 1865, Kekule provided another major advance when he suggested that carbon chains can double back on themselves to form rings of atoms. 

Although Kekule and Couper were correct in describing the tetravalent nature of carbon, chemistry w as still viewed in a two-dimensional way until 1874. In that year. Jacobus van t Hoff and Joseph Le Bel added a third dimension to our ideas about organic compounds w hen they proposed that the four bonds of carbon are not oriented randomly but have specific spatial directions. Van t Hoff went even further and suggested that the four atoms to 

Joseph Achille Le Bel (1847-1930) was born in Pechelbronn, France, and studied at the Ecole Polytechnique and the Sorbonne in Paris. Freed by his family s wealth from the need to earn a living, he established his own private laboratory. 

TABLE 1.1 Cround State Electron Configurations of Some Eiements 

Element number Configuration Element number Configuration  

Problem 1.1 Give the ground-state electron configuration for each of the following elements  

Friedrich August Kekule (1829-1896) was born in Darmstadt, Germany. 

We know through observation that eight electrons (an electron octet) in an atom s outermost shell, or valence shell, impart special stability to the noble-gas elements in group 8A of the periodic table Ne (2 + 8) Ar (2 + 8 + 8) Kr (2 + 8 + 18 +8). We also know that the chemistry of main-group elements is governed by their tendency to take on the electron configuration of the nearest 

The munber of covalent bonds an atom forms depends on how many additional valence electrons it needs to reach a noble-gas configuration. Hydrogen has one valence electron (Is) and needs one more to reach the helium configuration (Is ), so it forms one bond. Carbon has four valence electrons (2s 2p ) and needs four more to reach the neon configuration (2s 2p ), so it forms four bonds. Nitrogen has five valence electrons (25 2p ), needs three more, and forms three bonds oxygen has six valence electrons (2s 2p ), needs two more, and forms two bonds and the halogens have seven valence electrons, need one more, and form one bond. 

One bond Four bonds Three bonds Two bonds 

How many hydrogen atoms does phosphorus bond to in forming phosphine, PH Strategy 

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To justify such a description within quantum mechanics we are led into a consideration of ionic and covEilent contributions to an approximate wave function. If covalent contributions are minor, the bond is said to be ionic in character, and the electrostatic model is considered to be applicable. Unfortunately, the terms ionic character and covalent character are used with various meanings. This is so, in part, because the rapid development of chemical bond theory has caused a drift of the meanings of these terms over the past two decades. Pauling s definitions, as presented in his book (1585, p. 48), no doubt represent the intent of most workers as of 1940. He concluded that there is a covalent bond between two atoms X and Y if the dissociation energy of X—is the mean of the dissociation energies of X— X and Y— Y. If the dissociation energy of X— Y exceeds this mean, the excess is attributed to additional ionic character of the bond. This criterion furnishes the basis for his scale of electronegativity, and ionic character is inter-... 

Note that the concept of aromatidty continues to be a valuable stimulus for the development of chemical bond theory, and that some aspects of aromatidty are still controversially discussed (a) Pogodin, S. and Agranat, I. (2007) J. Org. Chem., 72,10096-107 ... 

Atomic Structure The Nucleus Atomic Structure Orbitals 4 Atomic Structure Electron Configurations 6 Development of Chemical Bonding Theory 7 The Nature of Chemical Bonds Valence Bond Theory sp Hybrid Orbitals and the Structure of Methane 12 sp Hybrid Orbitals and the Structure of Ethane 13 sp2 Hybrid Orbitals and the Structure of Ethylene 14 sp Hybrid Orbitals and the Structure of Acetylene 17 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur 18 The Nature of Chemical Bonds Molecular Orbital Theory 20 Drawing Chemical Structures 21 Summary 24... 

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