Forming Chemical Bonds

The force that holds two atoms together is called a chemical bond. Chemical bonds form because of attractions between oppositely charged atoms, called ions, or between electrons and nuclei. The outermost, or valence, electrons of atoms are the ones mainly involved in the formation of bonds. The elements within a group of the periodic table typically have the same number of valence electrons. 

Elements tend to react so as to achieve the stable electron configuration of a noble gas, typically an octet of electrons. A cation, or positive ion, is formed when an atom loses one or more electrons. 

An anion, or negative ion, is formed when an atom gains one or more electrons. The periodic table is useful in predicting the charges of ions typically formed by various atoms. 

Calcium (Ca, atomic number 20) is an element in group 2A of the periodic table. Write the electron configuration for a neutral atom of calcium. Tell how many electrons this atom readily tends to gain or lose to form an ion. Predict the charge on the ion, write its formula, and tell whether it is a cation or an anion. Finally, write the electron configuration of this ion. 

A neutral atom of element 20 would have 20 electrons, giving it the electron configuration ls 2s 2p 3s 3p 4s. To achieve the stable electron configuration of a noble gas, the atom would tend to lose its two valence electrons, producing a cation with a charge of 2-I- and the formula Ca. The configuration of this ion would be ls 2s 2p 3s 3p.  

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Chromium is able to use all of its >d and As electrons to form chemical bonds. It can also display formal oxidation states ranging from Cr(—II) to Cr(VI). The most common and thus most important oxidation states are Cr(II), Cr(III), and Cr(VI). Although most commercial applications have centered around Cr(VI) compounds, environmental concerns and regulations ia the early 1990s suggest that Cr(III) may become increasingly important, especially where the use of Cr(VI) demands reduction and incorporation as Cr(III) ia the product. 

Let s return for a last look at Pasteur s pioneering work. Pasteur took an optically inactive tartaric acid salt and found that he could crystallize from it two optically active forms having what we would now call the 2R,3R and 2S,3S configurations. But what was the optically inactive form he started with It couldn t have been meso-tartaric acid, because meso-tartaric acid is a different chemical compound and can t interconvert with the two chiral enantiomers without breaking and re-forming chemical bonds. 

Organic compounds make up more than 95% of all the chemical compounds known to exist. One reason for this is that carbon is unlike all other elements. It can form chemical bonds to connect (become bonded) with four other atoms. This ability to connect with other atoms (form bonds) is called valence. Carbon is said to have a valence of 4. The most unique feature of carbon is that it readily forms bonds with other carbon atoms to form what are usually called carbon chains. It also readily bonds to other elements, particularly hydrogen, oxygen, and nitrogen. 

A hydrogen cation is a hydrogen atom that has lost its single electron, leaving a bare hydrogen nucleus. A bare hydrogen nucleus is a proton. Thus, any reaction in which H moves from one species to another is called a proton-transfer reaction. Protons readily form chemical bonds. In aqueous solution, they associate with water molecules to form hydronium ions. 

Atoms can form chemical bonds with one another to construct molecules. As we point out in Chapter 2, this is one of the fundamental features of the atomic theory. The details of chemical bonding appear in Chapters 9 and 10. 

Metal cations in aqueous solution often form chemical bonds to anions or neutral molecules that have lone pairs of electrons. A silver cation, for example, can associate with two ammonia molecules to form a silver-ammonia complex ... 

For complexation purposes, a ligand is a species that has lone pairs of electrons available to donate to a metal atom or cation. Water molecules possess lone pairs of electrons, so water is a ligand that readily forms complex ions with metal cations. Although solubility and complexation equations show uncomplexed metal ions in solution, dissolved cations actually form chemical bonds to water molecules of the solvent. For example, q) bonds to six water... 

The electronegativity of an element indicates the relative ability of its atoms to attract electrons to form chemical bonds. According to the graph, as yon move across a period in the periodic table —... 

Transition metals are used extensively as reforming catalysts and the variation in the catalytic activity can be determined by the differences in the strength of the adsorbate-surface interaction with various metals. One of the fundamental properties of a metal surface is in fact its ability to bond or to interact vflth surrounding atoms and molecules. The bonding ability determines the state of the metal surface when exposed to a gas or liquid and it determines the ability of the surface to act as a catalyst. During catalysis, the surface forms chemical bonds to the reactants and it helps in this way the breaking of intramolecular bonds and the formation of new bonds. 

Whereas Pt in an acidic solution saturated with H2 acquires the reversible potential of the hydrogen electrode, this is not the case for the same Pt electrode in an acidic solution saturated with O2. This is related to the high activation energies involved in breaking and forming chemical bonds. Thus the O2 reaction is known to be highly irreversible. In particular, a Pt electrode in 02-saturated solution acquires a potential 0.9V (SHE) rather than 1.23 V. Hence an overpotential of >0.3 V can already be expected from an analysis of the equilibrium conditions. 

I can say, for example, that it tends to form chemical bonds to five other atoms at a time, but can tolerate fewer and, at a push, more. It is a metal, probably quite a soft one, heavier than iron but lighter than lead. Many of its compounds - its combinations with other elements - will be coloured. It will be apt to form bonds to other niobium atoms - so-called metal-metal bonds. It will behave chemically in a similar manner to the element vanadium, but will be more similar still to tantalum. 

Silicon is chemically similar to carbon. An atom of silicon, like an atom of carbon, has four electrons in its outermost shell, and these electrons are available for forming chemical bonds. Like carbon, silicon can bond with four other elements to create a huge range of different compounds. This fact alone, some say, makes silicon a possible backbone for biological molecules. However, a more detailed examination raises doubts about the ability of silicon to form the kinds of structures necessary to build or sustain any form of life as we know it. 

The presence of unbalanced attractions at the surface of a solid—say, a metal such as nickel—means that small molecules will tend to become rather loosely attached to the surface in one or (more likely) several molecular layers with an exothermic adsorption energy ranging to about —20 kJ mol-1 for nonpolar molecules. (The term adsorption is used to denote surface sorption without penetration of the bulk solid, which would be called absorption.) No chemical bonds are formed or broken. This state is usually called physical adsorption or physisorption. If, however, the adsorbate forms chemical bonds with the surface atoms, the adsorption process is called chemisorption. Chemisorption can be quite strongly exothermic (—40 to —800 kJ mol-1) but involves only the first monomolecular layer of adsorbate. 

Electrons in the outermost occupied shell of any atom may play a significant role in that atoms chemical properties, including its ability to form chemical bonds. To indicate their importance, these electrons are called valence electrons (from the Latin valentia, strength ), and the shell they occupy is called the valence shell. Valence electrons can be conveniently represented as a series of dots surrounding an atomic symbol. This notation is called an electron-dot structure or, sometimes, a Lewis dot symbol, in honor of the American chemist G. N. Lewis, who first proposed the concepts of shells and valence electrons. Figure 6.2 shows the electron-dot structures for the atoms important in our discussions of ionic and covalent bonds. (Atoms of elements in groups 3 through 12 form metallic bonds, which we ll study in Chapter 18.)... 

Paired valence electrons are relatively stable. In other words, they usually do not form chemical bonds with other atoms. For this reason, electron pairs in an... 

Draw an analogy from this to atoms forming chemical bonds or molecules interacting. 

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