Nonmetals covalent bonding

Ionic and covalent bonding are two extreme models of the chemical bond. Most actual bonds lie somewhere between purely ionic and purely covalent. When we describe bonds between nonmetals, covalent bonding is a good model. When a metal and nonmetal are present in a simple compound, ionic bonding is a good model. However, the bonds in many compounds seem to have properties between the two extreme models of bonding. Can we describe these bonds more accurately by improving the two basic models ... 

Metals + nonmetals - ionic bonds Nonmetals + nonmetals — covalent bonds... 

Ionic bonding occurs between a metal and a nonmetal. Covalent bonding, on the other hand, occurs between two nonmetals. The properties of these two types of compounds are different. Ionic compounds are usually solids at room temperature, while covalently bonded compounds can be solids, liquids, or gases. There s more. Ionic compounds (salts) usually have a much higher melting point than covalent compounds. In addition, ionic compounds tend to be electrolytes, and covalent compounds tend to be nonelectrol5rtes. (Chapter 6 explains all about ionic bonds, electrolytes, and nonelectrol3rtes.)... 

We can divide chemical bonds into three general types ionic bonds, which occur between a metal and a nonmetal covalent bonds, which occur between two nonmetals and metallic bonds, which occur within metals. 

Nonmetal haUdes are generally hydroly2ed to a hydrogen haUde and to an oxy-acid containing the other element. The first row nonmetal haUdes, eg, CCI4, resist hydrolysis because the nonmetal element cannot expand its octet of electrons to form a bond to water before its bond to the haUde is broken. Hydrolysis requires either an energetic water molecule to strike the haUde or ioni2ation of the covalent nonmetal—halide bond, processes that tend to be quite slow (16). 

Earlier we referred to the forces that hold nonmetal atoms to one another, covalent bonds. These bonds consist of an electron pair shared between two atoms. To represent the covalent bond in the H2 molecule, two structures can be written ... 

These examples illustrate the principle that atoms in covalently bonded species tend to have noble-gas electronic structures. This generalization is often referred to as the octet rule. Nonmetals, except for hydrogen, achieve a noble-gas structure by sharing in an octet of electrons (eight). Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two). 

Diagrams of four types of substances (see text discussion). X represents a nonmetal atom, — represents a covalent bond, M+ a cation, X- an anion, and e an electron. 

The transition metals, unlike those in Groups 1 and 2, typically show several different oxidation numbers in their compounds. This tends to make their redox chemistry more complex (and more colorful). Only in the lower oxidation states (+1, +2, +3) are the transition metals present as cations (e.g., Ag+, Zn2+, Fe3+). In higher oxidation states (+4 to +7) a transition metal is covalently bonded to a nonmetal atom, most often oxygen. 

Because nonmetals do not form monatomic cations, the nature of bonds between atoms of nonmetals puzzled scientists until 1916, when Lewis published his explanation. With brilliant insight, and before anyone knew about quantum mechanics or orbitals, Lewis proposed that a covalent bond is a pair of electrons shared between two atoms (3). The rest of this chapter and the next develop Lewis s vision of the covalent bond. In this chapter, we consider the types, numbers, and properties of bonds that can be formed by sharing pairs of electrons. In Chapter 3, we revisit Lewis s concept and see how to understand it in terms of orbitals. 

Hydrogen is unusual because it can form both a cation (1I+) and ail anion (11 ). Moreover, its intermediate electronegativity (2.2 on the Pauling scale) means that it can also form covalent bonds with all the nonmetals and metalloids. Because hydrogen forms compounds with so many elements (Table 14.2 also see Section 14.2), we shall meet more of its compounds when we study the other elements. 

What Do We Need to Know Already It would be a good idea to review the information on periodic trends in Sections 1.15-1.22 and 14.1-14.2. Because the nonmetals form molecular compounds, it would also be helpful to review Lewis structures, electronegativity, and covalent bonding in Chapters 2 and 3. The bulk properties of nonmetallic materials are affected by intermolecular forces (Sections 5.1-5.5). 

B Aluminum forms an amphoteric oxide in which it has the oxidation state +3 therefore, aluminum is the element. 14.3B Hydrogen is a nonmetal and a diatomic gas at room temperature. It has an intermediate electronegativity (x — 2.2), so it forms covalent bonds with nonmetals and forms anions in combination with metals. In contrast, Group 1 elements are solid metals that have low electronegativities and form cations in combination with nonmetals. 

Binary compounds of two nonmetals are covalently bonded. However, strong acids in water form ions completely. 

The elements that are commonly part of organic compounds are all located in the upper right corner of the periodic table. They are all nonmetals. The bonds between atoms of these elements are essentially covalent. (Some organic molecules may form ions nevertheless, the bonds tvithin each organic ion are covalent. For example, the salt sodium acetate consists of sodium ions, Na+, and acetate ions, C2H,02. Despite the charge, the bonds within the acetate ion are all covalent.)... 

In addition to functioning as Lewis acids, boron halides undergo many other types of reactions. As is typical of most compounds containing covalent bonds between a nonmetal and a halogen, the boron halides react vigorously with water to yield boric acid and the corresponding hydrogen halide. 

The Si-Cl bonds will react in ways that are typical of covalent bonds between halogens and nonmetals. One such reaction is... 

Ionic bonding results from electrostatic interactions between ions, which can be formed by the transfer of one or more electrons from a metal to a nonmetal or group of nonmetals (forming a polyatomic ion, like N03 ). Covalent bonding, on the other hand, results from sharing one or more electron pairs between two nonmetal atoms. 

Ionic compounds are made up of positively charged ions (usually metal ions) and negatively charged ions (usually non-metal ions or polyatomic anions) held together by electrostatic forces of attraction. Molecular compounds are made up of discrete units called molecules. Generally they consist of a small number of nonmetal atoms held together by covalent bonds (sharing of electrons). 

We know that the electronegativity difference between atoms must be greater than 1.9 to form an ionic bond. But if the electronegativity values of the atoms are similar, the tendency of the atoms to take or give electrons will also be similar. The transfer of electrons is not possible between such atoms, so the atoms must share electrons to gain a stable octet. The bond that is formed as a result of electron sharing is called a covalent bond. Covalent bonds are generally formed between two nonmetals. 

If the atoms are widely separated on the periodic table (metals and nonmetals), the bonding is most likely ionic. Nonmetals close together are likely to form covalent bonds. 

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