Molecules covalent compound

Lewis and many other chemists had recognized the shortcomings of the ionic bond. When diatomic molecules, such as or Cl, were considered, there was no reason why one atom should lose an electron and an identical atom should gain an electron. There had to be another explanation for how diatomic molecules formed. We have seen how the octet rule applies to the formation of ionic compounds by the transfer of electrons. This rule also helps explain the formation of covalent bonds when molecules (covalent compounds) form. Covalent bonds result when atoms share electrons. Using fluorine, F, as a representative halogen, we can see how the octet rule applies to the formation of the molecule. Each fluorine atom has seven valence electrons and needs one more electron to achieve the stable octet valence configuration. If two fluorines share a pair of electrons, then the stable octet configuration is achieved ... 

Covalent compounds are composed of molecules in which atoms are bonded by electron sharing. Because of weak interparticle forces between molecules, covalent compounds tend to be gases or liquids at room temperature. They tend to be insoluble in water, although many are extremely soluble. 

Covalent compounds do not contain ions and therefore do not mix so readily with water molecules. Covalent compounds tend to dissolve readily in non-polar organic solvents, such as benzene, which also contain covalent molecules. 

Some compounds that consist of molecules (covalent compounds) are soluble in water and some are not. Some of those that are soluble form ions when they dissolve, but most do not. Let us now take a look at what happens at the molecular level in these situations. 

These apparent anomalies are readily explained. Elements in Group V. for example, have five electrons in their outer quantum level, but with the one exception of nitrogen, they all have unfilled (I orbitals. Thus, with the exception of nitrogen. Group V elements are able to use all their five outer electrons to form five covalent bonds. Similarly elements in Group VI, with the exception of oxygen, are able to form six covalent bonds for example in SF. The outer quantum level, however, is still incomplete, a situation found for all covalent compounds formed by elements after Period 2. and all have the ability to accept electron pairs from other molecules although the stability of the compounds formed may be low. This... 

In most covalent compounds, the strong covalent bonds link the atoms together into molecules, but the molecules themselves are held together by much weaker forces, hence the low melting points of molecular crystals and their inability to conduct electricity. These weak intermolecular forces are called van der WaaFs forces in general, they increase with increase in size of the molecule. Only... 

Both boron and aluminium chlorides can be prepared by the direct combination of the elements. Boron trichloride can also be prepared by passing chlorine gas over a strongly heated mixture of boron trioxide and carbon. Like boron trifluoride, this is a covalent compound and a gas at ordinary temperature and pressure (boiling point 285 K). It reacts vigorously with water, the mechanism probably involving initial co-ordination of a water molecule (p, 152). and hydrochloric acid is obtained ... 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

Aluminum hydroxide and aluminum chloride do not ionize appreciably in solution but behave in some respects as covalent compounds. The aluminum ion has a coordination number of six and in solution binds six molecules of water existing as [Al(H20)g]. On addition of a base, substitution of the hydroxyl ion for the water molecule proceeds until the normal hydroxide results and precipitation is observed. Dehydration is essentially complete at pH 7. 

Arsenic Halides. Arsenic forms a complete series of trihaUdes, but arsenic pentafluoride is the only well-known simple pentahaUde. AH of the arsenic haUdes, the physical properties of which are given in Table 2, are covalent compounds that hydrolyze in the presence of water. The trihaUdes form pyramidal molecules similar to the trivalent phosphoms analogues and may be prepared by direct combination of the elements. 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

A useful property of liquids is their ability to dissolve gases, other liquids and solids. The solutions produced may be end-products, e.g. carbonated drinks, paints, disinfectants or the process itself may serve a useful function, e.g. pickling of metals, removal of pollutant gas from air by absorption (Chapter 17), leaching of a constituent from bulk solid. Clearly a solution s properties can differ significantly from the individual constituents. Solvents are covalent compounds in which molecules are much closer together than in a gas and the intermolecular forces are therefore relatively strong. When the molecules of a covalent solute are physically and chemically similar to those of a liquid solvent the intermolecular forces of each are the same and the solute and solvent will usually mix readily with each other. The quantity of solute in solvent is often expressed as a concentration, e.g. in grams/litre. 

The UV spectra suggest that the equilibrium between the diazonium ion and the solvent, on the one hand, and an electron donor-acceptor complex (8.58) on the other, lies on the side of the complex. The latter may possibly exist also as a radical pair (8.60) or a covalent compound (8.59). Dissociation of this complex within a cage to form an aryl radical, a nitrogen molecule, and the radical cation of DMSO is slow and rate-determining. Fast subsequent steps lead to the products observed. 

Nonmetals form covalent molecular hydrides, which consist of discrete molecules. These compounds are volatile and many are Bronstcd acids. Some are gases— for example, ammonia, the hydrogen halides (HF, HC1, HBr, HI), and the lighter hydrocarbons such as methane, ethane, ethene, and ethyne. Liquid molecular hydrides include water and hydrocarbons such as octane and benzene. 

If a nonmetal reacts with another nonmetal no electrons are lost or gained, but are shared. We call such compounds covalent (molecular) compounds. These compounds contain small units we call molecules. Ammonia, NH3, water, H20, and methane, CH4, are examples of covalent compounds. 

Many of the reactions that you will study occur in aqueous solution. Water readily dissolves many ionic compounds as well as some covalent compounds. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions— solutions that conduct electrical current due to the presence of ions. We may classify electrolytes as either strong or weak. Strong electrolytes dissociate (break apart or ionize) completely in solution, while weak electrolytes only partially dissociate. Even though many ionic compounds dissolve in water, many do not. If the attraction of the oppositely charged ions in the solid is greater than the attraction of the water molecules to the ions, then the salt will not dissolve to an appreciable amount. 

In this section, you have used Lewis structures to represent bonding in ionic and covalent compounds, and have applied the quantum mechanical theory of the atom to enhance your understanding of bonding. All chemical bonds—whether their predominant character is ionic, covalent, or between the two—result from the atomic structure and properties of the bonding atoms. In the next section, you will learn how the positions of atoms in a compound, and the arrangement of the bonding and lone pairs of electrons, produce molecules with characteristic shapes. These shapes, and the forces that arise from them, are intimately linked to the physical properties of substances, as you will see in the final section of the chapter. 

The energy equation 3-3 may be used to define the ion energy level of ionic compound molecules. Fmthermore, this energy equation may also be used to define the ion energy level in covalently bonded compoimd molecules, though the physical meaning of the ion level in covalent compounds is a matter of discussion. 

According to Lewis s (1916) definition, a covalent bond exists whenever two atoms are linked in a stable molecule by sharing two outer electrons. We distinguish between homopolar and heteropolar covalent bonds on the basis of whether or not the bonded atoms are of the same type. Pauling (1960) has shown that in covalent compounds the distance between two elements A and B, independent of the nature of the two atoms, is the same in all compounds containing... 

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