Molecule polar attraction

The small lithium Li" and beryllium Be ions have high charge-radius ratios and consequently exert particularly strong attractions on other ions and on polar molecules. These attractions result in both high lattice and hydration energies and it is these high energies which account for many of the abnormal properties of the ionic compounds of lithium and beryllium. 

Polar molecules, like nonpolar molecules, are attracted to one another by dispersion forces. In addition, they experience dipole forces as illustrated in Figure 9.9, which shows the orientation of polar molecules, such as Id, in a crystal. Adjacent molecules line up so that the negative pole of one molecule (small Q atom) is as dose as possible to the positive pole (large I atom) of its neighbor. Under these conditions, there is an electrical attractive force, referred to as a dipole force, between adjacent polar molecules. 

The London interaction arises from the attraction between instantaneous electric dipoles on neighboring molecules and acts between all types of molecules its strength increases with the number of electrons and occurs in addition to any dipole-dipole interactions. Polar molecules also attract nonpolar molecules by weak dipole-induced-dipole interactions. 

Since both the silicone oil and the cloth or leather are composed of relatively nonpolar molecules, they attract each other. The oil thus adheres well to the material. Water, on the other hand is polar and adheres very poorly to the silicone oil (actually, the water is repelled by the oil), much more poorly, in fact, than it adheres to the cloth or leather. This is because the oil is more nonpolar than is the cloth or the leather. Thus, water is repelled from the silicone-treated cloth or leather. 

Figure 11.8 Dipole-dipole bonds in polar molecules such as HC1. The hydrogen (black) of one molecule is attracted to the chlorine (white) of another because of the permanent charge imbalance on the molecule.

Noble gases and non-polar molecules such as COz and CH4 do not have dipoles. In these molecules, the movement of electrons results in nonpolar molecules becoming temporarily polar an instantaneous dipole is formed. The molecule which becomes momentarily polar then causes its neighboring molecule to become polar. Thus a weak attraction occurs between the molecules. This attraction is named the van der Waals force. 

The Lewis structure indicates that KrF2 is nonpolar. Thus, it only has very weak London dispersion forces between the molecules. SeF2 is polar and the molecules are attracted by dipole—dipole attractions, which are stronger than London. SnF2 has the highest melting point, because of the presence of strong ionic bonds. 

The molecules are polar if they have polar bonds, and if these bonds do not act in opposite directions and counteract each other. Polar molecules are attracted to each other by dipole-dipole forces. 

Since positive and negative charges are attracted to each other, the oxygen end of a water molecule is attracted to the hydrogen end of other water molecules. The oxygen end is also attracted to other positively charged particles— like the sodium in salt. Because of this, water s polarity allows it to dissolve many substances that humans and animals need and to transport them to different parts of the body. 

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

Just as two polar molecules, like opposite ends of a magnet, are attracted to each other, a polar molecule may be attracted to an ion. This gives rise to an ion-dipole force. The negative ends of polar molecules are attracted to cations and the positive end to anions. The charge on the ion and the strength of the dipole moment determine the... 

Hydrogen bond. An attractive force, or bridge occurring in polar compounds such as water, in which a hydrogen atom of one molecule is attracted to two unshared electrons of another. The hydrogen atom is the positive end of one polar molecule and forms a linkage with the electronegative end of another such molecule. 

These are now the geometric covolume factors of Ref. 8. The argument for H 0 and NH is that they will not exhihit their strong polar attraction toward the nonpolar molecules which are present in much higher concentration. Consequently these species should be given effective covolume factors more like those of nonpolar molecules. 

The relative tendency of a bonded atom in a molecule to attract electrons is expressed by the term electronegativity. The higher the electronegativity, the more effectively does the atom attract and hold electrons. A bond formed by atoms of dissimilar electronegativities is called polar. A nonpolar covalent bond exists between atoms having a very small or zero difference in electronegativity. A few relative electronegativities are... 

The solubility of molecules can be explained on the basis of the polarity of molecules. Polar, e.g. water, and nonpolar, e.g. benzene, solvents do not mix. In general, like dissolves like i.e., materials with similar polarity are soluble in each other. A polar solvent, e.g. water, has partial charges that can interact with the partial charges on a polar compound, e.g. sodium chloride (NaCl). As nonpolar compounds have no net charge, polar solvents are not attracted to them. Alkanes are nonpolar molecules, and are insoluble in polar solvent, e.g. water, and soluble in nonpolar solvent, e.g. petroleum ether. The hydrogen bonding and other nonbonding interactions between molecules are described in Chapter 2. 

B) Delocalization effect Let us return again to figure (II). The dipole moment in the bond Ox—H is such that H carries a net positive charge. As a result it will attract the lone-pair electrons on 02 and will itself fall under the electron-repelling influence of these two electrons. In conventional terms we can say that the two molecules polarize each other, and thus we can represent the situation by saying... 

Figure 6.28 illustrates how polar molecules electrically attract one another and as a result are relatively difficult to separate. In other words, polar molecules can be thought of as being sticky, which is why it takes more energy to separate them and let them enter the gaseous phase. For this reason, substances composed of polar molecules typically have higher boiling points than substances composed of nonpolar molecules, as Table 6.3 shows. Water, for example, boils... 

Nonpolar grime attracts and is surrounded by the nonpolar tails of soap molecules, forming a micelle. The polar heads of the soap molecules are attracted by ion-dipole attractions to water molecules, which carry the soap-grime combination away. 

Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform mono-molecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species. 

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