Dipole Moment
A dipole moment is a measure of the separation of positive and negative electrical charges within a molecule. It arises when there is an uneven distribution of electrons, causing one end of the molecule to be more negatively charged and the other end to be more positively charged. Dipole moments are important in understanding the polarity of molecules and their interactions with other molecules.
5 Key excerpts on "Dipole Moment"
eBook - ePub- Jacob N. Israelachvili(Author)
- 2010(Publication Date)
- Academic Press(Publisher)
...4 Interactions Involving Polar Molecules 4.1 What Are Polar Molecules? Most molecules carry no net charge, but many possess an electric dipole. For example, in the HCl molecule the chlorine atom tends to draw the hydrogen’s electron toward itself, and this molecule therefore has a permanent dipole. Such molecules are called dipolar or simply polar molecules. The dipoles of some molecules depend on their environment and can change substantially when they are transferred from one medium to another, especially when molecules become ionized in a solvent. For example, the amino acid molecule glycine contains an acidic group on one side and a basic group on the other. In water at neutral pH, the NH 2 group acquires a proton and the OH group loses a proton to the solution to produce a dipolar molecule: Quite often the magnitude of the positive and negative charges are not the same, and these molecules therefore possess a net charge in addition to a dipole. Such molecules are then referred to as dipolar ions. Polarity can also arise from internal charge displacements within a molecule, producing zwitterionic molecules or groups. In larger molecules, or “macromolecules,” such as proteins the net Dipole Moment is usually made up of a distribution of positive and negative charges at various locations of the molecules. It should already be apparent that the interactions and the solvent effects of polar molecules can be very complex. The Dipole Moment of a polar molecule is defined as (4.1) where l is the distance between the two charges + q and – q. The direction of the Dipole Moment is as shown in the above figure. For example, for two electronic charges q = ± e separated by l = 0.1 nm, the Dipole Moment is u = (1.602 × 10 −19) (10 −10) = 1.6 × 10 −29 C m = 4.8 D. The unit of Dipole Moment is the Debye, where 1 Debye = 1 D = 3.336 × 10 −30 C m, which corresponds to two unit charges separated by about 0.2 Å (~0.02 nm)...
eBook - ePub- Atef Korchef(Author)
- 2022(Publication Date)
- CRC Press(Publisher)
...When 0 < δ < q, where q is the electric charge, atoms are held together by a polar covalent bond, such as for the HCl molecule. When the Dipole Moment μ = q× d, such as in the NaCl molecule (Figure 9.4), a transfer of electrons between the atoms occurs and an ionic bond is formed between them. FIGURE 9.4 In NaCl, the Dipole Moment μ = q × d. The sodium atom loses an electron, forming a cation (Na +) and the chlorine atom loses an electron, forming an anion (Cl −). An ionic bond is formed between the two ions. It is interesting to note that the Dipole Moment of a polyatomic molecule is the vectoral sum of the Dipole Moment of the different bonds in the molecule. If the individual bond Dipole Moments cancel one another, the Dipole Moment of the molecule equals zero.This can explain why certain molecules such as carbon dioxide (CO 2) and hydrocarbons (C n H 2n) are non-polar molecules. For example, in the CO 2 molecule, the two bonds C=O have a non-zero Dipole Moment because the C and O atoms have different electronegativities. However, the Dipole Moment of the CO 2 molecule equals zero (μ = 0). This can only be explained when the molecule presents a linear structure. However, the individual bond Dipole Moments of the different bonds in SO 2 molecule, for example, do not cancel one another since it has a V-shape. The Dipole Moment of SO 2 molecule is not equal to zero (μ ≠ 0) and it is a polar molecule (Figure 9.5). In the same manner, we can explain why O 3 molecule is a polar molecule. FIGURE 9.5 CO 2 is a non-polar molecule. The two C=O bonds have a non-zero Dipole Moment (μ CO ≠ 0), however μ CO2 = 0 because the molecule is linear. SO 2 is a polar molecule that has a V-shape and μ SO2 ≠ 0. 9.2.2 Intermolecular Interactions Intermolecular forces hold molecules to other molecules and include van der Waals forces, hydrogen bonds, ion–dipole interactions, and ion-induced dipole interactions...
eBook - ePub- Jeffrey Gaffney, Nancy Marley(Authors)
- 2017(Publication Date)
- Elsevier(Publisher)
...atoms. Diamagnetic having the property of being weakly repelled by a magnetic field. Dipole a bond (or molecule) that has a partial positive charge on one end and partial negative charge on the other. Dipole-dipole forces an attractive force between molecules with permanent Dipole Moments. Dipole Moment a measure of the polarity of a covalent bond. Double bond a covalent bond that results from the sharing of two electron pairs between two atoms. Equatorial the atoms that lie along the horizontal plane of a molecule. Hybridization a mixing of s and p atomic orbitals when molecular orbitals are formed. Hydrogen bonding a type of dipole-dipole force that occurs between molecules containing a covalent bond between a hydrogen atom and a very electronegative atom with at least one lone pair of electrons, usually fluorine, oxygen, or nitrogen. Intermolecular forces the forces of attraction and repulsion between molecules. Ionic bond electrostatic attractions between oppositely charged positive and negative ions which holds the atoms together to form an ionic compound. Ionic radius the distance from the nucleus to the outermost occupied electron orbital in an ion. London dispersion force a weak attractive force arising from the formation of an induced instantaneous polarization in molecules that do...
eBook - ePubLiquid Crystal Displays
Fundamental Physics and Technology
- Robert H. Chen(Author)
- 2011(Publication Date)
- Wiley(Publisher)
...To differentiate, the term “charge polarization” will indicate the charge separation in molecules, and the single word “polarization” will be reserved for light whenever there is danger of confusion. As the story of liquid crystals unfolds, understanding of the two phenomena will increase, and the polarization in question will be clear in context, rendering particularization unnecessary. As described above, a liquid crystal’s molecules have a dipolar intrinsic charge polarization structure that can be described by a dipole, which can be described mathematically as just the separation of positive and negative charge (q) times the distance and direction between the charges expressed by a vector lever arm (l); this is the intrinsic electric Dipole Moment p = q l, as shown schematically in Figures 3.1 and 3.2 for between the charges and farther away from the charges respectively. Figure 3.1 Dipole field between the charges. Figure 3.2 Dipole field away from the charges. When no external electric field is applied, the intrinsic electric Dipole Moments in the liquid crystal are randomly oriented with no particular orientational order, and thus cancel each other out to have no net gross effect (the liquid crystal is non-polar). But applying an electric field will cause the electric Dipole Moments to tend to align with the field to produce a gross electric dipole field, that from a large distance (“large” meaning compared with the length of the dipole’s lever arm) manifests the response of the liquid crystal to the external electric field. In truth, since the human eye cannot perceive the microscopic individual dipoles, the gross average effect is the phenomenon that is observed anyway. So the molecules’ Dipole Moments produce an average Dipole Moment field that manifests the liquid crystal’s structural anisotropy...
- Kevin Reel, Derrick C. Wood, Scott A. Best(Authors)
- 2014(Publication Date)
- Research & Education Association(Publisher)
...The partial charges are represented by the lower-case Greek symbol delta (δ). The element that has the greater electronegativity will be slightly negative (δ −) and the other element will be slightly positive (δ +). • Dipoles are molecules that contain polar covalent bonds that are not completely canceled out by other polar bonds. The polar bonds add together like vectors to create a total Dipole Moment on the molecule, which results in the molecule having separate centers of positive and negative charge. Network Covalent Bonding • Network covalent crystals are groups of nonmetal atoms held together by covalent bonds. • The bonding in a network covalent solid is extensive, and repeats throughout the crystalline structure. • Network covalent crystals tend to have very high melting and boiling points, and a large amount of energy is needed to break apart the crystal. • The most common example of a network covalent solid is a diamond, which is a network covalent crystal of sp 3 hybridized carbon atoms bonding to one another over and over again. (See Figure 6.1.) Figure 6.1. Structure of a Diamond TEST TIP Network covalent solids are usually the same type of atom bonded over and over again. Look for chemical formulas such as S 8 or C 60 or common names such as diamond or graphite. Metallic Bonds DID YOU KNOW? Graphite conducts electricity because the sp 2 hybridized orbitals allow for one double bond per carbon atom. This double bond is delocalized and moves freely through-out the structure, thus conducting electricity. In addition, the planar sheets of graphite move easily past one another which makes it a good lubricant. • Metals compose the majority of elements on the periodic table, and all elements to the left of the metalloids on the periodic table (except hydrogen) are metals. • Metallic bonding can be thought of as cations surrounded by a sea of electrons...
