Charge, attraction between molecules

This expression applies to the transport of any conserved quantity Q, e.g., mass, energy, momentum, or charge. The rate of transport of Q per unit area normal to the direction of transport is called the flux of Q. This transport equation can be applied on a microscopic or molecular scale to a stationary medium or a fluid in laminar flow, in which the mechanism for the transport of Q is the intermolecular forces of attraction between molecules or groups of molecules. It also applies to fluids in turbulent flow, on a turbulent convective scale, in which the mechanism for transport is the result of the motion of turbulent eddies in the fluid that move in three directions and carry Q with them. 

Interactions such as surface tension also become more influential at smaller scales. Surface tension is due to attractions between molecules, such as electrical charges and hydrogen bonds, which cause them to bunch together and resist separation. For example, such forces create a tension on the surface of water because the molecules stick together, and this force is strong enough to let a bug known as the water strider walk on water. In Zetd s motor, the smaller drop grows imtil it reaches... 

Electrostatic forces provide a generalized attraction between molecules in chemical reactions. In the reaction between chloride anions and sodium cations described above, the way in which these two spherical species approached one another was unimportant because the charges attracted one another from any angle. In most organic reactions the orbitals of the nucleophile and electrophile are directional and so the molecular orbitals of the reacting molecules exert important control. If a new. bond is to be formed as the molecules collide, the orbitals of the two species must be correctly aligned in space. In our last example, only if the sp3 orbital of the lone pair on nitrogen points directly at the empty orbital of the BF3 can bond formation take place. Other collisions will not lead to reaction. In the first frame a successful collision takes place and a bond can be formed between the orbitals. In the second frame are three examples of unsuccessful collisions where no orbital overlap is possible. There are of course many more unproductive collisions but only one productive collision. Most collisions do not lead to reaction. 

Dispersion forces n. (1) Weak forces between atoms or molecules due to momentary fluctuations in their electronic charge-cloud distributions. (2) The force of attraction between molecules possessing no permanent dipole. The interaction energy is given by... 

In Chapter 9, we saw that bonding (intramolecular) forces are due to the attraction between cations and anions (ionic bonding) or between nuclei and electron pairs (covalent bonding). But the physical behavior of the phases and their changes are due primarily to intermolecular (nonbonding) forces, which arise from the attraction between molecules with partial charges or between ions and molecules. Coulomb s law explains why the two types of forces differ so much in magnitude ... 

There are three important contributions to the molecular forces present in a soap film that have been investigated and explain some of the equilibrium properties of soap films, particularly the properties of the thicker equilibrium film, the common black film. These are the van der Waals attraction between molecules and ions, the electrostatic repulsion due to the double layers of charge at each surface, and the Born repulsion due to the hard cores of the molecules and ions. In addition there are other molecular forces in the soap film that are not fully understood. They are probably partly steric in nature, arising from the rigidity of the surfactant. The forces due to hydrogen bonding between the water molecules are not significant. They would become important if the equilibrium thickness of the soap film was less than 20 A. 

The forces that hold atoms together within a molecule (intramolecular forces) are fundamentally electrostatic in nature the positively-charged nucleus of one atom is attracted to the negatively-charged electrons of another atom. Likewise, the forces of attraction between molecules (intermolecular forces) are also electrostatic in nature. This section is about three kinds of intermolecular forces dipole-dipole forces, hydrogen bonding, and London forces. 

The electrical charges in permanent dipoles fluctuate slightly in their distribution. Vhen dipole-dipole forces are present between two molecules, those fluctuations also cause London forces to be present. In general the London forces will be weaker than the dipole-dipole forces, but both contribute to the attractions between molecules. If a molecule also exhibits hydrogen bonding, dipole-dipole and London forces will both be present and contribute to the attractions between molecules. 

The CH3 group of methanol does not participate in dipole-dipole attractions between molecules because there is not sufficient polarity in any of its bonds to lead to significant partial positive or negative charges. This is due to the small electronegativity difference between the carbon and hydrogen in each of the C-H bonds. 

We have two interaction potential energies between uncharged molecules that vary with distance to the minus sixth power as found in the Lennard-Jones potential. Thus far, none of these interactions accounts for the general attraction between atoms and molecules that are neither charged nor possess a dipole moment. After all, CO and Nj are similarly sized, and have roughly comparable heats of vaporization and hence molecular attraction, although only the former has a dipole moment. 

When a molecule is isolated from external fields, the Hamiltonian contains only kinetic energy operators for all of the electrons and nuclei as well as temis that account for repulsion and attraction between all distinct pairs of like and unlike charges, respectively. In such a case, the Hamiltonian is constant in time. Wlien this condition is satisfied, the representation of the time-dependent wavefiinction as a superposition of Hamiltonian eigenfiinctions can be used to detemiine the time dependence of the expansion coefficients. If equation (Al.1.39) is substituted into the tune-dependent Sclirodinger equation... 

Many biological processes involve an associa tion between two species in a step prior to some subsequent transformation This asso ciation can take many forms It can be a weak associ ation of the attractive van der Waals type or a stronger interaction such as a hydrogen bond It can be an electrostatic attraction between a positively charged atom of one molecule and a negatively charged atom of another Covalent bond formation between two species of complementary chemical re activity represents an extreme kind of association It often occurs in biological processes in which aide hydes or ketones react with amines via imine inter mediates... 

A range of plasticizer molecule models and a model for PVC have been generated and energy minimized to observe their most stable conformations. Such models highlight the free volume iacrease caused by the mobiHty of the plasticizer alkyl chains. More detailed models have also been produced to concentrate on the polar region of the plasticizer and its possible mode of interaction with the polymer. These show the expected repulsion between areas on the polymer and plasticizer of like charge as weU as attraction between the negative portions of the plasticizer and positive portions of the PVC. 

The attractions between the water molecules interacting with, or hydrating, ions are much greater than the tendency of oppositely charged ions to attract one another. The ability of water to surround ions in dipole interactions and... 

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