Adhesion and Cohesion
Adhesion and cohesion are attractive forces between material bodies. A distinction is usually made between an adhesive force, which acts to hold two separate bodies together (or to stick one body to another) and a cohesive force, which acts to hold together the like or unlike atoms, ions, or molecules of a single body. However, both forces result from the same basic properties of matter. A number of phenomena can be explained in terms of adhesion and cohesion. For example, surface tension in liquids results from cohesion, and capillarity results from a combination of adhesion and cohesion. The hardness of a diamond is due to the strong cohesive forces between the carbon atoms of which it is made. Friction between two solid bodies depends in part upon adhesion.
Intermolecular forces, forces that are exerted by molecules on each other and that, in general, affect the macroscopic properties of the material of which the molecules are a part. Such forces may be either attractive or repulsive in nature. They are conveniently divided into two classes: short-range forces, which operate when the centers of the molecules are separated by 3 angstroms or less, and long-range forces, which operate at greater distances. Generally, if molecules do not tend to interact chemically, the short-range forces between them are repulsive. These forces arise from interactions of the electrons associated with the molecules and are also known as exchange forces. Molecules that interact chemically have attractive exchange forces; these are also known as valence forces. Mechanical rigidity of molecules and effects such as limited compressibility of matter arise from repulsive exchange forces.
Long-range forces, or van der Waals forces as they are also called, are attractive and account for a wide range of physical phenomena, such as friction, surface tension, adhesion and cohesion of liquids and solids, viscosity, and the discrepancies between the actual behavior of gases and that predicted by the ideal gas law. Van der Waals forces (weak, nonspecific forces between molecules ) arise in a number of ways, one being the tendency of electrically polarized molecules to become aligned. Quantum theory indicates also that in some cases the electrostatic fields associated with electrons in neighboring molecules constrain the electrons to move more or less in phase.
Surface tension, tendency of liquids to reduce their exposed surface to the smallest possible area. A drop of water, for example, tends to assume the shape of a sphere. The phenomenon is attributed to cohesion, the attractive forces acting between the molecules of the liquid. The molecules within the liquid are attracted equally from all sides, but those near the surface experience unequal attractions and thus are drawn toward the center of the liquid mass by this net force. The surface then appears to act like an extremely thin membrane, and the small volume of water that makes up a drop assumes the shape of a sphere, held constant when an equilibrium between the internal pressure and that due to surface tension is reached. Because of surface tension, various small insects are able to skate across the surface of a pond, objects of greater density than water can be made to float, and molten lead when dropped into a cool liquid forms suddenly into shot.
capillarity or capillary action, phenomenon in which the surface of a liquid is observed to be elevated or depressed where it comes into contact with a solid. For example, the surface of water in a clean drinking glass is seen to be slightly higher at the edges, where it contacts the glass, than in the middle. Capillarity can be explained by considering the effects of two opposing forces: adhesion, the attractive (or repulsive) force between the molecules of the liquid and those of the container, and cohesion, the attractive force between the molecules of the liquid (see adhesion and cohesion). Adhesion causes water to wet a glass container and thus causes the water’s surface to rise near the container’s walls. If there were no forces acting in opposition, the water would creep higher and higher on the walls and eventually overflow the container. The forces of cohesion act to minimize the surface area of the liquid (see surface tension); when the cohesive force acting to reduce the surface area becomes equal to the adhesive force acting to increase it (e.g., by pulling water up the walls of a glass), equilibrium is reached and the liquid stops rising where it contacts the solid. In some liquid-solid systems, e.g., mercury and glass or water and polyethylene plastic, the liquid does not wet the solid, and its surface is depressed where it contacts the solid. Capillarity is one of the causes of the upward flow of water in the soil and in plants.
Friction, resistance offered to the movement of one body past another body with which it is in contact. In certain situations friction is desired. Without friction the wheels of a locomotive could not “grip” the rails nor could power be transmitted by belts. On the other hand, in the moving parts of machines a minimum of friction is desired; an excess of friction produces heat, which in turn causes expansion, the locking of the moving parts, and a consequent breakdown of the machinery. Lubrication is important in minimizing friction as are also such devices as ball and roller bearings.
Factors Affecting Friction
Friction depends partly on the smoothness of the contacting surfaces, a greater force being needed to move two surfaces past one another if they are rough than if they are smooth. However, friction decreases with smoothness only to a degree; friction actually increases between two extremely smooth surfaces because of increased attractive electrostatic forces between their atoms. Friction does not depend on the amount of surface area in contact between the moving bodies or (within certain limits) on the relative speed of the bodies. It does, however, depend on the magnitude of the forces holding the bodies together. When a body is moving over a horizontal surface, it presses down against the surface with a force equal to its weight, i.e., to the pull of gravity upon it; an increase in the weight of the body causes an increase in the amount of resistance offered to the relative motion of the surfaces in contact.
The Nature of Fluid Friction
Fluid friction is observed in the flow of liquids and gases. Its causes are similar to those responsible for friction between solid surfaces, for it also depends on the chemical nature of the fluid and the nature of the surface over which the fluid is flowing. The tendency of the liquid to resist flow, i.e., its degree of viscosity, is another important factor. Fluid friction is affected by increased velocities, and the modern streamline design of airplanes and automobiles is the result of engineers’ efforts to minimize fluid friction while retaining speed and protecting structure.
The Coefficient of Friction
The coefficient of friction is the quotient obtained by dividing the value of the force necessary to move one body over another at a constant speed by the weight of the body. For example, if a force of 20 newtons is needed to move a body weighing 100 newtons over another horizontal body at a constant speed, the coefficient of friction between these two materials is 20/100 or 0.2. Different materials in contact yield different results; e.g., different resistances are felt if one pushes a block of wood over surfaces of wood, steel, and plastic. A different coefficient of friction must be calculated for each different pair of materials.
There is more than one coefficient of friction for a given pair of materials. More force is needed to start a body moving across a surface than is needed to keep it in motion once started. Thus the coefficient of static friction (describing the former case) for a pair of substances is greater than the coefficient of kinetic friction (describing the latter case) for the substances. Similarly, sliding friction is greater than rolling friction. The force of friction between two materials can be calculated by multiplying the coefficient of friction between these materials (determined experimentally and listed in engineering handbooks) by the force holding them together (e.g., the weight of the moving body).
Mechanism whereby atoms combine to form molecules. There is a chemical bond between two atoms or groups of atoms when the forces acting between them are strong enough to lead to the formation of an aggregate with sufficient stability to be regarded as an independent species. The number of bonds an atom forms corresponds to its valence. The amount of energy required to break a bond and produce neutral atoms is called the bond energy. All bonds arise from the attraction of unlike charges according to Coulomb’s law; however, depending on the atoms involved, this force manifests itself in quite different ways. The principal types of chemical bond are the ionic, covalent, metallic, and hydrogen bonds. The ionic and covalent bonds are idealized cases, however; most bonds are of an intermediate type.
The Ionic Bond
The ionic bond results from the attraction of oppositely charged ions. The atoms of metallic elements, e.g., those of sodium, lose their outer electrons easily, while the atoms of non-metals, e.g., those of chlorine, tend to gain electrons. The highly stable ions that result retain their individual structures as they approach one another to form a stable molecule or crystal. In an ionic crystal like sodium chloride, no discrete diatomic molecules exist; rather, the crystal is composed of independent Na+ and Cl– ions, each of which is attracted to neighboring ions of the opposite charge. Thus the entire crystal is a single giant molecule. (the electrostatic bond between two ions formed through the transfer of one or more electrons. Also called electrovalence, electrovalent bond (Also called polar valence. the valence of an ion, equal to the number of positive or negative charges acquired by an atom through a loss or gain of electrons)).
The Covalent Bond
A single covalent bond is created when two atoms share a pair of electrons. There is no net charge on either atom; the attractive force is produced by interaction of the electron pair with the nuclei of both atoms. If the atoms share more than two electrons, double and triple bonds are formed, because each shared pair produces its own bond. By sharing their electrons, both atoms are able to achieve a highly stable electron configuration corresponding to that of an inert gas. For example, in methane (CH4), carbon shares an electron pair with each hydrogen atom; the total number of electrons shared by carbon is eight, which corresponds to the number of electrons in the outer shell of neon; each hydrogen shares two electrons, which corresponds to the electron configuration of helium.
In most covalent bonds, each atom contributes one electron to the shared pair. In certain cases, however, both electrons come from the same atom. As a result, the bond has a partly ionic character and is called a coordinate link. Actually, the only purely covalent bond is that between two identical atoms.
Covalent bonds are of particular importance in organic chemistry because of the ability of the carbon atom to form four covalent bonds. These bonds are oriented in definite directions in space, giving rise to the complex geometry of organic molecules. If all four bonds are single, as in methane, the shape of the molecule is that of a tetrahedron. The importance of shared electron pairs was first realized by the American chemist G. N. Lewis (1916), who pointed out that very few stable molecules exist in which the total number of electrons is odd. His octet rule allows chemists to predict the most probable bond structure and charge distribution for molecules and ions. With the advent of quantum mechanics, it was realized that the electrons in a shared pair must have opposite spin, as required by the Pauli exclusion principle. The molecular orbital theory was developed to predict the exact distribution of the electron density in various molecular structures. The American chemist Linus Pauling introduced the concept of resonance to explain how stability is achieved when more than one reasonable molecular structure is possible: the actual molecule is a coherent mixture of the two structures.(coordinate bond a type of covalent bond between two atoms in which the bonding electrons are supplied by one of the two atoms. Also called dative bond.)
Metallic and Hydrogen Bonds
Unlike the ionic and covalent bonds, which are found in a great variety of molecules, the metallic and hydrogen bonds are highly specialized. The metallic bond is responsible for the crystalline structure of pure metals. This bond cannot be ionic because all the atoms are identical, nor can it be covalent, in the ordinary sense, because there are too few valence electrons to be shared in pairs among neighboring atoms. Instead, the valence electrons are shared collectively by all the atoms in the crystal. The electrons behave like a free gas moving within the lattice of fixed, positive ionic cores. The extreme mobility of the electrons in a metal explains its high thermal and electrical conductivity.
Hydrogen bonding is a strong electrostatic attraction between two independent polar molecules, i.e., molecules in which the charges are unevenly distributed, usually containing nitrogen, oxygen, or fluorine ((N, O, F)). These elements have strong electron-attracting power, and the hydrogen atom serves as a bridge between them. The hydrogen bond, which plays an important role in molecular biology, is much weaker than the ionic or covalent bonds. It is responsible for the structure of ice.
valence, combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound. Atoms are called monovalent, divalent, trivalent, or tetravalent, according to whether they form one, two, three, or four bonds
Also called addition compound. Chem. a combination of two or more independently stable compounds by means of van der Waals’ forces, coordinate bonds, or covalent bonds. (inclusion complex).
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