Forces holding crystals together

The free energy in a crystal is a minimum with respect to the arrangement of molecules and ions within it. A useful but approximate indication of the forces between components in a crystal is given by the energy needed to evaporate crystals into their separate molecules or ions. Typical values are the following for the ionic crystals lithium fluoride and sodium chloride, the energies required to break up the crystal into component anions 

Molecular crystals contain molecules that have aggregated by virtue of van der Waals interactions, hydrogen bonding, and other forces. If two molecules are polar, that is, they have a dipole moment, they 

TABLE 15.1. Types of intermolecular forces in crystal structures. 

Three oxygen atoms, each shared by two silicon atoms 

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The forces holding ions together in ionic solids are electrostatic forces. Opposite charges attract each other. These are the strongest intermolecular forces. Ionic forces hold many ions in a crystal lattice structure... 

The forces holding atoms together in molecules and crystals have been interpreted (p. 81) in terms of the electron density. In recent years further... 

What is the force holding ions together in a crystal ... 

Such an attractive force holds crystals of sodium chloride together. Here sodium is positively charged and chloride is negatively charged. 

Ion-ion forces (Section 2.13A) Strong electrostatic forces of attraction between ions of opposite charges. These forces hold ions together in a crystal lattice. 

Metallic Solids Most metallic elements crystallize in one of the two closest packed structures (Figure 12.30). In contrast to the weak dispersion forces in atomic solids, powerM metallic bonding forces hold atoms together in metallic solids. The properties of metals— high electrical and thermal conductivity, luster, and malleability— result from their delocalized electrons (Section 9.1). Melting points and hardnesses of metallic solids are also related to packing efficiency and number of valence electrons. We discuss bonding models that explain these metallic properties in the next two subsections. 

As the bulkiness of the substituents increases, the chains are prevented from coming into intimate contact in the crystal. The intermolecular forces which hold these crystals together are all London forces, and these become weaker as the crystals loosen up owing to substituent bulkiness. Accordingly, the value for the heat of fusion decreases moving down Table 4.2. 

Referring to Tables 5-1 and 5-II, we find that both sodium chloride and copper have extremely high melting and boiling points. These two solids have little else in common. Sodium chloride has none of the other properties that identify a metal. It has no luster, rather, it forms a transparent crystal. It does not conduct electricity nor is it a good heat conductor. The kind of forces holding this crystal together must be quite different from those in metals. 

At temperatures only slightly below the liquefaction temperatures, the liquids freeze. The solids are all simple crystals in which the atoms are close-packed in a regular lattice arrangement. The narrow temperature range over which any one of these liquids can exist suggests that the forces holding the crystal together are very much like the forces in the liquid. 

A sodium chloride crystal contains equal numbers of Na" cations and Cl anions packed together in an alternating cubic array. Figure 2-24 illustrates a portion of the sodium chloride array. Electrical forces hold the cations and anions in place. Each Na cation attracts all the nearby Cl anions. Likewise, each Cl anion attracts all its Na neighbors. Positive cations and negative anions group together in equal numbers to make the entire collection neutral. 

The best solvent for a molecular solid Is one whose Intermolecular forces match the forces holding the molecules in the crystal. For a solid held together by dispersion forces, good solvents are nonpolar liquids such as carbon tetrachloride (CCI4) and cyclohexane (Cg H12) For polar solids, a polar solvent such as acetone works well. Example provides some practice in recognizing solubility types. 

As one moves down the alkali metal family, one would expect the attractive forces holding the crystal structure together to decrease due to this last factor. 

In the crystalline solid state, there is little vibrational or translational freedom, and hence diffusion into a crystalline lattice is slow and difficult. As the temperature of a solid is raised by the input of heat, vibrational and translational motion increases. At a particular temperature - termed the melting point - this motion overcomes the attractive forces holding the lattice together and the liquid state is produced. The liquid state, on cooling, returns to the solid state as crystallization occurs and heat is released by the formation of strong attractive forces. 

The value of the effective van der Waals radius of an atom in a crystal depends on the strength of the attractive forces holding the molecules together, and also on the orientation of the contact relative to the covalent bond or bonds formed by the atom (as discussed below) it is accordingly much more variable than the corresponding covalent radius. In Table 7-20 there are given the ionic radii of nonmet llic elements for use as van der Waals radii. They have been rounded off... 

They are usually solids at room temperature, with high melting points. This is due to the strong electrostatic forces holding the crystal lattice together. A lot of energy is therefore needed to separate the ions and melt the substance. 

The reader is probably familiar with a simple picture of metallic bonding in which we imagine a lattice of cations M"+ studded in a sea of delocalised electrons, smeared out over the whole crystal. This model can rationalise such properties as malleability and ductility these require that layers of atoms can slide over one another without-undue repulsion. The sea of electrons acts like a lubricating fluid to shield the M"+ ions from each other. In contrast, distortion of an ionic structure will necessarily lead to increased repulsion between ions of like charge while deformation of a molecular crystal disrupts the Van der Waals forces that hold it together. It is also easy to visualise the electrical properties of metals in... 

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