Brad Gietman's List: Sept 11

• Binary ionic compounds are not too common, but the existence of polyatomic ions greatly extends the number of ionic substances.
• Because oppositely charged ions are held in crystal lattices by strong coulombic forces, physical properties of ionic compounds include hardness, brittleness, and having high melting and boiling points. The majority of them dissolve in water, and in solution each ion exhibits its own chemical properties. Ionic compounds obey the octet rule, which explains why ion are generally in a noble-gas electron configurations.
• covalent bond. Sharing one or more pairs of electrons between two atoms attracts the nuclei together and usually results in an octet around each atom
• Covalent bonding often produces individual molecules, like CO₂ or CH₃CH₂OH, which have no net electrical charge and little attraction for each other. Thus covalent substances often have low melting and boiling points and are liquids or gases at room temperature.

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• The energy required in step 1 to remove an electron completely from an isolated atom is called the ionization energy
• The ionization energy of lithium is 520 kJ mol^–1. In other words, 520 kJ of energy is needed to remove a mole of 2s electrons from a mole of isolated lithium atoms in order to form a mole of isolated lithium ions
• The energy released when an electron is acquired by an atom is called the electron affinity.
• When oppositely charged particles move closer to each other, their potential energy decreases and they release energy. The energy released when lithium ions and hydride ions come together to 160 pm under the influence of their mutual attraction is 690 kJ mol^–1, more than enough to offset the 447 kJ mol^–1 needed to transfer the electron. Thus there is a net release of (690 – 447) kJ mol^–1 = 243 kJ mol^–1 from the overall process. The transfer of the electron from lithium to hydrogen and the formation of an Li⁺H^– ion pair results in an overall lowering of energy.

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• under the influence of their mutual attractions and repulsions, they collect together in much larger aggregates, eventually forming a three-dimensional array like that shown in the following figure. On the macroscopic level a crystal of solid lithium hydride is formed.

• The simple example of lithium hydride is typical of all ionic compounds which can be formed by combination of two elements. Invariably we find that one of the two elements has a relatively low ionization energy and is capable of easily losing one or more electrons. The other element has a relatively high electron affinity and is able to accept one or more electrons into its structure
• The ions formed by this transfer of electrons almost always have an electronic structure which is the same as that of noble gas, and all electrons are paired in each ion
• The resulting compound is always a solid in which the ions are arranged in a three-dimensional array or crystal lattice

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• because formation of a binary ionic compound places quite severe restrictions on the elements involved. One element must be a metal and must have a very low ionization energy. The other element must be a nonmetal and must have a very high electron affinity.
• lthough it is impossible to draw an exact line of demarcation, a good working rule is that essentially all binary compounds involving metals from periodic group IA, group IIA, group IIIB (Sc, Y, Lu), and the lanthanoids will be ionic
• he transition metals to the right of group IIIB in the periodic table form numerous binary compounds which involve covalent bonding, so they cannot be included in our rule
• The only other elements which form monatomic anions under normal circumstances are hydrogen (which forms H^– ions) and nitrogen (which forms N^3– ions)

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• A convenient method for doing this is to regard the compound as being formed from its atoms and to use Lewis diagrams. The octet rule can then be applied. Each atom must lose or gain electrons in order to achieve an octet. Furthermore, all electrons lost by one kind of atom must be gained by the other.

• Because each ion in a crystal lattice, like those shown for lithium hydride and calcium fluoride, is surrounded by many oppositely charged nearest neighbors, each ion is held fairly tightly in its allotted position. At room temperature each ion can vibrate slightly about its mean position, but much more energy must be added before an ion can move fast enough and far enough to begin to escape the attraction of its neighbors.
• A characteristic feature of such a molten ionic compound is that it conducts electricity. If two wires are placed in the melt and are connected to a battery, the positive ions in the melt will gradually move toward the negatively charged wire, while the negative ions will move toward the positively charged wire. Movement of charged ions through the melt constitutes passage of an electrical current just as movement of electrons through a metal wire does. Its occurrence can be detected by an appropriate electrical meter.
• The crystals of ionic substances are transparent, hard, brittle, and have characteristic shapes. They also are easily cleaved
• Perhaps the most important property of ionic solids is that most of them dissolve in water.
• A characteristic feature of these solutions is that they all conduct electricity, showing that the ions retain their charges in solution and are also free to move about in i

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• The most important chemical characteristic of ionic compounds is that each ion has its own properties. Such properties are different from those of the atom from which the ion was derived.

• The seven crystal systems include all the basic types of unit cells. Closest-packed structures are important because they are ways of squeezing the maximum number of atoms into the minimum volume. There are two types: hexagonal closest packed ( hcp) and cubic closest packed ( ccp).
• Some liquids become so viscous just above their melting points that their particles no longer can move. In such a case a glass or amorphous material, in which there is no long-range order, is formed.
• Generalizations about phase transitions, where a solid changes to liquid, liquid to gas, or solid to gas can also be made. All require energy because they involve separation of particles which attract one anothe
• A saturated solution is one in which solute is in equilibrium with solution at a given temperature. If a supersaturated solution is prepared, it is unstable. Adding a seed crystal permits solute molecules to precipitate until the rate of solution equals the rate of precipitation. The solubility of a solute or the composition of any solution which is to be used quantitatively is usually reported in terms of concentration (amount of solute/volume of solution), mass fraction (mass of solute/mass of solution), or mole-fraction (amount of solute/amount of solution).
• The solubilities of various substances can be related to the molecular structures of solute and solvent. The most important rule is that like dissolves like.

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• This parallel-sided figure is the unit cell.

• Without some experience, it is quite easy to pick the wrong unit cell for a given structure. Some incorrect choices are shown immediately below the correct choice in the figure. Note in particular that the unit cell for structure b, in which each circle is surrounded by six others at the comers of a hexagon, is not a hexagon, but a parallelogram of equal sides (a rhombus) with angles of 60 and 120°.

  Figure 2 illustrates the space lattice and the unit cell for a real three-dimensional crystal structure—that of sodium chloride.

  

• The unit cell of sodium chloride contains four sodium ions and four chloride ions. In arriving at such an answer we must bear in mind that many of the ions are shared by several adjacent cells (part c of Figure 2 shows this well). Specifically, the sodium ions at the centers of the square faces of the cell are shared by two cells, so that only half of each lies within the unit cell. Since there are six faces to a cube, this makes a total of three sodium ions. In the middle of each edge of the unit cell is a chloride ion which is shared by four adjacent cells and so counts one-quarter. Since there are twelve edges, this makes three chloride ions. At each comer of the cube, a sodium ion is shared by eight other cells. Since there are eight comers, this totals to one more sodium ion. Finally there is a chloride ion in the body of the cube unshared by any other cell. The grand total is thus four sodium and four chloride ions.

  A general formula can be derived from the arguments just presented for counting N, the number of atoms or ions in a unit cell. It is

  

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• An important class of crystal structures is found in many metals and also in the solidified noble gases where the atoms (which are all the same) are packed together as closely as possible
• Each sphere will be touching twelve of its fellows since it is surrounded by six in the same plane and nestles among three in the plane above and three in the plane below. We say that each sphere has a coordination number of 12. It is impossible to make any other structure with a larger coordination number, that is, to pack more spheres within a given volume. Accordingly the structure just described is often referred to as a closest-packed structure.

• The first step in this process is to draw a skeleton structure to show which atoms are linked to which. Next, determine how many valence electrons are available by adding the periodic group numbers for all atoms in the molecule. Finally, allocate the available electrons either as shared pairs or as lone pairs so that each atom has an octet (or in the case of H, a 1s² pair).

• Our discussion of ionic compounds was confined to monatomic ions. However, more complex ions, containing several atoms but still having a positive or negative charge, occur quite frequently in chemistry. Well-known examples of such polyatomic ions are
• the sulfate ion (SO₄^2–)
• the hydroxide ion (OH^–),<script type="text/javascript"> jmolInitialize("http://144.92.39.89/~xavier/jmol/"); </script>
• the hydronium ion (H₃O⁺),
• and the ammonium ion (NH₄⁺).<script type="text/javascript"> jmolInitialize("http://144.92.39.89/~xavier/jmol/"); </script>
• The atoms in these ions are joined together by covalent electron-pair bonds, and we can draw Lewis structures for the ions just as we can for molecules. The only difference is that the number of electrons in the ion does not exactly balance the sum of the nuclear charge

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