scan0007 - 8.2 Ionic Bonding formation of a crystalline...

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Unformatted text preview: 8.2 Ionic Bonding; formation of a crystalline lattice of sodium and chloride ions is a large release of energy. A” Na- + -c1: —> [Na]® + [zillile —+ [Nal®i::l‘le EXERCISE 8.2 lonic Compound Formation Calcium chloride, CaClz, is spread on our sidewalks to melt ice. Because of its abil— ity to absorb moisture, it is used to dry gases and prevent dust buildup in mining and highway maintenance. Using Lewis dot symbols, show the transfer of electrons to form CaClZ from calcium metal and chlorine atoms. Solution we start by drawing the Lewis dot symbols for the calcium and chlorine atoms. Then we show the transfer of an electron from the less electronegative calcium to the more electronegative chlorine atom (arrow). We show a similar transfer with the other calcium electron to a different chlorine atom. We end with three ions (Ca2+ and two Cl‘) that have noble gas electron configurations. Note that the ions end up with either a completely full valence electron configuration (and become iso— electronic with a noble gas) or a completely empty valence shell (and become isoelectronic with a noble gas). This is the octet rule in action. c2a + \4-(21: PRACTICE 8.2 Use the Lewis dot structure model to show the formation of sodium oxide from atomic sodium and oxygen atoms. See Problems 21, 22, and 27—30. Medicinal chemists, biochemists, and pharmacognocists know about the dri- Ving force for the formation of ionic bonds. Ethidium bromide, shown in Figure 8.5, is a toxic compound that is used by researchers to stain DNA in order to make it easier to see. This compound exhibits its effects by inserting itself into DNA stands, causing structural changes to the DNA molecule. The negative charges on the DNA and the positive charge of the ethidium cation help hold the com— pound in place. These forces of attraction explain why this compound has such a 511ng association with DNA. Another example of the importance of ionic bond- ing accounts for the source of calcium in milk. Casein, one of the major proteins Build in milk, contains phosphate anions that form ionic bonds with calcium canons. Sizes of the Ions Water from many wells typically contains relatively high concentrations of alcium, often along with some magnesium and iron. When the water is heated, isolved calcium bicarbonate, Ca(HCO3)2, decomposes to form rock-like de- posits of calcium carbonate, CaC03, also known as boiler scale. The boiler scale Ca(HCO3)2(aq) —> CaCO3(5) + H200) + C02(g) coats the inside of the pipes that carry the water in the hot-water heater and throughout the house, so such water is called “hard.” Hard water is not hazardous 309 ElEURF§£ Ethidium bromide. The stabilizing force that holds the molecule in place in DNA is the attraction between the anionic charges on the DNA and the positive charge on ethidium bromide. Application ...
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