Ch. 5 Atoms and Bonding Section 4. Bonding in Metals

1. Ch. 5 Atoms and Bonding Section 4. Bonding in Metals Objectives: I will explain how the properties of metals and alloys compare, describe how metal atoms are bonded in solid metal and explain how metallic bonding results in useful properties of metals.

2. A. Metals and Alloys • Metal that are used every day are made of more than one element. • An alloy is a mixture made of two or more elements that has the properties of metal. • Alloys are generally stronger and less likely to react with air or water than are the pure metals from which they are made.

3. a. Physical Properties 1. Properties of an alloy can differ greatly from those of its individual elements. 2. Depending on how they are mixed, alloys also retain many of the physical properties of metals 3. For example, pure gold is shiny, but it is soft and easily bent and for that reason, gold jewelry and coins are made of an alloy of gold mixed with a harder element, such as copper or silver.

4. b. Chemical Properties 1. Iron is an extremely strong metal that would be good for making tools. 2. However, iron objects rust when they are exposed to air and water, for this reason, iron is often alloyed with one or more other elements to make steel. 3. Stainless steel – an alloy of iron, carbon, nickel, and chromium

5. B. Metallic Bonding 1. The properties of solid metals and their alloys can be explained by the structure of metal atoms and the bonding between those atoms. 2. The loosely held electrons in metal atoms result in a type of bonding that is characteristic of metals. 3. A metal exists as crystals. 4. Metal atoms are very close together and in specific arrangements. 5. The atoms are positively charged ions.

6. 6. Their valence electrons are free to drift among the ions 7. Each metal ion is held in the crystal by a metallic bond 8. Metallic bond is an attraction between a positive metal ion and the electrons surrounding it. 9. A metal or metal alloy consists of positively charged metal ions embedded in a “sea” of valence electrons. 10. The more valence electrons an atom can add to the “sea,” the stronger the metallic bonds will be.

7. C. Metallic Properties • Why is it when you touch an aluminum pan and a wooden tabletop, the aluminum pan feels colder, when both objects are at room temperature? • You feel the difference because aluminum conducts heat away from your hand much faster than wood does. • Metal fins called a “heat sink” are used inside many electronics to cool their insides. • The “sea of electrons” model of solid metals explain the ease with which they can change shape, their ability to conduct electric current, their luster, and their ability to conduct heat.

8. a. Changes in Shape 1. Metal ions move easily, metals are ductile, which means that they can be bent easily and pulled into thin strands or wires. 2. Metals are flexible and can be reshaped easily (stretched, pushed or compressed). 3. Metals act this way because the positive ions are attracted to the loose electrons all around them rather than to other metal ions.

9. 4. However, the metallic bonds between the ion and the surrounding electrons keep the metal from breaking. 5. Because the metal ions move easily, metals are ductile, which means they can be bent easily and pulled into thin strands or wires. 6. Metals are also malleable – able to be rolled into thin sheets, as in aluminum foil, or beaten into complex shapes.

10. b. Electrical Conductivity 1. Recall that when charged particles are free to move an electric current is possible. 2. Metals conduct current easily because the electrons in a metal can move freely among the atoms. 3. When connected to a device such as a battery, there is a current into the metals at one point and out at another.

11. c. Luster 1. Polished metals exhibit luster, that is, they are shiny and reflective. 2. A metal’s luster is due to its valence electrons. 3. When light strikes these electrons, they absorb the light and then give it off again. 4. This property makes metals useful for making products as varied as mirrors, building, jewelry, and astronaut helmets.

12. d. Heat Conductivity 1. Thermal energy travels through materials as the greater motion of the particles in the warmer parts of the material is passed along to the particles in the cooler parts. 2. This transfer of thermal energy is known as heat. 3. Metals conduct heat easily because of the valence electrons’ freedom of motion within a metal or metal alloy.

13. D. Questions 1. How do covalent bonds form? - Covalent bonds form when two atoms share electrons. 2. What are molecular compounds? - Molecular compounds are compounds that consist of molecules bonded with covalent bonds. 3. How does unequal sharing of electrons affect the atoms in molecular compounds? - Unequal sharing of electrons causes the bonded atoms to have slight electrical charges.

14. 4. In a fluorine molecule, how many valence electrons does each fluorine atom have? - eight 5. How many valence electrons does oxygen have before and after bonding with hydrogen? - Six before; eight after. 6. How many valence electrons does nitrogen have before and after bonding with hydrogen? - Five before; eight after 7. How many hydrogen atoms would you expect to bind to an atom with four valence electrons? - four

15. 8. If two oxygen molecules were to share just two electrons instead of four, how many valence electrons would each oxygen atom have? (Refer to page 168 figure 16 in Physical Science book) - six 9. Why must two oxygen atoms share four electrons? (Refer to page 168 figure 16 in Physical Science book) - So that each atom has eight 10. What happens to the charge of oxygen if it pulls electrons more strongly? - It becomes negative

16. 11. What happens to the charge of hydrogen if it pulls electrons less strongly? - It becomes positive 12. Which do you think has a stronger positive charge, hydrogen atoms in a water molecule or sodium ions in a crystal of sodium chloride? - Sodium ions; the hydrogen atoms do not actually gain or lose electrons as ions do. 13. How do the shapes of the two molecules modeled in Figure 18 differ? (Refer to page 170 in Physical Science book) - Carbon dioxide has a straight-line shape. Water has a bent shape.