Ionic Compounds

1. Ionic Compounds 7.4 Metallic Bonds and the Properties of Metals

2. Metallic Bonds • Objectives: • Describe a metallic bond • Relate the electron sea model to the physical properties of metals • Define alloys and categorize them into two basic types

3. Metallic Bonds • Properties of materials are based on bonding, and the bonding in both metals and ionic compounds is based on the attraction of particles with unlike charges. • Metals are not ionic but share several properties with ionic compounds.

4. Metallic Bonds • As solids, metal atoms form lattices, similar to ionic crystal lattices. • Each metal atom is surrounded by 8-12 other metal atoms. • Although metal atoms have at least one valence electron, they do not share these electrons with neighboring atoms nor do they lose electrons to form ions.

5. Metallic Bonds • Instead, the outer energy levels of the metal atoms overlap. • The electron sea model proposes that all metal atoms in a metallic solid contribute their valence electrons to form a “sea” of electrons.

6. Metallic Bonds • The electrons are not held by any specific atom and can move easily from one atom to the next. • Because they are free to move they are often referred to as delocalized electrons, with the rest of the atom called a metallic cation.

7. Metallic Bonds

8. Metallic Bonds • A metallic bond is the attraction of a metallic cation for delocalized electrons.

9. Properties of Metals In general, metals have • moderately high melting points. • Cations and electrons are mobile; it does not require extreme amounts of energy to move them past each other. • Melting points of metals do vary greatly, however. • high boiling points. • Atoms must be separated from the group of cations and electrons; requires a lot of energy.

10. Properties of Metals • Malleable, meaning they can be hammered into sheets, and ductile, meaning they can be drawn into wire. • Mobile particles can be pushed or pulled past each other. • Durable • Electrons even though mobile, are strongly attracted to the metallic cations. They are not easily removed from one another.

11. Properties of Metals • Good conductors • Delocalized electrons in metal are free to move, keeping metallic bonds intact. • Delocalized electrons move heat quickly from one place to another. • Electrons themselves move as part of an electric current when an electric potential is applied to the metal • Lustrous • Delocalized electrons interact with light

12. Properties of Metals • The mobile electrons in transition metals consist of s and d electrons • As the number of delocalized electrons increases, so do the properties of hardness and strength.

13. Metallic Bonds • Strong metallic bonds are found in transition metals like chromium, iron and nickel.

14. Metallic Bonds • Whereas alkali metals are considered soft because they have only one delocalized electron.

15. Metal Alloys • An alloy is a mixture of elements that has metallic properties. • See Table 13, pg. 228

16. Metal Alloys • The properties of alloys differ somewhat from the properties of the elements they contain. • Steel is iron mixed with at least one other element. • Properties of iron are present but steel is much stronger than iron alone.

17. Metal Alloys • Alloys most commonly form when the elements involved are either similar in size or the atoms of one element are considerably smaller than the atoms of the other. • Two basic types of alloys exist.

18. Metal Alloys • Substitutional Alloy: Atoms of the original metallic solid are replaced by other metal atoms of similar size. • Ex. sterling silver – Cu atoms replace some Ag atoms.

19. Metal Alloys • Brass, pewter and 10-carat gold are also examples of substitutional alloys.

20. Metal Alloys • An interstitial alloy is formed when small holes (interstices) in a metallic crystal are filled with smaller atoms. • Carbon steel (see next slide)

21. Metal Alloys • Carbon steel • holes in the iron crystal are filled with carbon atoms • the alloy formed is harder and stronger, but less ductile, than pure iron