Ionic Compounds

7.4 Metallic Bonds and the

Properties of Metals

Metallic BondsObjectives:

Describe a metallic bond Relate the electron sea model to the

physical properties of metals Define alloys and categorize them into two

basic types

Metallic BondsProperties 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.

Metallic BondsAs 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.

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.

Metallic BondsThe 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.

Metallic Bonds

Metallic BondsA metallic bond is

the attraction of a metallic cation for delocalized electrons.

Properties of MetalsIn 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.

Properties of MetalsMalleable, 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.

Properties of MetalsGood 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

Properties of MetalsThe mobile electrons in transition

metals consist of s and d electronsAs the number of delocalized

electrons increases, so do the properties of hardness and strength.

Metallic Bonds

Strong metallic bonds are found in transition metals like chromium, iron and nickel.

Metallic BondsWhereas alkali metals are considered

soft because they have only one delocalized electron.

Metal AlloysAn alloy is a

mixture of elements that has metallic properties.

See Table 13, pg. 228

Metal AlloysThe 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.

Metal AlloysAlloys 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.

Metal AlloysSubstitutional

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.

Metal AlloysBrass, pewter and

10-carat gold are also examples of substitutional alloys.

Metal AlloysAn interstitial alloy is formed when small

holes (interstices) in a metallic crystal are filled with smaller atoms. Carbon steel (see next slide)

Metal AlloysCarbon steel

holes in the iron crystal are filled with carbon atoms

the alloy formed is harder and stronger, but less ductile, than pure iron