Lecture 7.0 - Metals

8/14/2019 Lecture 7.0 - Metals

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MetalsMetals


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Properties of Metals Luster

Malleability

Ductility

High thermal and

electrical conductivity

Hardness

These properties are the

result of non directional

covalent bonding found

in metallic crystals 2


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Packing in MetalsPacking in Metals

Model:Model: Packing uniform, hard spheres to best usePacking uniform, hard spheres to best useavailable space. This is calledavailable space. This is called closest packingclosest packing. Each. Eachatom has 12 nearest neighbors.atom has 12 nearest neighbors.

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Metal atoms slide or roll past one another

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Closest PackingClosest Packing

HolesHoles

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Gold atom packing

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Metal atoms arrange themselves

so that they are as close together

as possible (lowest possible PE),

while minimizing the repulsionbetween their nuclei

Each metal has a characteristic, lowest energy

arrangement of atoms which is called its

crystal structure


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Electron Sea Model

Metal atoms become cations

All valence electrons are delocalized andpooled together

A network of positive ions is immersed in a

mobile sea of electrons

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Mobile e- flowthroughout metal

crystal, enabling

metallic properties


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Band Model

Electrons travel within molecular orbitalsformed by the valence orbitals of the metallicatoms

When many metal atoms interact, the largenumber of resulting molecular orbitals becomemore closely spaced and finally form a virtualcontinuum of levels, called bands

Molecular orbitals occupied by conducting

electrons are called conduction bands

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orma on o an energy an n ummetal

As more and more Li atoms are added to the growing molecule, Li2, Li3, . . . ,

additional energy levels are added and the spacing between levels becomes

increasingly smaller. In an entire crystal ofNatoms, the energy levels merge

into a band ofNclosely spaced levels 11


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In band theory of conductivity, delocalized electrons move

freely through bands formed by overlapping molecular orbitals.

Mg 1s22s22p63s2 or [Ne]3s2

e-


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Energy Gaps Between Valence and Conduction Bands in

Metals, Semiconductors and Insulators

e-

e-


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Metals are malleable and ductile

Malleability: ability to pound a piece of metal into a thin sheet.

Ductility: ability to draw a piece of metal into a thin wire.

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When a piece of metal is hammered or

stretched, the atoms slide or roll past one

another. The metal atoms rearrangethemselves within the electron sea to

assume the new shape without breaking

their metallic bonds.


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Metals are good conductors of electricity

When an electric current (a stream ofe-) is

passed through a metal, the e- are free to move

from metal ion to metal ion.

The more freely they move through the metal,

the better conductor it is. Ag is the best conductor

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Metals are good conductors of heat

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Metals have luster

When light hits a metal

surface, the free valence e-on the surface absorb andthen re-emit the lightenergy,

which makes the

surface of the metal

appear shiny 17


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Hardness and Melting Points

The hardness and melting point of a metal depend to a

large extent on how many valence electrons the metal hasto participate in metallic bonding.

The greater the number of valence electrons, the more

firmly the metal ions are held in place.

Moving left to right across a period, the hardness andmelting points of metals generally increases as the

number of valence electrons available for metallic bonding

increases.

Alkali metals (Li, Na, K) have only one valence electron sothey are very soft metals with low melting points. Alkaline

Earth (Mg, Ca, Sr) have two valence electrons, so they are

harder than the Alkali metals and have higher melting

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Density The density of metals depends on the average

atomic mass of the atoms, and also on how the

metal ions are packed together to form its crystal

structure.

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osmium, Os, 22.61 g/cm3

iridium, Ir, 22.56 g/cm3

platinum, Pt, 21.4 g/cm3

rhenium, Re, 21.0 g/cm3

neptunium, Np, 20.4 g/cm3

plutonium, Pu, 19.8 g/cm3

gold, Au, 19.3 g/cm3

tungsten, W, 19.3 g/cm3

mercury, Hg, 13.53 g/cm3

lead, Pb, 11.4 g/cm3


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Why is mercury a liquidWhy is mercury a liquid

at STP?at STP?

Mercury hangs on to its valence 6s

electrons very tightly.

Mercury-mercury bonding is very

weak because its valence

electrons are not shared readily.

http://antoine.frostburg.edu/chem/senese/101/periodic/faq/why-is-mercury-

liquid.shtml

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Floating Billiard Ball?!Floating Billiard Ball?!

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Li id Mi th M ?Li id Mi th M ?


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Liquid Mirrors on the Moon?Liquid Mirrors on the Moon?

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3.7-m diameter liquid mirror at Laval University. The liquid is mercury.

There are no detectable mercury vapors in the air because a thin transparent

layer of oxide covers the surface.


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A mineralis a naturally occurring substance with a range of

chemical compositions.

An ore is a mineral deposit concentrated enough to allow

economical recovery of a desired metal.

Metallurgyis the science and technology of separating metals

from their ores and of compounding alloys.

An alloyis a solid solution either of two or more metals, or of a

metal or metals with one or more nonmetals.

Recovery of a metal from its ore:

1. Preparation of the ore

2. Production of the metal

3. Purification of the metal

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Mn nodules on the ocean floor

All i t


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Alloys: mixturesof metals that

give uniqueproperties

Alloys are solutions!

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Usually much harder and

stronger than pure metals

because:

1. additional valence electronsincreases the strength of the

metallic bonding

2. Smaller atoms will interfere

with how easily the metal atoms

can slide past one another


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Metal Alloy TypesMetal Alloy Types

Substitutional Alloy::

some metal atomsreplaced by others

of similar size.

brass = Cu/Zn

Interstitial Alloy:

Interstices (holes) in

closest packed metalstructure are occupied by

small atoms.

steel = iron + carbon


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Examples of Alloy Composition

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Coins are alloys too

Copper is used in all U.S. coins.

By 1982, the price of copper rose to where it was costingthe U.S. Treasury Department more than 1 cent to make a

penny.

Since then, to conserve copper and reduce costs, the

penny has been made of 97.5% zinc and 2.5% copper,

with the zinc core sandwiched between two thin layers of

copper.

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$10,000 penny?!


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Where did all the old quarters go?

Before 1965, dimes, quarters, and half-dollar US coins contained

90% silver &10% copper.

Since 1965, they are made of a copper (90%) and nickel (10%)

alloy

People started hoarding the silver versions almost as soon as the

new coins began to roll out from the mints.

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$2 quarter?!


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10 oz Pt Bar

12/07/09 @ 11:25 am :

$ 14 31

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Lecture 7.0 - Metals