Metals and Metallic Properties 4

Metallic Substances

• lattice of positive nuclei surrounded by delocalised electrons (a sea of electrons)

• metals hold onto their valence electrons very weakly

Shared Properties of Most Metals

• relatively high melting points

• relatively high boiling points

• good conductors of heat

• good conductors of electricity

• malleable

• ductile

Conduction of Electricity

• metals conduct electricity

• electrons are free to move through the solid since not fixed to positive nuclei

Conduction of Heat

• metals conduct heat

• positive nuclei gain energy ∴ move faster → since they are closely packed together they hit the particle next to them so the energy is transferred along the metal

• electrons also gain energy and since position is not fixed → it can flow along the metal

Malleability and Ductility

• malleable - hammered into shape (able to be bent)

• ductile - drawn into wires

• when hit with a hammer → the layers of lattice just slide over each other → ∴ does not break

• electrons allow positive nuclei to slide by without them repelling each other

Metallic Bond

• special bond type found in metals

• hold metal atoms together very strongly

• formed from atoms of metallic elements

• electron cloud around atoms form a mobile ‘sea’

• good electrical conductors at all states

• lustrous

• very high melting points

• e.g. Na, Fe, Al, Au

Metals Form Alloys

• metals do not combine with metals

• form alloys - solution of a metals mixed (but not reacted) with other metals

• e.g. steel, brass, bronze

Reactivity with Water

• the way metals react with water can indicate their relative reactivity

• increases down a group and decreases across the period from left to right

• transition metals are generally less reactive with water than group 1 and group 2 metals are

Reactivity with Acids

• increases down a group and decreases across the period from left to right

• metals are normally more reactive with acids than with water

• more metals react with acids and the reactions tend to be more energetic

Reactivity of Oxygen

• group 1 metals react rapidly with oxygen

• heat is usually required to start the reaction with group 2 metals → does not react as rapidly as group 1 metals

• many transition metals needed by society cannot be found in nature as pure elements but often exist as oxides

Reasons for Different Reactivities of Metals

• in general, the reactivity of main group metals increases going down a group and decreases across a period

• can be explained in terms of the relative attractions of valence electrons to the nucleus of atoms

  • when metals react, their atoms tend to form positive ions by donating one or more of their valence electrons to other atoms

  • the metal atoms that require less energy to remove electrons tend to be the most reactive

  • the most reactive metals tend to be those with the largest atomic radii and therefore the lowest ionisation energies (bottom-left corner of periodic table)

Modifying Metals

• metal can be determined by the metal’s physical and chemical properties

• most metals need to be modified to make them more useful (some are valuable in their pure state)

• metals can be modified by:

  • through alloy production

  • by heat treatment

  • by the formation of nano-sized structures

Alloy Production - way of modifying metals

• alloy - resultant of when metals are mixed and melted together with small amounts of another substance (metal/carbon)

• by varying the composition of alloys → can obtain materials with specific properties

• harder and melts at a lower temperature than the pure metal (generally)

  • because atoms of different sizes are now included in the metal lattice

  • the atoms do not pack in the same way as the main metal → don’t allow the lattice to shift and bend in the same way

  • also accounts for the lowered melting point

Steel - alloys of iron

• all the iron mined around the world is used to make the alloy steel

• simplest steel made is carbon steel → adds a small amount of carbon to iron

  • type of interstitial alloy

• small proportion of an element with significantly smaller atoms is added to a metal

• added atoms sit in interstices (very small spaces) between metal cations in the metallic lattice

• harder and less malleable than pure iron (generally)

• varying the amount of carbon in the mixture produces steels with different properties

  • allows the steel with the most suitable properties to be used in specific applications

Altering the Structure of Metals

• work hardening

  • hammering/working cold metals causes the crystals to rearrange as they are pushed and deformed

  • results in the hardening of the metal as the crystals are flattened out and pushed closer together

• heat treatment

  • physical properties of a metal can be altered by controlled heating and cooling

  • when metals are heated above a critical temp., the individual crystals merge → reform when allowed to cool

  • quenching and tempering → increases the strength and wear properties

  • annealing → reduces the strength/hardness, increases uniformity of crystals → reduces stresses and restores ductility

  • annealing - metal is heated to a moderate temp. and allowed to cool slowly

    • larger metal crystals form

    • metal is softer with improved ductility

  • quenching - metal is heated to a moderate temp. and cooled quickly (e.g. by plunging into water)

    • tiny metal crystals form

    • metal is harder and brittle

  • tempering - quenched metal is heated (to a lower temp than quenching) and allowed to cool

    • crystals of intermediate size form

    • metal is hard but less brittle

Crystals of Metals

• crystal - region in a solid where the particles are arranged in a regular way

• each crystal is a continuous regular arrangement of cations surrounded by a sea of delocalised electrons → but the arrangement is random

• the way a metal behaves (malleability and brittleness) depends on the size and arrangement of the crystals

• smaller crystals result in harder metals as there is more free movement of layers of cations over each other (generally)

  • also have more areas of disruption between them → metals with be more brittle