metallic bonding and crystallography

describe Hexagonal close packed structures (HCP)?

• ABA arrangement

• packing sequence of 1

• third layer sisters directly over first

• so here’s occupy 74% of total volume

what is a HCP unit cell ?

• the smallest parallel-sided volume element of a crystal structure (atom arrangement )

what does a HCP unit cell look like?

describe close cubic packing? (CCP)

• ABC or ABCA

• packing sequence of 2

• third layer sits over holes in first layer

• four layer sits directly over first layer

• 74% of the total volume is taken up by the spheres

what is a CCP unit cell?

• the unit cell sits with one corner on an A layer and four (ABCA ) required to complete the cell

• the CCP unit cell is also Face- centred cubic (FCC)

describe the Body-centred cubic (BCC) ?

• spheres occupy 68% of the total volume

• the arrangement spheres are in layers based on square rows NOT triangular

• second layer sits so a sphere is directly over the gap in the first layer and pushes the spheres slightly apart

• third layer sists DIRECTLY over first

what is the unit cell for a BCC ( body centred cubic )?

coordination number = 8

what is the overall definition of a unit cell ?

• the smallest spatial segment of an entire crystal structure that fully represents the whole structure

what is the coordination number for an atom in a hexagonal close packed structure ?

12

do HCP And CCP have the same coordination numbers ?

YES

what is the coordination number for BCC?

8

what is a crystalline solid

• atoms , ions or molecules are in a lattice

• have regular arrangement of atoms,ions or molecules

name examples of crystalline solids

• NaCl

• CuSO4.5H2O

• diamond

• any metak

what is an amorphous solid ?

• non crystalline

• atoms, ions or molecules have a random arrangement

• similar in character to liquids

name examples of amorphous solids

charcoal , rubber, glass

what are the 5 types of crystalline solids?

• atomic

• molecular

• ionic

• metallic

• network covalent

what is an atomic crystalline solid held together by and what is the strength of its bonding?

• dispersion forces

• weak bonding ∴ low melting boiling points

what is an example of an atomic crystalline solid?

noble gases

Identify the forces in molecular solids and their lattice structure

• intermolecular forces

  i.e dispersion, dipole, dipole, H bonds

• lattice structure: lattice points are occupied by individual molecules

give one example of molecular crystalline solid and what is its lattice point

• methane in an FCC structure

• C on each lattice point

what bonding is an ionic crystalline solid and how is the solid held together?

• ionic bonding : electrons are transferred between atoms to form cations and anions

• solid is held together by electrostatic forces

give examples of ionic crystalline solids ?

NaCl, AgI , CdS, CaF2, ZnS

what is the structure of crystalline metallic solids and what is it held together by?

• sea of electrons surrounding an array of cations held together by electrostatic interactions between electrons and cations

how do metals conduct electricity freely?

via motion of electrons

what is the structure of network covalent crystalline solids?

• atoms joined by strong cov bonds enxtending throughout the solid forming a network

what materials do network covalent solids form?

• hard rigid materials

give examples of network covalent solids?

diamond

graphite

what packing structure do metals adopt

HCP

CCP ( FCC)

BCC

in metallic structures what does delocalised electrons give rise to?

• non directional bonding

why are metallic structures ductile and malleable?

• delocalised electrons give rise to non directional bonding

why are metallic structures electrical conductors and thermal conductors?

• their electrons are delocalised and mobile

what is an alloy

• blend of metals ( and othe elements ) that is prepares by mixing molten components at an atomic level then cooling

what is a solid solution?

a solid with element mixed at the atomic level

what is the role of Zn in brass?

it increases the strength

what is the role of Cu in sterling silver

• increases strength

what is the role of each Cr , Ni and C in stainless steel ?

Cr = helps stop corrosion

Ni = increases strength

C= improves strength in a diff way

what is a substitutional alloy ?

• atoms of a solute metal randomly occupy sites in the solvent metal lattic

what is the criteria for a substitutional alloy?

• metal radii must be similar within 15% of each other

• solute metal must tolerate the coordination environment of the host lattice

• elctropositivty of two metals must be similar

why must the electropositivity be similar of the two metals in substitutional alloy?

• increased likely to be charge transfer and formation of a compound

what is an interstitial alloy?

• holes ( interstices) between metals are occupied by another element

• 26% of space is available even in a close packed structure

what is a crystal structure?

• name given to the unit cell and the particular arrangement of atoms within

what is a structure type?

• a structure type is a defined arrangement of atoms

what are unit cells defined by?

• three vectors ( a,b,c)

• α, β and γ

how do you count atoms in a unit cell

by using their positions

corner : 1/8 per unit cell

edge : ¼ per unit cell

face: ½ per unit cell

count the atoms of Na and Cl in NaCl

label the angles and sides in a cubic crystal system

Cubic (a = b = c, α = β = γ = 90°)

label the angles and sides in a hexagon crystal system

Hexagonal (a = b ≠ c, α = β = 90°, γ = 120°).

what is centring in relation to a unit cell and what does it do?

• translational symmetry

• it generates new atom positions linked by the structural symmetry of the unit cell

what is a primitive cell?

• has lattice points only on the corners of its unit cell

• has no centring

• EVERY LATTICE SYSTEM has a primitive lattice one within it

What is a Body-Centred Cell?

A body-centred cell (symbol I) has atoms shifted by ½ in all three directions (a,b,c), placing an atom at the body centre of the unit cell.

What is the translation vector for a Body-Centred Cell?

(½, ½, ½) — this translates the origin lattice point to the body centre.

What is a Face-Centred Cell?

: A face-centred cell (symbol F) has atoms shifted by ½ along two axes ( a and b ) , (a and c ) ( b and c )

what unit cell does this plan view represent?

primitive (P)

what unit cell does this plan view represent?

body centred cubic (BCC)

what unit cell does this plan view represent?

face centred cubic (FCC)

what unit cell does this plan view represent?

hexagonal close packed (HCP)