Structures and Types of Solids

Two types of solids  1. Crystalline solids  highly regular arrangement of their particles  crystals- at microscopic level

How are they crystals?



Lattice

- 3-D system of points indicating position of ions, atoms or molecules that make up the substance

Unit cell  Smallest repeating pattern of the lattice extend in all directions for the structure

3 types of unit cells  A. Simple cubic  polonium metal  B. Body centered cubic  Uranium metal  C. Face centered cubic  Gold metal

Two types of solids  2. Amorphous solids considerable disorder in their structure  EX- glass- liquid that is “frozen in place”

Crystalline solids??

 How do we determine the structure of the solid?

 X-ray diffraction!!

Bragg equation  1915 Noble prize in physics  n  = 2dsin   where n= integer  = wavelength of x-rays d= distance b/w atoms  = angle of incidence

Types of crystalline solids  Easy to classify based on what particle is at lattice point

Types of crystalline solids  Explains why solids have different properties  melting point  conductivity  ductility

Three types  1. Ionic solids- have ions at the points of the lattice  NaCl

Three types  2. Molecular solid- have discrete covalently bonded molecules at lattice points  ice

Three types  3. Atomic solids substances that have atoms at the lattice points  C, B, Si and all metals

Three types of atomic solids  A. metallic solids delocalized nondirectional covalent bonding

Three types of atomic solids  B. network solids atoms bond with strong directional covalent bonds that lead to giant molecules (networks)

Three types of atomic solids  C. Group 8 solids noble gas elements are attracted by London dispersion forces

Structure and Bonding in Metals  Metals- high thermal and electric conductivity, malleability, ductility due to nondirectional covalent bonding

Closest packing  Spherical atoms packed together and bonded in all directions  spheres packed in layers-each surrounded by 6 others

Three arrangements 

Hexagonal closest packed structure (hcp)-

aba arrangement  has hexagonal unit cell

hcp structure  Every other layer has the same vertical position  EX: Mg, Zn

Three arrangements  

Cubic closest packed structure (ccp)-

abc arrangement  face-centered cubic cell

ccp structure  Every 4th level occupies the same vertical position  EX: Al, Fe, Cu, Co, Ni

Three arrangements 

Body centered cubic unit cell (bcc)-

spheres touch along the body diagonal of the cube

bcc unit cell  Most spread apart arrangement  EX: alkali metals

Counting atoms...

 Need to know number of atoms in a unit cell

Face-centered  8 cubes share one cell  1/8 x 8 corners +  1/2 x 6 faces =  net 4 whole spheres

Density of ccp solid  Ag crystallizes in a ccp structure. The radius of a silver atom is 144pm. Calculate the density of solid silver.

Bonding models for metals  Model must account for physical properties:  A. shape can be changed fairly easily malleable and ductile

Bonding models for metals  B. durable  C. high melting points

Bonding models for metals  Indicates the bonding is STRONG and NONDIRECTIONAL  or difficult to separate metals atoms, but easy to move them

The model is...

 Electron “sea”model  metal cations in sea of e (mobile e for conductivity and cations can be moved around when hammered

Last topic-metal alloys!

 Metals introduced into the crystal structure of other metals

Alloy  A substance that contains a mixture of elements and has metallic properties

Two types of alloys 

Substitutional alloy-

some of the host metal atoms are

replaced

by other metal atoms of similar size

Substitutional alloys  EX: Brass (1/3 of copper atoms replaced with zinc)  Pewter (85%Sn, 7%Cu, 6%Bi, 2%At)

Two types of alloys 

Interstitial alloy-

formed when some of the holes in a close packed metal structure are occupied by smaller atoms

Interstitial alloy  EX: Steel (carbon atoms into iron)  Strengthens iron by adding strong directional bonds