Structure of metals (chap 2) part 1 Flashcards
Atoms can be packed together in what two ways
sparse, random packing structure and dense, ordered packing structure
which of the two packing structures have lower energy levels
dense, ordered packing
why are dense ordered packing have lower energy levels?
This could be sure to the fact that the intraatomic forces such as van der Waals forces (non-metal) / metallic bonding (metals) are maximized due to the particles being in close proximity to one another thus contributing to their stability and low energy.
what is a crystalline material?
A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances
What base materials contain crystalline structures
- all metals many ceramics and some polymers
characteristics of crystalline materials
have long range order
atoms packed periodically , 3d array
crystalline materials depend on what?
the type of crystal structure formed
the form of a crystal structure depends on what factors
it depends on the chemistry of the fluid, the conditions and the ambient pressure.
How are crystals formed?
from the cooling of a liquid
give 2 examples of materials that have crystal structures
AL, Ti, ice nacl, cscl, polyethylene polytetrafluoroethylene (Teflon)
Why is dense order lower in energy
Dense order is more stable, this stablity comes from the atoms being in close proximity to each other maximising the interatomic forces. they dont need as much energy to stay together.
non crystalline solids are also called what?
amorphous
what is a non crystalline solid?
this is one in which the atoms are not arranged in a systematic and regular arrangement over large atomic distances
apart from amorphous what are non crystalline solids called and why?
super cooled liquids, because their atomic structure resembles liquids
when are non crystalline solids formed?
they are formed when rapid cooling to freezing temperatures occurs faster than the atoms or molecules can organize themselves into a more thermodynamically favorable crystalline state.
what is a unit cell
a unit cell in the smallest repeating unit of a crystal lattice
unit cells can be in one of two shapes for most atoms what are the two shapes
parallelepipeds or prisms
what do unit cell affect
they affect how the crystal structure is described.
a crystalline structure can be FCC,HCP,BCC,SC what do these stand for?
Face-centered cubic, hexagonal close packed, body-centered cubic and simple cubic
Metallic crystal structures
The atomic bonding in this group of materials is metallic and thus nondirectional in nature.
why are metallic crystal structures dense
there are minimal restrictions as to the number and position of nearest neighbor atoms; this leads to relatively large numbers of nearest neighbors and dense atomic packings for most metallic crystal structure.
pure metals adopt one of the following
FCC,HCP,BCC
in a metallic crystal structure the spheres represent what?
ion cores
the atomic radius is defined by?
the ion core radius which is typically 0.1 - 0.2 nm
There are strong bonds to nearest neighbor atoms true of false?
- false, their is small to lower bond energy
what do electron clouds do?
they shield ion cores from each other to avoid repulsive forces.
characteristics of metallic bonds
- no restrictions to the numbers and positions of newest neighbor atoms
-non directional
-low bond energy
Density formula and what they mean
n = number of atoms associated with each unit cell
A = atomic weight (g/mol)
Vc = Volume of the unit cell (cm^3)
NA= Avogadro’s number
order of density
density metals> density ceramics> density polymers
order of density of why
metals have close-packing often large atomic masses, ceramics have less dense packing often lighter elements, polymers have low density often amorphous lighter elements (c,h,o), composites have intermediate values.
