Structure of metals (chap 2) part 1

Question 1

Atoms can be packed together in what two ways

Answer 1

sparse, random packing structure and dense, ordered packing structure

Question 2

which of the two packing structures have lower energy levels

Answer 2

dense, ordered packing

Question 3

why are dense ordered packing have lower energy levels?

Answer 3

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.

Question 4

what is a crystalline material?

Answer 4

A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances

Question 5

What base materials contain crystalline structures

Answer 5

• all metals many ceramics and some polymers

Question 6

characteristics of crystalline materials

Answer 6

have long range order
atoms packed periodically , 3d array

Question 7

crystalline materials depend on what?

Answer 7

the type of crystal structure formed

Question 8

the form of a crystal structure depends on what factors

Answer 8

it depends on the chemistry of the fluid, the conditions and the ambient pressure.

Question 9

How are crystals formed?

Answer 9

from the cooling of a liquid

Question 10

give 2 examples of materials that have crystal structures

Answer 10

AL, Ti, ice nacl, cscl, polyethylene polytetrafluoroethylene (Teflon)

Question 11

Why is dense order lower in energy

Answer 11

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.

Question 12

non crystalline solids are also called what?

Answer 12

amorphous

Question 13

what is a non crystalline solid?

Answer 13

this is one in which the atoms are not arranged in a systematic and regular arrangement over large atomic distances

Question 14

apart from amorphous what are non crystalline solids called and why?

Answer 14

super cooled liquids, because their atomic structure resembles liquids

Question 15

when are non crystalline solids formed?

Answer 15

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.

Question 16

what is a unit cell

Answer 16

a unit cell in the smallest repeating unit of a crystal lattice

Question 17

unit cells can be in one of two shapes for most atoms what are the two shapes

Answer 17

parallelepipeds or prisms

Question 18

what do unit cell affect

Answer 18

they affect how the crystal structure is described.

Question 19

a crystalline structure can be FCC,HCP,BCC,SC what do these stand for?

Answer 19

Face-centered cubic, hexagonal close packed, body-centered cubic and simple cubic

Question 20

Metallic crystal structures

Answer 20

The atomic bonding in this group of materials is metallic and thus nondirectional in nature.

Question 21

why are metallic crystal structures dense

Answer 21

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.

Question 22

pure metals adopt one of the following

Answer 22

FCC,HCP,BCC

Question 23

in a metallic crystal structure the spheres represent what?

Answer 23

ion cores

Question 23

the atomic radius is defined by?

Answer 23

the ion core radius which is typically 0.1 - 0.2 nm

Question 23

There are strong bonds to nearest neighbor atoms true of false?

Answer 23

• false, their is small to lower bond energy

Question 23

what do electron clouds do?

Answer 23

they shield ion cores from each other to avoid repulsive forces.

Question 24

characteristics of metallic bonds

Answer 24

• no restrictions to the numbers and positions of newest neighbor atoms
  -non directional
  -low bond energy

Question 25

Density formula and what they mean

Answer 25

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

Question 26

order of density

Answer 26

density metals> density ceramics> density polymers

Question 27

order of density of why

Answer 27

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.