Exam 2- Chem of the elements

Closed pack structures

the arrangements of atoms in layers

Hexagonal closed packing layers

ABABAB (two repeating layers)

Cubic closed packing (FCC)

ABCABC (three repeating layers)

in each structure (closed packed), how many closest neighbors does each sphere have

each sphere has 12 nearest neighbors (12 spaces touch this given sphere)

Coordination number of each sphere

12

Neighbors of closed pack (where are they located)

6 neighbors in the same plane, 3 in the upper plane, and 3 in the lower plane

How much of the space is occupied by atoms in a closed pack structure

74% of the space, 26% of the remaining space is holes

Tetrahedral and octahedral holes

in closed packed structures, the spheres lie either at the corners of tetrahedra or octahedra. The spheres pack so that there are octahedral and tetrahedral holes between them

Are octahedral or tetrahedral holes bigger?

octahedral holes (twice as many octahedral holes than octahedral ones in a close-packed array)

Non-close packed arrangement

not organized in layers

how much of the space is occupied by spheres?

BCC: 68%

SC: 52%

Coordination number of SC

6

Coordination number of BCC

8

Polymorphism

many metals exhibit more than one crystalline form depending on temperature and/or pressure conditions

Metals are rarely

found uncombined in nature, they tend to combine to form alloys

Alloys

a mixture of metals or of metals and non metals generates alloys with improved physical properties such as hardness, resistance to corrosion, and ductility.

Example of an alloy

stainless steels (don't become stained with rust)

How is an alloy manufactured

by combining the different elements in the molten state followed by cooling

Why is the speed of the cooling process important?

-if the cooling is done rapidly, the distribution of the metal atoms in the solid solution is random. The element in excess is termed the solvent and the minor component is the solute.

-if the cooling is done more slowly, it results in a more ordered distribution of the solute atoms.

substitutional alloys

contain guest solute atoms which have the same size and coordination number as those of the host atoms.

example of substituional alloy

sterling silver used for silver cutlery and jewerly (92.5% Ag and 7.5% Cu)

Interstitial alloys

contain atoms which occupy the octahedral holes while smaller atoms occupy the tetrahedral holes

Carbon steel is an example of

an interstitial alloy

Intermetallic compounds

when mixtures of metals solidify, the structure of the formed alloy may be different from those of the pure metals.

Brass is an example of

an intermetallic compound

Band theory

used to describe the bonding in metals nd semiconductors and takes into account the mobility of the electrons over the whole crystalline structure.

band theory- metals

the MOs are close in energy and form a band of orbitals

band theory- insulator

if there is a significant energy difference between two bands, a band gap occurs

band theoy- semiconductors

if a fully occupied band is separated from an unoccupied band by a small band gap

intrinsic semiconductors

the occupation of the upper band (conduction band) increases with increasing temperature. The electrons in the upper band act as charge carriers and the semiconductor is able to conduct electricity.

The removal of electrons from the lower valence band creates positive holes into which electrons can move, and that leads to the ability to conduct charge

extrinsic semiconductors

A semiconductor that has been doped with an impurity element to increase its conductivity.

p-type semiconductor

positive semiconductor (Ga-doped Si), electrical conductivity arises from thermal population of an acceptor level which leaves vacancies (positive holes) in lower band

n-type semiconductor

a donor level is close in energy to the conduction band (As-Si)