Chap 10 Part 3a.notebook December 12, 2017

Transcription

1 Metallic Bonding and Semiconductors Chapter 10 Sect 4 Metallic Bonding positive metal ions surrounded by a "sea of electrons" Bonding is strong and nondirectional Iron, Silver, alloys, Brass, Bronze Forces Holding Units Together: Metallic; electrostatic attraction among metal ions and electrons Typical Properties: Malleable ductile good electric conductivity as solid and liquid good heat conductivity wide range of hardness and melting points

2 Alloy a substance that contains a mixture of elements and has metallic properties Substitutional alloy some of the "host" metal atoms are replaced by other metal atoms of similar size, the resulting alloy tends to be less malleable and ductile that the pure metals brass 1/3 of copper have been replaced by zinc Interstitial alloy some of the holes between atoms are occupied by smaller atoms Steel Carbon atoms are between iron atoms Carbon forms strong directional bonds making the alloy harder, stronger, and less ductile than pure iron Often the surface of a metal or alloy is changed through a chemical reaction. An example is formation of a chemically inert oxide layer in stainless steel, through reaction with oxygen in the air.

3 Silicon is a semiconductor a substance that does not conduct electricity very well Semiconductors conduct electricity better at higher temperatures (metals decrease in conductivity with increasing temperature) Doping semiconductors with other elements can improve conductivity n type semiconductors n for negative, add an element with one more valence electron than Si (arsenic) p type semiconductors p for positive, add an element with one less valence electron than Si (B) Silicon is a covalent network solid and a semiconductor. 1. Silicon forms a three dimensional network similar in geometry to a diamond. 2. Silicon s conductivity increases as temperature increases. 3. Periodicity can be used to understand why doping with an element with one extra valence electron converts silicon into an n type semiconducting (negative charge carrying) material, while doping with an element with one less valence electron converts silicon into a p type semiconducting (positive charge carrying) material. Junctions between n doped and p doped materials can be used to control electron flow, and thereby are the basis of modern electronics

4 Network solids or Network Covalent Solids covalently bonded compounds that form networks instead of discrete molecules graphite and diamonds SiO 2 (Silicates) sand, quartz, talc, mica Very hard and rigid, high melting points CO 2 melting point 78 C SiO 2 melting point 1600 C Graphite Diamonds Both are pure carbon allotropes! Quartz SiO 2 Network Solids are giant molecules Due to their size bonds need to be broken for them to melt unlike covalent compounds that form discrete (small) molecules! Covalent network solids have properties that reflect their underlying 2 D or 3 D networks of covalent bonds. Covalent network solids generally have extremely high melting points and are hard.

5 Amorphous Solids literally without a shape, solids that do not have regular structure at the atomic level leads to non specific melting points Includes glass, plastics (polymers) such as nylon and polyethylene Polymers tend to be long chained carbon compounds

6 Round up of Solids Ionic positive and negative ions, no discrete molecules but extended networks/lattices Metal + Nonmetal or polyatomic ions NaCl, CaCl 2, (NH 4 ) 3 PO 4 Forces Holding Units Together: Ionic strength based on Coulomb's Law Typical Properties Hard brittle high melting points poor electric conductivity as solids (ions can't move in a solid so electricity which is just e s can't move) good as liquid/molten often water soluble

7 Molecular discrete molecules held together by covalent bonds (nonmetals!) H 2, H 2 O, CO 2, CH 4, CH 3 OH Forces Holding Units Together: Dispersion forces, dipole dipole forces, Hydrogen bonds Typical Properties Low to moderate melting and boiling points (depends on IMFs) soft poor electric conductivity in solids and liquids Network Covalent Atoms held in an infinite one, two or three dimensional network graphite, diamond, quartz, silicates (think carbon and SiOs) Forces Holding units Together: Covalent bonds Typical Properties Wide range of harnesses and melting pointscan be very high poor electric conductivity with some exceptions Amorphous covalently bonded networks with no long range regularity plastics, glass, nylon Forces: covalent bonds Typical Properties noncrystalline wide temperature range for melting poor electric conductivity with some exception

8 Identify the type of solid Metal, molecular, ionic, Network covalent Cl 2 NaCl C 2 H 6 Fe C (graphite) (NH 4 ) 2 CO 3