semiconductors

MM 2006 AH Chemistry Unit 1

Semiconductors

2. Increasing temperature has what effect on the conductivity of :

i) metals ii) superconductors iii) semiconductors?

1. Where in the Periodic Table would you find semiconducting elements?

4. What is the conductor band?

5. Will conduction occur with filled, partially filled or empty bands?

6. Elements from which group are used in p-type doping?

Conduction involves the movement of what in p-type semiconductors?

3. Which term describes the highest occupied energy band?

7. Elements from which group are used in n-type doping?

What moves during conduction?

MM 2006 AH Chemistry Unit 1

Oxides, chlorides and hydrides of the second and third periods

properties relate to position in the periodic table

trends in physical properties and chemical reactions with water can be explained in terms of the changing nature of the bonding moving across the periods

MM 2006 AH Chemistry Unit 1

OxidesGenerally can be prepared by direct combination of

elements with oxygen

spontaneous

slowly at room temperature

occurs on heating

high temperature

e.g. alkali metals

e.g. magnesium

e.g. carbon

e.g. nitrogen

MM 2006 AH Chemistry Unit 1

Melting points of period 2 & 3 oxides

3000

2500

2000

1500

1000

500

0

-500

Melting point (oC)

Group

Series 1

Series 2

1 2 3 4 5 6 7

Period 2

Period 3

MM 2006 AH Chemistry Unit 1

Types of bonding in oxides

Electrical conductivity

Melts or solutions of ionic oxides

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

ionic lattice

ionic/covalent network

covalent network

covalent molecular

MM 2006 AH Chemistry Unit 1

‘Reaction’ of oxides with water

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

no reaction/insoluble

forms an acidslightly soluble but no reaction

forms an alkali e.g. Na2O(s) + H2O (l) NaOH(aq)

e.g. NO2 (s) + H2O (l) H NO3(aq)

forms a weak alkali

MM 2006 AH Chemistry Unit 1

Acid/base character of oxides

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

basic

amphoteric

acidic

neutral

Al2O3 + 6HCl 2AlCl3 + 3H2O

Al2O3 + 3H2O + NaOH 2NaAl(OH)4

weakly basic

MM 2006 AH Chemistry Unit 1

ChloridesCan be prepared by direct combination of elements with

chlorine or by reacting metals with hydrochloric acid

Bonding present in periods 2 and 3 varies in a similar way to that of the oxides

MM 2006 AH Chemistry Unit 1

Types of bonding in chlorides

Electrical conductivitymelts or solutions of ionic chlorides

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

ionic lattice

partially covalent network

covalent molecular

MM 2006 AH Chemistry Unit 1

‘Reaction’ of chlorides with water

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

soluble

insoluble/immiscible

produces fumes of hydrogen chloride

e.g. PCl5 (s) + 4H2O (l) H3PO4(aq) + 5HCl (aq)

MM 2006 AH Chemistry Unit 1

Acid/base character of chlorides

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

weakly acidic

acidic

neutral

MM 2006 AH Chemistry Unit 1

HydridesH- ion very strong base/reducing agent e.g.

2KH(s) + 2H2O(l) H2(g) + 2KOH(aq)

Reaction with sodium hydride produces the metal from

many metal oxides e.g.

CuO + NaH Cu + NaOH

Lithium aluminium hydride (LiAlH4) and sodium

tetrahydroborate (NaBH4) used in organic chemistry

when strong reducing agents required

MM 2006 AH Chemistry Unit 1

Types of bonding in hydrides

Electrical conductivitywhen molten

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

ionic lattice

ionic/covalent

polymeric

covalent molecular

good non-conductors poor

MM 2006 AH Chemistry Unit 1

‘Reaction’ of hydrides with water

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

forms hydroxide and hydrogen

insoluble

soluble

forms hydroxide

MM 2006 AH Chemistry Unit 1

Acid/base character of hydrides

Na

Li

Mg

Be

Al

B C N O F

Si P S Cl

strongly alkaline

alkaline

strongly acidic

neutral

weakly alkaline

weakly acidic