Main group chemistry

Oxidation states down a group

Positive states become more common because metallic behaviour increases

Why is multiple bonding common for second period elements?

They are smaller in size so, p oribtals are closer so π overlap is stronger

Alkali metals that form oxides O²⁻

Lithium

Alkali metals that form peroxides O₂²⁻

Sodium

Alkali metals that form superoxides O₂⁻

Potassium, Rubidium, Cesium

Boron

• Has covalent network structure, B12

• High mp

• Hard, black

• Metalloid

Why does Ga have w smaller radius than Al?

It has a filled 3d subshell which shields e- poorly

Inert pair effect

Explains why +1 is Thalliums most stable oxidation state

Boron group mp (increasing-decreasing)

B, Al, Ga

Natural sources of boron

Borate minerals such as borax

How is elemental boron extracted?

Converting borate minerals to B2O3 followed by reduction with Mg or Al (chemical reduction)

Three centre two electron bonds

• In boron compounds

• e- can be delocalized over more than two atoms

Boric acid H3BO3

• White

• Used as a disinfectant or insecticide

• Starting material for fibreglass

Boron containing glasses

• Borosilicate glass

• Resistant to thermal shock

• Used as cookware or lab glassware

Aluminium

• Most abundant metal in earth’s crust

• Typical metal

Bauxite

Hydrated form of Al2O3

Converting Al2O3 to Al

• Takes place through a reduction process carried out by electrolysis

• Requires melting the Al to a very high temperature

Hall-Héroult process

Cryolite added to Al2O3 lowers the melting point to a more accessible temperature (electrolytic reduction)

Aluminium alloys

• Strong

• Lightweight

• Resistant to corrosion when anodized

Pnictogen oxidation states

From -5 to +3. Negative states less stable down the group, +3 is only common for Bismuth.

Rubis and Sapphires

Aluminium is present in?

How is Nitrogen separated from air?

The liquid distillation of air

Nitrogen

• 80% of the atmosphere

• Highly unreactive

• Used as an inert blanketing gas

• Used as a cryogen to store sperm & eggs

Phosphorus

• Major source is phosphate rock, which contain things like fluoroapatite (forms part of tooth enamel)

• Phosphate rock is reported in terms of Calcium Phosphate present because it’s the quantity of interest when making fertilizers

Common phosphorus oxidation states

+3 and +5

P4O6

• Phosphorus “trioxide”

• White waxy toxic solid

• Formed with limited oxygen

N2O

• Laughing gas

• Colourless & sweet smelling

P4O10

• White solid

• Phosphorus “pentoxide”

• Powerful drying solid

• Formed with excess oxygen

NO

• Colourless & paramagnetic

• Air pollutant & neural transmitter

NO2

• Brown toxic gas

• In photochemical smog

• NO + O2 → NO2

Extracting phosphorus

Requires reduction, and carbon is used as the reducing agent

PH3

• Phosphine

• Colourless highly toxic gas

• No basic properties but can act as a base in organic reactions

• When ingested rat poisons hydrolyze to release phosphine

HNO2

• Nitrous acid

• Weak acid

• Useful reagent in organic synthesis

HNO3

• Nitric acid

• Strong acid

• Used to make ammonium nitrate which is a fertilizer

• Very strong oxidizing agent

H3PO3

• Weak acid

• Phosphorous

• Diprotic acid, NOT a triprotic one!!

• A hydrogen is bonded to the P atom

Ostwald process

Oxidation of NH4 to form HNO3

H3PO4

• Phosphoric acid

• Triprotic acid

• Strong acid

• Starting material for fertilizers

Sulfur oxidation states

-2 to +6

Selenium and Telluride applications

Semiconductors (photocopying machines also use selenium)

How is oxygen extracted?

Fractional distillation of liquid air

Oxygen applications

Steel making & chemical manufacturing

O3

• Ozone

• Diamagnetic gas

• Blue colour only visible at very high concentrations

• In the lower atmosphere, it is an air pollutant

Sulfur

• Has more allotropes than any other element

• Frequently bonds with itself in long chains or rings

Problem with sulfide minerals

Often contaminated with sulfur containing compounds (ex: H2S) that combust into SO2, which eventually oxidized into SO3 which creates acid rain when reacted with water

H2SO4

• Sulfuric acid

• Most important chemical substance in industry

• Used as a strong acid in chemical manufacture (especially fertilizer)

• Strong dehydrating & oxidizing agent