Main Group Chemistry

Question 1

In which blocks of the periodic table do the main-group elements sit?

Answer 1

The s- and p-blocks

Question 2

Second vs third period elements

Answer 2

Second period elements are often anomalous while third period elements are more representative of their groups

Question 3

The anomalous behaviour of second period elements can be traced to:

Answer 3

1) small size and high electronegativity
2) maximum of four covalent bonds (no d-orbitals)
3) more common occurrence of multiple bonding (small atoms can be closer, allowing for good Pi-overlap, and Pi-bonds occur between p-orbitals with the same n values)

Question 4

Alkaline earth metals

Answer 4

Compared to alkali metals, these are harder, have higher melting points and are less reactive.
They occur naturally as ionic compounds containing the M2+ cation
Ex. Beryl (Be)
Magnetite (Mg)
Limestone (Ca)

Question 5

Preparation of alkaline earth metals

Answer 5

Mg reduces Be compounds such as BeF2 to elemental Be (this is because Mg is a stronger reducing agent and more readily gives up electrons)
Mg is isolated from compounds such as MgCl2 by electrolysis as elements heavier than Mg, though better reducing agents, tend to be very expensive

Question 6

Uses of alkaline earth metals

Answer 6

Be and Mg are used in strong, lightweight alloys for aircraft, cars, and bikes, and to make tools that won’t spark (Be0.002Cu0.98 and Mg0.9Al0.1)
CaCO3 is an important structural material (concrete, cement)
MgO, with v high mp, is used to line furnaces
And the reaction of Mg and O is very exothermic and used to make flashbulbs

Question 7

Reactivity of alkaline metals

Answer 7

Easily form M2+ cations in ionic compounds
Only Ca, Sr, Ba reduce water
M + H20 -> M(OH)2(aq) + H2(g)
(Be and Mg have a protective coating)

Question 8

Which alkaline earth metal is unique? Why?

Answer 8

Beryllium
BeO is amphoteric (acts as an acid or a base)
Be compounds have covalent character in bonding ex. BeCl2 doesn’t dissolve in water as it’s molecule with an extended structure.
Beryllium compounds are often electron deficient

Question 9

Group 13 elements-Boron

Answer 9

Boron (B) is a metalloid with a covalent network structure; it’s black, hard, high mp
Occurs naturally as the mineral borax
To prepare, it’s converted to B2O3 then reduced with Mg to elemental B
Used in Pyrex (a mix of B2O3 and SiO2) which is resistant to thermal shock
Sodium perborate is a colour safe bleach, used as a teeth whitening agent
Boric acid is a disinfectant, eyewash, contact solution
Boron forms many electron deficient compounds

Question 10

What is special about borane, B2H6?

Answer 10

It forms 3-centered, 2e- bonds (delocalized electrons) called a banana bond

Question 11

Group 13 elements-Aluminium

Answer 11

Al is a typical metal
Occurs naturally as mineral bauxite, which has a very high mp (extracted by electrolysis until the Hall-Héroult process, which added cryolite to reduce mp)

Question 12

Nitric acid

Answer 12

HNO3

Acid anhydride: N2O5

Question 13

Nitrous acid

Answer 13

HNO2

Acid anhydride: N2O3

Question 14

Phosphorous acid

Answer 14

H3PO3

Acid anhydride:P4O6

Question 15

Phosphoric acid

Answer 15

H3PO4

Acid anhydride: P4O10

Question 16

Group 16 elements (chalcogens)-Oxygen

Answer 16

Oxygen occurs as two allotropes; O2(dioxygen)
-colourless, paramagnetic gas with bp 183°C and making up 21% of the atmo
O3(ozone)
-bluish when liquified, this gas is pungent, diamagnetic, has a bp of 112°C and a concentration of ~8ppm in the strato
Extracted by fractional distillation of liquid air
Most comes from photosynthesis
CO2 + H2O -> O2 + (CH2O)n
Major uses: steel production, sewage treatment, chemical manufacture

Question 17

Group 16 elements (chalogens)-sulfur

Answer 17

Most important allotrope is S8(s), yellow sulfur, found in earths crust w a crown-shaped structure
Natural gas and crude oil contain H2S and organosulfur compounds. When sour gas/dirty oil is burned, sulfur is oxidized
S -> SO2 -> SO3 -> H2SO4
Sulfuric acid is the source of acid rain, and used as a cheap, strong acid. It’s also a strong dehydrating agent. (Sugar snake demo)

Question 18

Group 18 elements-noble gases

Answer 18

All monoatomic gases
Very low bps
With completely filled valence shells, they are unreactive
A compound of Xe, the first noble gas compound is used as a cryogen, cooling magnets used in MRI machines, it’s rarity is why MRIs are so expensive