Core 3: Main Group

Question 1

What are the 3 Fajans rules of polarisability?

Answer 1

• Small, highly charged cations have polarising ability
• Large, highly charged anions are easily polarised
• Cations that dont have a noble-gas electron configuration are easily polarised

Question 2

Why does phosphorus form P₄ shapes in preference to P≡P such as in N₂?

Answer 2

Due to the strength of the triple bonds, P₄ is more favourable than P₂. In N₂ there is very strong pi overlap of orbitals but lone pair repulsions weaken the single bond.

Question 3

What solid state structure will Cs form with 18-crown-6?

Answer 3

Cs⁺ with an e^- counter ion

Question 4

What is the structures of borazine rings and cyclophosphazenes? Name a reaction which shows the reactivity of borazines compared to benzene.

Answer 4

A 6-membered ring of B(H)-N(H)=B(H)-N(H) and P(Cl₂)=N-P(Cl₂)=N.

Borazines will react with water to remove all double bonds.

Question 5

What molecular structure will LiBuⁿ resemble in the presence of TMEDA (bidentante ligand)?

Answer 5

Borane structrue with TMEDA ligands as the terminal positions and 2 lithium atoms sharing 2 Buⁿ molecules.

Question 6

Describe the structure of 18-crown-6, specifically the heteroatoms.

Answer 6

A crown ring of 2 carbon atoms then an oxygen atoms, there is 18 atoms in the ring, 6 of which are oxygen. The oxygen atoms are slightly offset above and below the plane of the crown.

Question 7

Define isoelectronic.

Answer 7

Where two fragments have the same number of electrons or the same electron configuration.

Question 8

What happens when sodium is placed in liquid EtNH₂ with crypt-2,2,2-and?

Answer 8

A complex forms where Na⁺ is surrounded by the cryptand and Na^- exists in solution.

Question 9

What are the 4 oxidation states for group 15? Where are they common?

Answer 9

-III, 0, +III, +V

+III is common in all elements, +V is common in the middle

Question 10

What is the reaction between EX₄ and an excess of water where E is a group 14 element? Any exceptions?

Answer 10

EO₂ will vigourously form except for carbon which is less likely to form the 5-coordinate intermediate which is required for it to form.

Question 11

What is the term for an atom that has more than 8 valence electrons?

Answer 11

Hypervalent.

Question 12

What is the stength of the polarising effect of lithium?

Answer 12

Very polarising as its size is very small so it distorts electron clouds that are much bigger than itself e.g Cl^-.

Question 13

How can cyclophosphazenes bonding be rationalised?

Answer 13

Using the sp² and s orbitals of N and the d orbitals of P overlap can be shown.

Question 14

What are the possible fluorides of sulfur? Describe some differences in their reactivity.

Answer 14

S₂F₂, SF₂, SF₄ and SF₆ are possible. SF₄ is highly reactive and toxic, SF₆ is extremely inert, non-toxic and used as electrical insulating gas.

Question 15

How do graphene sheets line up with one another in graphite? Is this the same for hexagonal boron nitride?

Answer 15

The carbon atoms in graphite line up with a gap above it. Hexagonal boron nitride is different and stacks with a boron atom directly underneath a nitrogen atom.

Question 16

Why does PX₅ have a structure where all positions are equal?

Answer 16

The shape undergoes ‘Berry pseudorotation’ sp all postions are equivalent.

Question 17

Why does Z_eff increase across the period?

Answer 17

Electrons in the same type of orbital do not shield each other well.

Question 18

What nitrogen hailides can form? How do these compare to phosphorus halides? Are these lewis basic or acidic?

Answer 18

N₂F₂, N₂F₄, NF₃ (other NX₃ compounds are contact explosives due to weaker N-X bonds), [NH₄]⁺ and ONH₃.

All these halides are possible as well as stable PX₃ and PX₅. All states except +V are lewis basic.

Question 19

Which noble gases are possible to form compounds with and what are some examples?

Answer 19

The elements later in the group where ionisation energies are lower can form compounds with strong oxidising elements.

XeF₂, XeF₄, XeF₆ and XeOF₄ have all been formed as well as KrF₂ and HArF however these are unstable.

Question 20

What happens when alkali metals are placed in liquid ammonia? What explains this observation?

Answer 20

A conducting, paramagnetic, bright blue solution is formed as an electron is released from the metal which is then solvated and has a virtual 1s→2p transition resulting in the blue colour.

Question 21

How will RMgX react with LiR and HgR₂? Describe its bonding

Answer 21

It will form MgR₂. A 3c2e structure similar to borane forms.

Question 22

Which halides, and with which elements, will oxidation states other than -I be formed?

Answer 22

All but F will form the hailides and will form with O or F.

Question 23

What is the trend for oxidation states of the group 13 elements?

Answer 23

At the top +III is most common but the inert pair effect means that +I becomes common down the group

Question 24

Name 3 similarities between Li and Mg, elements with a diagonal relationship

Answer 24

1. Both will form nitrides, Li₃N and Mg₃N₂ unlike other group 1 elements.
2. Their carbonates will decompose to CO₂ at much lower temperatures.
3. Both will form covalent organometallic compounts.

Question 25

What is the chemical name for silicones?

Answer 25

Polysiloxanes

Question 26

What oxidation effect do group 1 elements have?

Answer 26

They are strong reducing agents and especially in water they will reduce water to H₂ and OH^- and form M⁺ ions. This is a violent reaction.

Question 27

What is the IUPAC definition of oxidation state?

Answer 27

Oxidation state of an atom is the charge of this atom after ionic aproximation of its heteronuclear bonds.

Question 28

What structure will BeCl₂ take in vapour phase?

Answer 28

It will form dimers where 2 Cl atoms form bonds (with covalent character) to Be atoms. This leaves the Be atoms the original 2 Cl atoms and an additional Cl bond.

Question 29

What is the standard state of sulfur?

Answer 29

S₈ alpha-orthorhombic sulfur rings.

Question 30

How do you form the classic Mg organometallic and what solvent is used?

Answer 30

RX + Mg → RMgX.solvent

Solvent=Et₂O or THF

Question 31

How are lithium aromatics formed? What solvent is used?

Answer 31

From alkyls: LiBuⁿ + Arl → LiAr + BuⁿI in diethylether

Question 32

How are lithium alkyls formed? What solvents are suitable? Does this method work for heavier alkali metals.

Answer 32

2Li + RX →LiR + LiX, where R is an alkyl such as Bu or Bu^t

Dry solvents such as alkanes, benzene, ether and the lithium will reduce water.

This method is also suitible for heavier alkali metals.

Question 33

What reaction occurs when organometallic compounds react to carbonyls?

Answer 33

The additon of the carbonyl to form a R-O-M structure.

Question 34

What oxidation states do group 16 form?

Answer 34

All form -II, all but oxygen form +IV and +VI.

Question 35

What is size matching for a crown or cryptand?

Answer 35

The size of the ion which is being bound has to match with the size of the crown or cryptand. If the sizes match well then the equilibrium will be favoured to the complex with a large K_c value, called the binding constant.

Question 36

What is the structure of Li[C-2,1,1]⁺?

Answer 36

A cryptand where 3 carbon chains with an oxygen atom every 2 carbons surrounds a metal. The numbers indicate how many oxygens are in the chains.

Question 37

Why don’t N and Bi form +V oxidation states?

Answer 37

The ionisation enthalpy for N is high and will only occur for NO₃^-, not NH₅ or NX₅

The inert pair effect is strong in Bi.

Question 38

What hydrides exist for group 13? What is their structure? What is the trend and cause of their stability

Answer 38

MEH₄ where M=a group one metal and E=B, Al or Ga

A EH₄ ion and a M⁺ ion will form. These get less stable down the group due to poor orbital overlap. They are a useful of H^- but must be used in EtO₂.

Question 39

Which group 1 metal is the most reducing? Why?

Answer 39

Lithium even though it has the highest 1st I.E because it has a very high enthalpy of hydration. This is because it has a very high charge density due to its low ionic radius so it is strongly attracted to the water molecules.

Question 40

What shape will BeCl₂ take in the solid state, what can this be equated to?

Answer 40

It forms a chain structure with each chlorine atom forming a coordinate bond to an adjacent Be atom. This shape is similar to Al₂Cl₆.

Question 41

Why do graphite and hexagonal boron nitride have different reactivities?

Answer 41

Hexagonal boron nitride has a diople between the N and B where graphite doesn’t.

Question 42

What interhalide compounds are possible?

Answer 42

XY, XY₃ and XY₅ are formed where Y=F and X=Cl, Br and I. IF₇ is also possible under forcing conditions.

Question 43

Why do group 1 elements form metallic bcc lattices?

Answer 43

They have large metallic radii so they form lower density lattices.

Question 44

Why doesn’t BF₃ dimerise like BH₃?

Answer 44

BF₃ has some pi stability from Fs lone pairs

Question 45

What is the structure of LiMe soild and LiBuⁿ in alkane solution?

Answer 45

LiMe forms a tetrameric structure where the Li atoms form a triangular based pyramid and the Me atoms bond to the faces of the pyramid with interactions with the 3 near Li atoms. LiBuⁿ does the same but in a hexameric (hexagonal prisms) structure.

Question 46

What oxygen fluorides can form? What other halogen oxides can form?

Answer 46

OF₂ and O₂F₂.

XO₂, X₂O₆ and X₂O₇ can be formed with Cl, Br and I.