Core 3: Main Group Flashcards
What are the 3 Fajans rules of polarisability?
- 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
Why does phosphorus form P4 shapes in preference to P≡P such as in N2?
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.
What solid state structure will Cs form with 18-crown-6?
Cs+ with an e- counter ion
What is the structures of borazine rings and cyclophosphazenes? Name a reaction which shows the reactivity of borazines compared to benzene.
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.
What molecular structure will LiBun resemble in the presence of TMEDA (bidentante ligand)?
Borane structrue with TMEDA ligands as the terminal positions and 2 lithium atoms sharing 2 Bun molecules.
Describe the structure of 18-crown-6, specifically the heteroatoms.
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.
Define isoelectronic.
Where two fragments have the same number of electrons or the same electron configuration.
What happens when sodium is placed in liquid EtNH2 with crypt-2,2,2-and?
A complex forms where Na+ is surrounded by the cryptand and Na- exists in solution.
What are the 4 oxidation states for group 15? Where are they common?
-III, 0, +III, +V
+III is common in all elements, +V is common in the middle
What is the reaction between EX4 and an excess of water where E is a group 14 element? Any exceptions?
EO2 will vigourously form except for carbon which is less likely to form the 5-coordinate intermediate which is required for it to form.
What is the term for an atom that has more than 8 valence electrons?
Hypervalent.
What is the stength of the polarising effect of lithium?
Very polarising as its size is very small so it distorts electron clouds that are much bigger than itself e.g Cl-.
How can cyclophosphazenes bonding be rationalised?
Using the sp2 and s orbitals of N and the d orbitals of P overlap can be shown.
What are the possible fluorides of sulfur? Describe some differences in their reactivity.
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.
How do graphene sheets line up with one another in graphite? Is this the same for hexagonal boron nitride?
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.
Why does PX5 have a structure where all positions are equal?
The shape undergoes ‘Berry pseudorotation’ sp all postions are equivalent.
Why does Zeff increase across the period?
Electrons in the same type of orbital do not shield each other well.
What nitrogen hailides can form? How do these compare to phosphorus halides? Are these lewis basic or acidic?
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.
Which noble gases are possible to form compounds with and what are some examples?
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.
What happens when alkali metals are placed in liquid ammonia? What explains this observation?
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.
How will RMgX react with LiR and HgR2? Describe its bonding
It will form MgR2. A 3c2e structure similar to borane forms.
Which halides, and with which elements, will oxidation states other than -I be formed?
All but F will form the hailides and will form with O or F.
What is the trend for oxidation states of the group 13 elements?
At the top +III is most common but the inert pair effect means that +I becomes common down the group
Name 3 similarities between Li and Mg, elements with a diagonal relationship
- Both will form nitrides, Li3N and Mg3N2 unlike other group 1 elements.
- Their carbonates will decompose to CO2 at much lower temperatures.
- Both will form covalent organometallic compounts.
What is the chemical name for silicones?
Polysiloxanes
What oxidation effect do group 1 elements have?
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.
What is the IUPAC definition of oxidation state?
Oxidation state of an atom is the charge of this atom after ionic aproximation of its heteronuclear bonds.
What structure will BeCl2 take in vapour phase?
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.
What is the standard state of sulfur?
S8 alpha-orthorhombic sulfur rings.
How do you form the classic Mg organometallic and what solvent is used?
RX + Mg → RMgX.solvent
Solvent=Et2O or THF
How are lithium aromatics formed? What solvent is used?
From alkyls: LiBun + Arl → LiAr + BunI in diethylether
How are lithium alkyls formed? What solvents are suitable? Does this method work for heavier alkali metals.
2Li + RX →LiR + LiX, where R is an alkyl such as Bu or But
Dry solvents such as alkanes, benzene, ether and the lithium will reduce water.
This method is also suitible for heavier alkali metals.
What reaction occurs when organometallic compounds react to carbonyls?
The additon of the carbonyl to form a R-O-M structure.
What oxidation states do group 16 form?
All form -II, all but oxygen form +IV and +VI.
What is size matching for a crown or cryptand?
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 Kc value, called the binding constant.
What is the structure of Li[C-2,1,1]+?
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.
Why don’t N and Bi form +V oxidation states?
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.
What hydrides exist for group 13? What is their structure? What is the trend and cause of their stability
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.
Which group 1 metal is the most reducing? Why?
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.
What shape will BeCl2 take in the solid state, what can this be equated to?
It forms a chain structure with each chlorine atom forming a coordinate bond to an adjacent Be atom. This shape is similar to Al2Cl6.
Why do graphite and hexagonal boron nitride have different reactivities?
Hexagonal boron nitride has a diople between the N and B where graphite doesn’t.
What interhalide compounds are possible?
XY, XY3 and XY5 are formed where Y=F and X=Cl, Br and I. IF7 is also possible under forcing conditions.
Why do group 1 elements form metallic bcc lattices?
They have large metallic radii so they form lower density lattices.
Why doesn’t BF3 dimerise like BH3?
BF3 has some pi stability from Fs lone pairs
What is the structure of LiMe soild and LiBun in alkane solution?
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.
What oxygen fluorides can form? What other halogen oxides can form?
OF2 and O2F2.
XO2, X2O6 and X2O7 can be formed with Cl, Br and I.
