Chemsitry Of The Elements: Structure And Bonding

Question 1

Why do sigma bond energies tend to be greater than pi bond energies in homonuclear bonds and for which elements is this an exception?

Answer 1

Pi bonding is side-on and corresponds to less net overlap. Both tend to decrease going down a group due to weaker bonds formed between more diffuse (high energy) orbitals.

This is not true for N and O- sigma bonding unusually weak because of short bonds and electron-electron repulsion between lone pairs.

Question 2

Why do elements at the top of groups prefer to adopt structures containing sigma and pi bonds whereas elements that follow prefer structures containing predominantly sigma bonding?

Answer 2

Pi bond energy is comparable (if not greater than) to sigma bond energy.

Question 3

Why is nitrogen described as a true non-metal?

Answer 3

It only exists as a diatomic structure

Question 4

What are the possible forms of sigma bonded phosphorous allotropes?

Answer 4

White (Triangular pyramid, highly strained)
Red (polymerised form of white)
Black (most thermodynamically favourable, releases strain by buckling)

Question 5

What is the most common allotrope of sulphur?

Answer 5

Polymeric S(infinity)
It is helical
Sigma bonds
Chiral

Question 6

What is ionic lattice energy proportional to?

Answer 6

Charge on anion x charge on cation / sum of ionic radii

Question 7

What is the n=3 effect on heteronuclear bond energies?

Answer 7

X-F bond strength generally decreases down groups 13-15, but for groups 14-15 there is an increase in the third period.
This may be because these X atoms have access to d orbitals for pi bonding with fluorine, while those above do not.

Question 8

What is the Stevenson-schomaker equation?

Answer 8

rABb= rA + rB -9DX (takes account of polarity)

Question 9

What is the isoelectronic principle?

Answer 9

The idea that isoelectronic species are commonly also isostructural. i
i.e. s2p2 = tetrahedral diamond
s2p3 = Black phosphorus 
s2p4 = helical S
s2p5 = I2

Question 10

Why is boron nitride (hexagonal form) ‘eclipsed’ rather than staggered despite being isoelectronic with graphite?

Answer 10

B-N bonds are polarised so weak electrostatic (not just vdw) interactions develop between layers. This also means it is brittle, rather than soft like graphite.

Question 11

What are the 4 basic rules of the VSEPR model?

Answer 11

1. Most favourable arrangement of electron pairs in the valence shell of an atom is that which maximises the distance between them.
2. Lone pair repulsion > Bonding pair repulsion
3. Bonding pairs of electrons are distributed towards the most electronegative atom.
4. The two or three electron pairs of double and triple bonds repel more than the electron pair of a single bond.

Question 12

Why is (H3Si)3N not tetrahedral? (It is trigonal bipyramidal)

Answer 12

Possible due to d-p pi bonding between the filled 2p orbital on N and the 3d orbital on Si. This is called BACK DONATION.

Question 13

In PCl4F, why does F adopt the axial position?

Answer 13

It is the most sterically crowded, so the most electronegative elements adopt it (electrons closer to them than central atom, minimises repulsion).

Question 14

What does it mean to say a molecule is fluxional?

Answer 14

It has mobility on the timescale of the spectroscopic technique. This can occur if there are alternative structures which are close in energy to one another available.

(E.g. BERRY PSEUDOROTATION in PF5)

Question 15

What is a limitation of VSEPR?

Answer 15

Assumes electron localisation in 2c-2e bonds

Question 16

What is the consequence of little covalency in an ionic lattice?

Answer 16

No requirement for directionality in the anion-cation interaction, coordination numbers of the ions will be dependent on the ratio of the sizes of the ions involved.

Question 17

What are the ranges of limiting ratios (r+/r-) used to determine the geometry of an ionic lattice based on the assumption that the cation is at least big enough to prevent the anions from touching each other?

Answer 17

0. 225-0.414= tetrahedral, cn=4
1. 414-0.732= octahedral, cn=6
2. 732-1= cubic, cn=8

Question 18

What is lattice energy?

Answer 18

The molar enthalpy change when gaseous cations and anions are condensed into a solid lattice structure.

Question 19

In metals, how does conductivity vary with temperature?

Answer 19

Decreases with increasing temperature (due to increasing vibrations)

Question 20

What is band theory?

Answer 20

A model for the bonding in metals.

As the number of atoms in a metallic lattice increases, the energy gap between MOs decreases until they can essentially be thought of as 2 bands- bonding (valence band) and antibonding (conducting band).
The more compact the orbitals are (i.e. more electronegative the atom) the bigger the gap between these bands (and the worse the material is at conducting- fewer electronic transitions into upper band).