Inorganic and Materials Chem 1 Flashcards
what are the three factors which determine how well AOs combine
1) Constituent AOs must have suitable symmetry to interact
2) The constituent AOs must have similar energies to for significant bonding and antibonding interactions to occur
3) Even if the orbitals are close in energy, higher degrees of overlap will give a better interaction - generally smaller orbitals
How is the periodic table laid out
- increasing atomic number
- aligned vertically according to number of valence electrons, these generally have similar chemical properties - groups
- arranged horizontally by highest energy level of electrons - periods
what do we generally treat the nucleus as when considering periodicity and the periodic table
- a point charge
name the blocks of the periodic table, what can they vaguely be thought of as, why is this not always the case
- s,p,d,f blocks
- they roughly correspond to the subshell of the highest energy valence electrons
- this is not always the case there can be exceptions so the groups often indicate similar properties e.g. Lu, Lr
why are He and H floating
- their chemical properties don’t match the s-block
explain briefly why Cu and Cr have unusual configurations, give the configurations
- ‘Exchange Interaction Energy’ is a quantum mechanical phenomena which explains the ‘Half-shell stability’ found in Cr and Cu
Cr: ….4s1, 3d5
Cu: ….4s1, 3d10
what does the strangeness of some of the d-block and f-block configurations tell us
that the s and d valence orbital energies are similar
that the s,d,f valence orbital energies can be similar for the f-block energies
roughly, what do the
‘main group’
‘transition metals’
‘rare earths’
refer to
- ‘main group’ = s,p block
- ‘transition metals’ = d block
- ‘rare earths’ = 4f elements
what should we consider in addition to the attractive electron-nucleus interactions to give the familiar arrangement of the periodic table
- we must also consider the repulsive electron-electron interactions
- the elements can attain a lower energy by having different orbital occupancies, this is where
4s2 3dn has higher energy than 4s0 3d(n+2)
6s2, 5d0, 4fn has lower energy than 6s0, 5d0, 4f(n+2)
what is a ‘rule of thumb’ to help remember the order of orbital filling
- the ‘Madelung rule’
- consider all n (principal quantum number) and all relevant l (angular momentum number; s = 0, p=1 etc.)
- the lowest n+l orbitals fill first
- if they have the same value then the lower n value ‘wins’
what are the 6 main ways that we will analyse energies and OEs in atoms
- calculated orbital energies using how well valence orbital energies match
- determining Zeff
- orbital energy is approximately equal to -1 x ionisation energy, this can be ,measured using photoelectron spectroscopy in the gas phase
- standard half cell potentials
- a cover all number called electronegativity can be determined
- we will also look at electron attachment enthalpy
what are the 5 key variables that affect each of the (orbital) energy related quantities
- screening
- penetration
- d and f block contractions
- relativistic effects
- exchange interaction
what should we remember when drawing MO diagrams (in terms of energy)
we can use the graph of orbital energies across the period to correctly draw the MO diagram
what are the two variables that orbital energy depends on, give the equation
E = -RH (Zeff^2 / n^2)
- the two variables are principal quantum number n and effective nuclear charge Zeff
explain screening and how it affects orbital energies/ contributes to the trends we can see
- Screening: electrons don’t make perfect screens.
- the energy of a given orbital decreases as the effective nuclear charge increases
there are two main trends:
1) moving down a group, orbital energy increases (less -ve) because n^2 increases faster than Zeff^2, lower orbitals are good at screening higher orbitals and the radius/RDF max for higher orbitals are further from the nucleus so electrostatic force is greater
2) moving across a period, orbital energies generally decrease, e.g. period 4:
from Sc to Cu, the 4s energy decreases slowly because the 3d orbital screens the 4s well
from Ga to Kr the 4s decreases rapidly because electrons are now added to the 4p, not screening effectively, i.e. screening is imperfect and Zeff increases
- OVERALL, Screening is always imperfect so Zeff increases
explain penetration and how it can lead to variations from the trend in orbital energies expected from screening
- Penetration: orbitals with the same n but different l have different rates of decrease in energy as Zeff increases
- for a given n, s,p,d.. orbital energies are NOT degenerate, because s orbitals penetrate the nucleus more than p orbitals, i.e. more of their RDF is closer to the nucleus
- this means for any given n, as Zeff increases, the energy of the s orbitals decreases faster than the energy of the p orbitals
- this explains why no s-p mixing occurs for higher Z atoms
explain d and f block contractions and how they affect the trend we normally expect in orbital energies
- d and f block contractions
- a d or f section in a given period causes lowered orbital energies for elements that come after
- adding a full d or f subshell means that although the same number of electrons as protons have been ‘added’ the following orbitals are lowered in energy because they do not screen perfectly so any ‘later’ subshells are lower in energy than expected
explain relativistic effects and how they affect the trend we normally expect in orbital energies
- relativistic effects:
- the 6s orbital energy is lowered in period 6 elements because of the very large nuclear charge that has lowered the 1s orbital energy
- 1s has its RDF max. closest to nucleus, QM says if 1s orbital’s energy and radius are decreased, all other s orbitals have lower energy and radius too
- the lowering of 1s orbital’s energy technically occurs generally for any higher nuclear charge but is only noticeable in s AOs once you reach 6s
- it comes from the Bohr-Sommerfeld model where electrons orbit at speed
c x Z/137
so the greater z is, the faster the electron travels so the greater its mass so the greater the Rydberg constant RH so the lower the orbital energy
what is a secondary consequence of relativistic effects on orbital energies
- secondary consequence is that lower energy s AOs are slightly better at screening so valence d and f orbitals (in particular) are raised in energy
explain exchange interaction energy and how it affects the trend we normally expect in orbital energies
- Exchange Interaction Energy:
- QM effect that means more parallel (unpaired) electron spins gives lower energy - from Pauli exclusion principle
- lowering of energy is related to nC2 where n is the number of parallel (unpaired) spins in the AOs
- this is particularly noticeable in Cr and Cu where they have ‘half filled shells’
why is it surprising that having a greater number of parallel (unpaired) electron spins gives lower energy (due to exchange interaction energy)
- the overall energy of the atom is lowered
- this is despite the extra energy required for an electron to sit in 3d rather than 4s, this is higher energy because 3d is anyway and it is more contracted than 4s so there’s more coulombic repulsion if 3d contains more electrons
- equally the 4s energy is raised slightly because the 3d screens better with more electrons
- HOWEVER, the reduction in exchange interaction energy is dominant
explain the relationship in energies of the 4s and 3d orbitals and hence explain why K and Ca have 4s1 and 4s2 not 3d1 and 3d2 as valence configs, explain why all of the 3d block elements have a full 4s orbital
- initially the 3d orbital is higher in energy than the 4s orbital, this explains why K and Ca have 4s1 and 4s2
- following that, the 3d orbital becomes lower in energy than 4s but we keep a 4s2 arrangement for all following atoms
- this is because the 3d AOs are quite contracted so there is more electron-electron repulsion (if they contain more electrons) than the 4s orbital
- hence, it is more energetically favourable to fill 4s first
why should we not be too concerned by the d and f blocks not having very predictable patterns
- the 5 energy factors can cause complex relationships which can deviate significantly even for the isoelectronic ions
what are the two main ways to determine orbital energies experimentally
- Ionisation energies
- Standard electrode potentials
explain how we can determine orbital energies from ionisation energies, why is it not a perfect method, what are some further challenges and alternatives
- experimentally, we can take ionisation energies but the issue is that when you remove an electron, all orbital energies change so the difference is not quite the same thing
- it is a lot of work to measure IEs, they are specific to the system and only measured for gas phase
- computational methods can be used but are difficult for large systems, it also has to calculate energies for all AOs and it can’t just add them without considering e- – e- repulsion
what can we say about the trends we expect in orbital energies by considering Zeff
E = -RH Zeff^2 / n^2
Zeff = sqrt(En^2/RH)
- in a given group Zeff increases top to bottom but n^2 increases faster than Zeff^2 so OEs become less -ve
- in a given period, Zeff increases left to right due to imperfect screening, so OEs become more -ve as n doesn’t increase
what do slaters rules allow us to determine, what is the basis equation we use
Slater’s rules are a background to computational methods for determining OEs
Slater suggested Zeff is equal to Z minus a screening constant S
what is the method for calculating Zeff using Slater’s rules (LEARN)
SETUP: group the orbitals by n such that s and p of the same n are together but d and f aren’t
RULES for calculating S:
1) ignore groups to the right (higher n,l) to what is being considered
2) 0.35 is added to S for each electron in the same group, apart from 1s where 0.3 is added
3) if the electron (we calculate S for) is in an s or p orbital, then electrons in n-1 contribute 0.85 to S, electrons in n-2 or lower contribute 1 to S
4) if the electron (we calculate S for) is in a d or f orbital, electrons in ANY group to the right (lower n,l) contribute 1 to S
CALCULATE:
Zeff = Z - S
E can be calculated from Zeff
what ideas are encapsulated by Slater’s rules
- electrons in higher orbitals don’t screen
- electrons in same shell don’t screen well
- electrons in next lower shell screen well
- electrons in much lower shell screen very well
Define ionisation energy
- the energy change for the process
A(g) —> Ag+(g) + e-
what is Koopmans’ theorem
Orbital energy = -IE
how good is Koopmans’ theorem at predicting orbital energies, what else can we learn from it
- it is a fairly good approximation for OEs, the correlation is strong
- the parts where there is deviation between the two quantities (IE and OE) can give us clues as to the electronic structure of the atoms
what is the usual explanation for deviations in the correlation between ionisation energy and orbital energies
- it is generally from exchange interaction energy
- e.g. ionising N to N+ requires more energy than ionising O to O+ because the exchange interaction energy is more favourable in N than N+ but not in O to O+