S/P-block Flashcards
How is the periodic table organised
- Atomic number
- Orbitals
- Energy
What are the trends in effective nuclear charge
Zeff increases across period
no. protons increases
not compensated for by e-
Zeff increases down group
no protons increases charge
not fully compensated for by e-
What are the trends in atomic radius
Decrease across period
increasing Zeff
Increase down group
increasing n
What are the anomalies in atomic radius
Li»_space; H
effective screening by 1s orbital
Al ~ Ga
d-block contraction
Tl ~ In
f-block contraction
What are the trends in first ionisation energy
Increasingly endothermic across period
increasing Zeff
no increase in n
Decreasingly endothermic down group
increase in n
∴ e- spends more time further from nucleus
∴ e- more weakly held
What are the anomalies in first ionisation energy
Be -> B
e- in p-orbital has higher energy
∴ less tightly bound than s-orbital
N -> O
e- paired in O
∴ lower than expected energy
What are the trends in e- gain energy
More exothermic across period
increase in Zeff
no increase in n
Less exothermic down group
decreasing Zeff
What are the properties of non-metals
High IE
-ve electron gain energy
𝜒 > 2.2
Small radius
Good orbital overlap
∴ share e-
∴ covalently bonded
What are the properties of metals
Low IE
𝜒 < 1.9
Large radius
Weak orbital overlap
∴ sea of e-
∴ metallic bonding
What are the trends in bond strength of binary hydrides across a period
Mean bond enthalpy increases
Li -> F
increasing covalency
∵ increasing mean 𝜒
C -> F
increasing bond polarity
∵ increasing Δ𝜒
N doesn’t follow trend
∵ e- pair effect
What is the equation for electron density
Ed(AB) = ( (Ed(AA) + Ed(BB))/2 ) + (𝜒A - 𝜒B)^2
What is the trend in bond strength of binary hydrides down a group
Decreases down group
decreasing Δ𝜒
∴ reduced electrostatic contribution
increasing bond length
∵ reduced orbital overlap
increasing mismatch orbital energy
What are the structures of group 1 & 2 hydrides
1 = predominantly ionic w/ CCP
2 = predominantly ionic w/ rutile
What is the trend in MP/BP of binary hydrides
Van der Waal interactions increase across period
2nd row hydrides stronger
∵ polarisation leads to h-bonds
HF more polarised
∴ stronger dipole
∴ able to form fewer H-bonds
What is the trend in acidity of binary hydrides
Increases across period
increasing Δ𝜒
∴ increasing polarisation
Increases down group
decreasing Δ𝜒
∴ decreasing bond strength
Why does water have greater conductivity than ammonia
Greater bond polarisation
Higher pKa
Better stoichiometry
What is the isotope effect
Heavier mass
affects vibrational modes
affects bond strength
∴ MP/BP & pKa higher
Why does reactivity increase down group 1
Lower IE
∴ lower barrier to reaction
Weaker metalic bonding
∴ lower MP/BP
Lower MP & higher density
increases contact w/ water
Faster rate of reaction
∴ more heat generated
∴eventually explodes
What is the comparison of group 1 & 2
2 has higher 1st IE
∵ increased Zeff
2 has higher MP & ΔatH
∵stronger metallic bonding
∵presence 2 delocalised e-
What are the trends in ionic radius
Cations smaller than neutral
loss of e-
∴ higher Zeff on remaining e-
loss of outer shell
Cation size decrease across period
∵ increasing valency
Anions larger than neutral
gain e-
∴ smaller Zeff on e-
Size ions increases down group
e- in higher energy shell
∴ spend more time further from nucleus
What is the lattice dislocation energy
When 1 mol ionic solid
converted to gaseous ions
Endothermic
+ve enthalpy
What is the Gibbs energy of solution
Energy change
when 1 mol ionic solid
is dissolved in water
to give solution of infinite dilution
What affects energy of hydration
Smaller ions
w/ higher charge multiplicities
have higher charge density
∴ interact more strongly
w/ water molecule
leading to higher energy of hydration
How do hydrated salts form
Lone pair e- of H2O
coordinate strongly
to charge dense metal ions
form highly ordered inner coordination sphere
What are the trends in Gibbs energy of solution
Salts w/ large anions
solubility in water decreases down group 1 & 2
Salts w/ small anions
solubility in water increases down group 1 & 2
Group 2 have higher lattice enthalpy
compounds generally less soluble
enthalpy hydration also higher
What are the trends in oxidation state
Expansion of octet not possible
so max at N +5
Max OS easier down group
∵ IE & 𝜒 decrease
Period 4/5 often favour lower OS
∵ inert pair effect
What is the inert pair effect
Promoting e- from 2s -> 2p
allows extra bond to form
Gap from 4s -> 6s increases
∵ d/f-contraction
Promotion E increases slightly down group
∵ relativistic effects & d/f-contraction
Bond H decreases down group
so E released from forming new bonds decreases
∴ payback to cost of promotion lower
What are the relativistic effects
Velocity e- in heavier elements
approaches appreciable fraction of c
∴ higher nuclear charge
Leads to increased mass e-
& contraction of orbitals
Leads to higher promotion energies
What are the trends in σ-bonds
Repulsion between e- lone pairs
weakens σ-bonds
Decrease down group
∵ increasing atomic radius
& reduced orbital overlap
What are the trends in bond enthalpies
Increase across period
∵ increasing 𝜒
lone pair repulsion weakens bonding
for N2, O2 & F2
Decrease down group
as nuclear radii increases
& orbitals become more diffuse
∴ overlap weaker
What are the trends in covalency in p-block
Bond enthalpy decreases down group
∵ larger more diffuse orbitals
A-F enthalpies > A-Cl
∵ better orbital overlap
& greater difference in 𝜒
∴ Stronger polar covalent bonds for F
2nd row F bonds weak
∵ short inter atomic distances
∴ repulsion between lone pairs
What are the trends in π-bonding
Orbital size increases down group
σ-bond length increases
∴ rapid drop-off in overlap
between perpendicular orbitals
For Si 2σ-bonds strongly favoured
Double/triple bonds only seen
when stabilised by bulky groups
to prevent rearrangement
What is the bonding in O/N
Single bonds particularly weak
∵ repulsion between lone pairs
Multi bonds energetically favoured
What are the allotropes of P
Common allotropes form 3 σ-bonds
Reactivity decreases from white -> black
∵ reduced strain
Solubility/volatility decreases
∵ formation increasingly extended structures
What are the trends in IE for group 13
IE increases w/ every e- lost
∵ increasing Zeff
1st IE lower as higher E p-orbital
IE generally decreases down group
but alternating effect
due to d/f-contraction
What are the trends in p-block oxides
Average 𝜒 increases across row
∵ difference in 𝜒 decreases
∴ increasingly covalent bonding
What is back bonding
Back donation of e-
from filled orbital of attached ligand
to empty orbital of central atom
Gives rise to pπ-pπ bonding
What is the effect of lewis acid/bases on back bonding
Addition of lewis acid distorts shape
& disrupts pπ-pπ bonding
What are the trends in bonding of group 18 B halides
Lewis acidity increases down group 17
BF3 < BCl3 < BBr3 < BI3
∵ weaker back bonding
∵ orbital size & energy mismatch
What are the trends in noble gases
e- gain energy generally increases across period
group 18 have full shell so unfavourable
1st IE Ar similar to O
How do high oxidation states become easier to achieve in hypervalent halogens
As size central atom increases down group
& 𝜒/IE decrease
As size peripheral atom decreases
& becomes more 𝜒
F particularly good due to weak F-F bonds
What are the forms of P pentahalides
PF5 stable
PCl5 exists as gas
disproportionates in solid state
to form PCl4 cation & PCl6 anions
Br/I do the same
form P4 cation & I-
∵ larger size
What are the trends in bond angle
For AH3
decreases down group 15
size A allows H - A - H
to be squashed more by lone pairs
For AX3
Increases down halides
∵ reduced 𝜒 of bonding pair
