Chemistry of the elements L4/5

Question 1

Describe covalent trend going down group for s-block elements

Answer 1

Size of atom increases down group because even though Zeff increases so does n and n dominates
S block elements show regular increase of 0.2-0.5 A

Question 2

What is the group 13 anomaly?

Answer 2

The expected expansion in size is seen from B to Al
Ga is the same size as Al and there is only a small size increase Ga–> Tl
This is because d-block contraction of ~0.3 A occurs because 10 elements have been encountered so Zeff is higher
At Tl there is f- and d-block contraction and relativistic effects and thus Tl is the same size as Na
(NB: Tl+ is poison because disrupts Na+ metabolism)

Question 3

Why are 4d and 5d transition metals the same size within each group?

Answer 3

F- block contraction (so 5d not bigger than 4d as expected)

Also relativistic contraction of 6s which is valence orbital

Question 4

Define ionisation energy

Answer 4

The enthalpy change when one mole of atoms/ions in the gaseous state is oxidised by 1e-

Question 5

What are overall IE trends across period and down group?

Answer 5

Period- increased bc of Zeff

Group- n dominates, e- further from nucleus and easier to remove, decreases

Question 6

What happens when element has half full/ full electron configuration?

Answer 6

Takes more energy to remove outermost electron hence spikes in IE vs n graph

Question 7

Why are O, Ne and F below the trend line?

Answer 7

Quantum mechanical exchange energy, atoms prefer not to have pairing of electrons with opposite spins

Question 8

Electron Affinity define

Answer 8

Energy released when one mole gaseous atoms/ions is combined with 1 mole electrons
Defined as positive if exothermic, negative if endothermic

Question 9

Why are all values usually positive?

Answer 9

Because values are a balance between Zeff (attraction approaching electron feels) and repulsion of e- cloud and Zeff usually wins

Question 10

What is overall EA trend across period?

Answer 10

Increase due to increased Zeff

Question 11

Why is there a high value for Au?

Answer 11

Relativistic effects, Au- 5d10 6s2 v contracted

NB: remember from A level Au is s1 d10

Question 12

What is electronegativity?

Answer 12

Ability of an atom to attract electrons to itself within a molecule (NOT thermodynamic)

Question 13

Draw MO diagram to explain why electronegative atom becomes d-

Answer 13

(see notes for diagram)

Question 14

What did Pauling notice?

Answer 14

That the experimental value for heteronclear AB bond energy is higher than homoatomic AA/BB

Question 15

E(AB)=? in kJmol-1

Answer 15

1/2(E(aa)+E(bb)) + 96.5(chi(a)-chi(b))^2

Question 16

What is borderline of Pualing scale between ionic and covalent?

Answer 16

Difference in electronegativity of 1.8

Question 17

Draw a van Arkel-Ketalar diagram

Answer 17

(see notes for diagram)

Question 18

What does the low electronegativity of metal elements result in?

Answer 18

Higher energy, more diffuse orbitals and a tendency to delocalise e- in classic metallic structure (therefore drift to metallic character down the periodic table)

Question 19

High electronegativity if non-metals…

Answer 19

Leads to lower energy, more compact orbitals and tendency to localise e- in 2e- bonds (therefore there is a drift to covalent character across the periodic table)

Question 20

Why is there a maintenance of covalent character in group 13 compared to s-block?

Answer 20

Added shielding effect overwhelmed by d/f-block contraction and Relativistic Effects

Question 21

Why 4p,5p,6p higher ionisation energy than expected?

Answer 21

4p,5p,6p higher IE than expected due to d-block contraction so harder to take off electron because orbital energy lowered