9. E differences between s and p orbitals and SP mixing

Question 1

What are the 2s and 2p orbital E differences from H to F and what are the reasons for the change in E?

Answer 1

• 2s and 2p in H are of the same E because there is no shielding (1 e in H)
• 2s and 2p in Li are different due to:
• shielding - in Li 1s orbital is shielding 2s (more shielded - lower) and 2p (wavefunction go in different ways) - lower E
• increased nuclear charge - 2s and 2p E are lower in Li than in H because of increase in effective nuclear charge (nuclear charge increases further to F because of more protons → AOs decrease in E of the same principal number but 2p are shielded by 2s so stay relatively on the same E level → gap increases)
• going along from H to F - ΔE increases (gap gets bigger) - increased nuclear charge lowers 2s more than 2p due to shielding
• differences in 2s and 2p orbital E affects MO - s-p mixing

Question 2

Explain what is s-p mixing

Answer 2

• only orbitals of the same symmtry can combine
• some orbitals experience bonding effect → decrease in E, other experience antibonding effect → increase in E
• if close in E AOs → mix a lot, far apart → don’t mix so much
• along the period s-p mixing decreases - orbitals are further apart due to shielding and nuclear charge
• for O, Cl: 111221 (orbitals in E levels)
• for others left to O 112121 (orbitals in E levels)

Question 3

Explain MO of N₂

Answer 3

• concerned only with valence e (for N₂ 2s²2p³)
• same E for AOs in both N
• small E difference between 2s and 2p
• s AO interact in 2σ MO (not into pi as s is of the wrong symmetry - only sideways p orbitals for π) -> 2σg, 2σu*, 2σg => σ bond oder 3
• as AO interacts in 3π MO -> 1πu orbital formes - lower E than 2σg - σ weaker bond than π (usually the opposite - MO theory helps to explain)

Question 4

Explain the trend of 3σg orbital E in diatomic molecules of elements in period 2

Answer 4

Paramagnetic - unpaired e

• E goes down across a period due to inrcreased nuclear charge
• 3σg orbital E increase relatively faster going back in period than other MO because 2s orbitals are good at shielding