CHEM20312 Main Group Chemistry - Periodic Trends and Group Properties

Periodic Trends

• Periodic properties of the main group elements.
• Structures & Fundamentals of s- and p-block chemistry.
• ILO1 - describe the variation of atomic size, ionization energy, electron affinity, and electronegativity across the Periodic Table and explain these variations using electronic configurations and valence orbital energies.

Bohr Model and $Z_{eff}$

• For hydrogen: $E<em>n = -\frac{Z^2 R</em>H}{n^2}$ and $r = a_0 \frac{n^2}{Z}$
• For other elements, replace Z with $Z_{eff}$ (or $Z^*$).
• $Z_{eff} = Z - S$ where $S$ = shielding, estimated from Slater's rules:
  • Electrons in higher n shells contribute 0 to shielding.
  • Electrons with the same value of n each contribute 0.35.
  • Electrons in the n-1 shell each contribute 0.85.
  • Electrons in n-2 or below each contribute 1.00.
• Example:
  • Li: 1s² 2s¹
    $Z_{eff} = 3 - 2 \times 0.85 = 1.3$
  • Na: 1s² 2s² 2p⁶ 3s¹
    $Z_{eff} = 11 - 8 \times 0.85 - 2 \times 1.00 = 2.2$
  • K: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
    $Z_{eff} = 19 - 8 \times 0.85 - 10 \times 1.00 = 2.2$
• Slater's rules assume equal shielding from all orbitals (e.g., 3s, 3p, 3d) and all electrons below the n-1 shell.

$Z_{eff}$ Revisited

• Assuming shielding is only based on n is not accurate.
• s orbitals are more penetrating than p orbitals, which means there's some probability of a 2s electron being close to the nucleus.
• $Z_{eff}$ is now best determined from orbital energies.

$Z_{eff}$ Trends

• Electrons with the same n values are relatively poor at screening both s and p orbitals.
• Energy decreases across a row because $Z_{eff}$ increases.
• Clementi values for $Z_{eff}$ of the highest occupied orbital(s) of the atom are used.
• $Z_{eff}$ increases across a period and down a group.

Periodic Trends - Covalent and Ionic Radii

• Radii are expected to decrease across a period as $Z_{eff}$ increases from left to right.
• Electrons in the same shell are poor at shielding/screening nuclear charge.
• Radii increase down a group as n is the dominant effect.
• Ionic radii follow similar trends to covalent radii. Anions are larger due to lower $Z_{eff}$ & greater interelectron repulsion. Cations are smaller.

Periodic Trends - Ionization Energies

• Ionization energy (IE) is the energy required to remove an electron from the gaseous atom or molecule in its ground state.
• $M<em>{(g)} \rightarrow M^+</em>{(g)} + e^-_{(g)}$
• IE (given in kJ mol⁻¹) generally decreases down a group because the valence orbital is higher in energy and the valence quantum number n increases.
• Group 1 and Group 2 elements generally have well-behaved trends.
• 1st IE generally increases from left to right across a period as $Z_{eff}$ increases but not uniformly.
• 2s orbitals are lower in energy than 2p orbitals, so IE for B < Be.
• 3p orbitals are higher in energy than 3s orbitals, so IE for Al < Mg.

Ionization Energy - N/O and P/S kinks

• There are