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.

For hydrogen: En = -\frac{Z^2 RH}{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.

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.

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.

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.

Ionization energy (IE) is the energy required to remove an electron from the gaseous atom or molecule in its ground state.
M{(g)} \rightarrow M^+{(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.