Lead (Pb) - Atomic Structure and Properties

1. Atomic Structure of Lead (Pb)

Lead (Pb) is a heavy post-transition metal.

1.1. Atomic Model Details

• Atomic Number (Z): 82
  • Indicates 82 protons in the nucleus.
  • In a neutral atom, there are also 82 electrons.
• Atomic Mass (Average): 207.2 u (unified atomic mass units)
• Neutrons: For the most abundant isotope, $^{208}Pb$, the number of neutrons is 208 - 82 = 126. Lead has several stable isotopes (e.g., $^{204}Pb, ^{206}Pb, ^{207}Pb, ^{208}Pb$).
• Atomic Size (Metallic Radius): Approximately 175 pm (picometers).

1.2. Electronic Configuration

The electronic configuration of Lead is crucial for understanding its chemical behavior.

• Full Configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 4d^{10} 4f^{14} 5s^2 5p^6 5d^{10} 6s^2 6p^2$
• Condensed Configuration: $[Xe] 4f^{14} 5d^{10} 6s^2 6p^2$
  • This configuration shows that Lead is in the p-block, Group 14, and Period 6. The presence of filled $4f^{14}$ and $5d^{10}$ orbitals contributes to the lanthanide and d-block contractions, respectively, affecting its properties.

1.3. Shell Diagram Representation (Electrons per Shell)

The distribution of electrons in different principal energy shells is as follows:

• Shell 1 (K): 2 electrons
• Shell 2 (L): 8 electrons
• Shell 3 (M): 18 electrons
• Shell 4 (N): 32 electrons
• Shell 5 (O): 18 electrons
• Shell 6 (P): 4 electrons (Valence electrons, $6s^2 6p^2$)

2. Periodic Trends of Lead

Lead’s position in Group 14 and Period 6 influences its periodic properties, often showing deviations due to relativistic effects and poor shielding by inner d and f electrons.

• Ionization Enthalpy (1st Ionization Energy): 715.6 kJ/mol
  • This value is higher than expected when moving down Group 14 from Tin (Sn) to Lead (Pb), due to the lanthanide contraction and poor shielding by 5d electrons, which increases the effective nuclear charge on the valence electrons.
• Electronegativity (Pauling Scale): 2.33
  • Lead is relatively more electronegative than lighter elements in its group (e.g., Tin has 1.96), again due to the high effective nuclear charge resulting from inefficient shielding by $4f$ and $5d$ electrons and relativistic effects on $6s$ electrons. This makes its metallic character less pronounced than expected.
• Electron Gain Enthalpy: Approximately -35 kJ/mol
  • This value is slightly negative, indicating a very low, but slightly exothermic, tendency to accept an additional electron. For metals, electron gain enthalpy is generally low or slightly positive.
• Atomic Radius (Metallic): 175 pm
  • Despite being further down the group, the increase in atomic radius from Tin (Sn, 145 pm) to Lead (Pb) is smaller than expected due to the lanthanide contraction.

3. Key Physical Properties of Lead

Lead is characterized by a unique combination of physical properties.

• Density: 11.34 g/cm³ at 20°C
  • It is a very dense metal.
• Physical State at Room Temperature (25°C): Solid
• Color: Bluish-white when freshly cut, but quickly tarnishes to a dull grey upon exposure to air due to the formation of a thin layer of lead oxide.
• Melting Point (MP): 327.5 °C
  • Lead has a relatively low melting point for a metal.
• Boiling Point (BP): 1749 °C
  • Its boiling point is considerably higher than its melting point.