Cadmium (Cd) - Atomic Structure and Properties

Cadmium (Cd) - Atomic Structure and Properties

Cadmium (Cd) is a soft, silvery-white, malleable, and ductile metal belonging to Group 12 of the periodic table. It is often grouped with zinc and mercury due to similar properties, including a full d-subshell.

1. Atomic Model Details

Cadmium is characterized by the following atomic parameters:

• Atomic Number (Z): 48
  • This indicates 48 protons in the nucleus and 48 electrons in a neutral cadmium atom.
• Average Atomic Mass: 112.41 u
• Neutrons: The most abundant isotope, Cadmium-114 ($^{114}$Cd), has 66 neutrons (114 - 48). Other common isotopes include $^{112}$Cd (64 neutrons) and $^{110}$Cd (62 neutrons).
• Atomic Radius (Covalent): Approximately 161 pm
• Ionic Radius (Cd²⁺): 95 pm (for coordination number 6)

2. Electronic Configuration & Shell Diagram

The electronic configuration of Cadmium dictates its chemical behavior.

• Ground State Electronic Configuration: [Kr] 4d¹⁰ 5s²
  • This configuration shows a completely filled 4d subshell and a completely filled 5s subshell. This stable configuration contributes to its relatively inert nature compared to typical transition metals, and its preference for forming a +2 oxidation state.
• Orbital Diagram Representation (Textual Explanation):
  • The inner core ([Kr]) represents the configuration of Krypton (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶).
  • Following the Krypton core, the 4d subshell is completely filled with 10 electrons.
  • Finally, the 5s subshell contains 2 electrons.
• Shell-wise Electron Distribution:
  • K-shell (n=1): 2 electrons (1s²)
  • L-shell (n=2): 8 electrons (2s² 2p⁶)
  • M-shell (n=3): 18 electrons (3s² 3p⁶ 3d¹⁰)
  • N-shell (n=4): 18 electrons (4s² 4p⁶ 4d¹⁰)
  • O-shell (n=5): 2 electrons (5s²)

3. Periodic Trends

Cadmium exhibits distinct periodic trends within its group and period:

• Ionization Enthalpy (kJ/mol):
  • First Ionization Enthalpy (IE₁): 867.8 kJ/mol
  • Second Ionization Enthalpy (IE₂): 1631.4 kJ/mol
  • Trend: The first ionization enthalpy of Cadmium is higher than that of Zinc (Zn, 906.4 kJ/mol) due to the poor shielding effect of the filled 4d subshell electrons, which results in a greater effective nuclear charge experienced by the 5s valence electrons. This trend reverses for Mercury (Hg, 1007.1 kJ/mol) due to the lanthanide contraction.
• Electronegativity (Pauling Scale): 1.69
  • Trend: Electronegativity generally decreases down Group 12 from Zinc to Cadmium.
• Electron Gain Enthalpy: Cadmium has a very small positive or near-zero electron gain enthalpy (e.g., approximately +32 kJ/mol).
  • Trend: Like other metals, Cadmium generally does not readily accept electrons, reflecting its metallic character and tendency to lose electrons.
• Atomic Radius: 161 pm (covalent radius)
  • Trend: Atomic radius increases from Zinc (139 pm) to Cadmium (161 pm) due to the addition of an extra electron shell. However, it then slightly decreases or remains similar for Mercury (157 pm) due to the lanthanide contraction effect.

4. Key Physical Properties

Cadmium possesses the following notable physical characteristics:

• Density: 8.65 g/cm³ (at 20°C)
• State at Room Temperature: Solid
• Color: Silvery-white with a bluish tinge, lustrous
• Melting Point: 321.07 °C (594.22 K)
• Boiling Point: 767 °C (1040 K)
• Group: 12
• Period: 5
• Block: d-block