Krypton (Kr) - Atomic Structure & Properties

Atomic Model Details

Krypton (Kr) is a noble gas element located in Group 18 and Period 4 of the periodic table.

Subatomic Particles and Atomic Number

• Atomic Number (Z): 36
  • Indicates 36 protons in the nucleus.
  • In a neutral atom, it possesses 36 electrons.
• Mass Number (A): The most stable and abundant isotope is Krypton-84 ($^{84}\text{Kr}$).
  • Neutrons: For $^{84}\text{Kr}$, the number of neutrons is calculated as Mass Number - Atomic Number = 84 - 36 = 48 neutrons.

Atomic Size

• van der Waals Radius: 189 pm (picometers). This value is typically reported for noble gases due to their inability to readily form covalent bonds.

Electronic Configuration & Shell Diagram

Electronic Configuration

• Ground State Electronic Configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6$
• Condensed Electronic Configuration: $[Ar] 3d^{10} 4s^2 4p^6$
  • This configuration signifies a completely filled valence shell ($4s^2 4p^6$), representing a highly stable octet.

Shell Diagram Representation

• Electron Distribution per Shell:
  • K-shell (n=1): 2 electrons
  • L-shell (n=2): 8 electrons
  • M-shell (n=3): 18 electrons
  • N-shell (n=4): 8 electrons (valence shell)
• This arrangement distinctly illustrates the stable octet configuration in the outermost shell, a defining characteristic of noble gases.

Periodic Trends

Krypton’s classification as a noble gas profoundly influences its periodic properties, indicating its chemical inertness.

Ionization Enthalpy ($IE_1$)

• First Ionization Enthalpy: 1350.8 kJ/mol
• Trend: Extremely high. The stable, completely filled valence shell electronic configuration of Krypton requires a substantial amount of energy to remove an electron.

Electronegativity

• Electronegativity: Not generally assigned, or considered to be 0 on most scales (e.g., Pauling scale).
• Trend: Krypton exhibits negligible tendency to attract electrons in a chemical bond because it is chemically inert and does not readily form compounds.

Electron Gain Enthalpy ($\Delta H_{eg}$)

• Electron Gain Enthalpy: Approximately +96 kJ/mol
• Trend: Highly positive. Krypton already possesses a stable octet; consequently, the addition of an extra electron necessitates energy input, rendering it an energetically unfavorable process, hence the positive electron gain enthalpy.

Atomic Radius

• van der Waals Radius: 189 pm
• Trend: Atomic radius generally increases down a group and decreases across a period. Krypton is observed to be larger than Argon but smaller than Xenon within Group 18. When compared to elements in Period 4, its van der Waals radius is relatively large due to weak intermolecular forces, although its (hypothetical) covalent radius would be small.

Key Physical Properties

Krypton is characterized as a colorless, odorless, and tasteless monatomic noble gas.

• State at Room Temperature (25°C, 1 atm): Gas
• Color: Colorless
• Odor: Odorless
• Taste: Tasteless
• Density (at STP): 3.749 g/L
• Melting Point: -157.36 °C (115.79 K)
• Boiling Point: -153.22 °C (119.93 K)
• Solubility in Water: Slightly soluble.