Argon (Ar) Revision Guide: Atomic Structure & Properties

1. Atomic Model Details

Argon (Ar) is an inert gas belonging to Group 18 (Noble Gases) of the periodic table.

• Element Symbol: Ar
• Atomic Number (Z): 18
  • This indicates 18 protons in the nucleus.
  • In a neutral argon atom, there are also 18 electrons.
• Mass Number (A): Approximately 40 (for the most abundant isotope, $^{40}\text{Ar}$)
  • Neutrons: A - Z = 40 - 18 = 22 (for $^{40}\text{Ar}$)
• Atomic Size (van der Waals Radius): Approximately 188 pm (picometers).
  • Noble gases typically do not form covalent bonds, so their size is often characterized by their van der Waals radius.

2. Electronic Configuration & Shell Diagram

Argon’s electronic configuration explains its chemical inertness.

• Electronic Configuration (Full): $1s^2 2s^2 2p^6 3s^2 3p^6$
• Electronic Configuration (Noble Gas Notation): $[Ne] 3s^2 3p^6$
• Shell Distribution:
  • K-shell (n=1): 2 electrons ($1s^2$)
  • L-shell (n=2): 8 electrons ($2s^2 2p^6$)
  • M-shell (n=3): 8 electrons ($3s^2 3p^6$)
• Shell Diagram Explanation: The outermost M-shell contains 8 electrons, completing its octet. This stable electronic configuration makes Argon highly unreactive and gives it the characteristic properties of a noble gas.

3. Periodic Trends

The position of Argon in Group 18 and Period 3 dictates its periodic properties.

• Ionization Enthalpy (First): Approximately 1520.6 kJ/mol
  • Trend: Argon has a very high ionization enthalpy. This is due to its stable, fully filled outermost electron shell (octet) and a relatively small atomic radius. Significant energy is required to remove an electron from this stable configuration.
• Electronegativity: Not defined on the Pauling scale.
  • Trend: As noble gases do not readily form chemical bonds, their electronegativity is typically considered to be zero or negligible.
• Electron Gain Enthalpy: Approximately +96 kJ/mol
  • Trend: Argon has a positive electron gain enthalpy. This indicates that energy must be supplied to force an additional electron into its already stable, fully occupied valence shell, making the process energetically unfavorable.
• Atomic Radius (van der Waals): Approximately 188 pm
  • Trend: Within its period, the van der Waals radius of Argon is relatively larger than the covalent radii of elements like Chlorine. This is because van der Waals radius accounts for non-bonded interactions and is generally larger than covalent radius for comparable elements. Across a period, nuclear charge increases, generally leading to a decrease in atomic size, but the definition difference for noble gases is crucial.

4. Key Physical Properties

Argon exhibits typical physical properties of a noble gas.

• State at Room Temperature (25°C) and Pressure (1 atm): Gas
• Color: Colorless
• Odor: Odorless
• Taste: Tasteless
• Density (at STP): 1.784 kg/m³ (denser than air, which is ~1.225 kg/m³)
• Melting Point: -189.3 °C (or 83.8 K)
• Boiling Point: -185.8 °C (or 87.3 K)
• Solubility in Water: Slightly soluble (more soluble than Nitrogen, less soluble than Oxygen).