Strontium (Sr) - Atomic Structure & Chemical Properties

Atomic Model Details

Strontium (Sr) is an alkaline earth metal, located in Group 2 and Period 5 of the periodic table.

• Atomic Number (Z): 38
• Number of Protons: 38
• Number of Electrons: 38
• Number of Neutrons: Approximately 50 (for the most abundant isotope, Sr-88, where Mass Number = 88).
• Atomic Mass: 87.62 u (or g/mol)
• Atomic Radius (Metallic): 215 pm

Electronic Configuration & Shell Diagram

Strontium has 38 electrons distributed in its electron shells.

• Full Electronic Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s²
• Noble Gas Configuration: [Kr] 5s² (where [Kr] represents the electron configuration of Krypton)

Shell-wise Electron Distribution (Shell Diagram Representation): The electrons are arranged in shells as follows:

• K-shell (1st shell): 2 electrons
• L-shell (2nd shell): 8 electrons
• M-shell (3rd shell): 18 electrons
• N-shell (4th shell): 8 electrons
• O-shell (5th shell): 2 electrons (valence electrons)

The presence of two valence electrons in the outermost 5s orbital dictates its chemical reactivity as a Group 2 element, readily losing these two electrons to form a +2 ion (Sr²⁺).

Periodic Trends

Strontium exhibits trends characteristic of its position in Group 2 and Period 5.

• Ionization Enthalpy:
  • First Ionization Enthalpy (IE₁): 549.5 kJ/mol. It is relatively low due to the large atomic size and the ease of removing the outermost 5s electron.
  • Second Ionization Enthalpy (IE₂): 1064.2 kJ/mol. It is higher than IE₁ but still relatively low, reflecting the tendency to form a stable Sr²⁺ ion.
• Electronegativity: 0.95 on the Pauling scale. Strontium is a highly electropositive element, indicating its strong tendency to lose electrons rather than gain them.
• Electron Gain Enthalpy: Generally positive (endothermic), indicating that energy is required to add an electron to a strontium atom. This is typical for metals, especially alkaline earth metals, which prefer to lose electrons. (Approx. +50 kJ/mol, but often considered practically negligible or positive for alkaline earth metals).
• Atomic Radius: 215 pm (metallic radius). It is larger than elements in earlier periods within the same group (e.g., Calcium) and larger than elements in the same period but later groups. This large size contributes to its low ionization enthalpy.

Key Physical Properties

Strontium is a soft, silvery-white metal.

• Density: 2.64 g/cm³ at 20 °C.
• State at Room Temperature (25 °C): Solid.
• Color: Silvery-white, lustrous metal. It tarnishes quickly in air, forming a yellowish oxide layer.
• Melting Point: 777 °C.
• Boiling Point: 1382 °C.