Potassium (K) - Atomic Structure and Properties

Atomic Model Details of Potassium (K)

Potassium (K) is an alkali metal located in Group 1, Period 4 of the periodic table.

• Atomic Number (Z): 19
  • Indicates 19 protons in the nucleus.
  • In a neutral atom, it also indicates 19 electrons.
• Mass Number (A): Approximately 39 (for the most common isotope, K-39).
  • Neutrons: Mass Number - Atomic Number = 39 - 19 = 20 neutrons (for K-39).
• Atomic Size:
  • Metallic Radius: 227 picometers (pm). This is the radius of an atom in a metallic crystal.
  • Ionic Radius (K⁺): 138 pm. The cation K⁺ is significantly smaller than the neutral atom due to the loss of the outermost shell.

Electronic Configuration and Shell Diagram

Electronic Configuration

The electronic configuration describes the distribution of electrons in the atomic orbitals.

• Orbital Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
• Noble Gas Configuration: [Ar] 4s¹
  • This indicates that Potassium has a stable argon core with one valence electron in the 4s orbital.

Shell Diagram Representation

A shell diagram provides a simplified visual representation of electrons in different energy shells.

• K-shell (n=1): 2 electrons
• L-shell (n=2): 8 electrons
• M-shell (n=3): 8 electrons
• N-shell (n=4): 1 electron (this is the valence electron)

Potassium readily loses its single valence electron to form a stable K⁺ ion, achieving the noble gas configuration of Argon.

Periodic Trends of Potassium

As an alkali metal, Potassium exhibits characteristic trends within its period and group.

• First Ionization Enthalpy (IE₁): 418.8 kJ/mol
  • Potassium has a very low first ionization enthalpy, indicating that it requires a small amount of energy to remove its outermost electron. This is due to the large atomic size and the shielding effect of inner electrons.
• Electronegativity (Pauling Scale): 0.82
  • Potassium has a very low electronegativity, signifying its weak tendency to attract shared electrons in a covalent bond. It is among the least electronegative elements.
• Electron Affinity (EA): -48 kJ/mol
  • Electron affinity for Potassium is negative, meaning energy is released when an electron is added to form a gaseous K⁻ ion. However, forming a stable K⁻ ion is not common; this value primarily reflects the energy change for the gas-phase process. Generally, alkali metals have a very low tendency to gain electrons.
• Atomic Radius: 227 pm (Metallic Radius)
  • Potassium has one of the largest atomic radii in its period (Period 4) and increases down Group 1. This large size is attributed to the presence of four electron shells.

Key Physical Properties of Potassium

Potassium displays typical metallic properties with some unique characteristics of alkali metals.

• Density: 0.86 g/cm³ (at 20°C)
  • Potassium is less dense than water, causing it to float on water.
• State at Room Temperature (25°C): Solid
• Color: Silvery-white, lustrous (when freshly cut)
  • Rapidly tarnishes upon exposure to air due to oxidation.
• Melting Point (MP): 63.5 °C
  • Potassium has a relatively low melting point compared to most metals, indicating weak metallic bonding.
• Boiling Point (BP): 759 °C
  • Similar to its melting point, Potassium’s boiling point is also relatively low for a metal.