Rubidium (Rb) Revision Guide

Atomic Model of Rubidium (Rb)

Rubidium (Rb) is an alkali metal in Group 1 and Period 5 of the periodic table.

• Atomic Symbol: Rb
• Atomic Number (Z): 37 (Number of protons and electrons in a neutral atom)
• Mass Number (A): Approximately 85 (for the most abundant isotope, $^{85}$Rb)
• Number of Protons: 37
• Number of Electrons: 37 (in a neutral atom)
• Number of Neutrons: 48 (for $^{85}$Rb, calculated as Mass Number - Atomic Number)
• Atomic Radius (Metallic): 248 pm

Electronic Configuration & Shell Diagram

Electronic Configuration

The electronic configuration of Rubidium describes the arrangement of its 37 electrons in different energy levels and orbitals.

• Full Configuration: $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 5s^1$
• Condensed (Noble Gas) Configuration: $[Kr] 5s^1$

Shell Diagram Representation

The electron distribution in different principal energy shells (K, L, M, N, O) for Rubidium is as follows:

• K Shell (n=1): 2 electrons
• L Shell (n=2): 8 electrons
• M Shell (n=3): 18 electrons
• N Shell (n=4): 8 electrons
• O Shell (n=5): 1 electron (Valence electron)

This configuration clearly shows one valence electron in the outermost $5s$ orbital, characteristic of alkali metals, which readily loses this electron to form a stable $Rb^+$ ion.

Periodic Trends

Rubidium, being an alkali metal, exhibits distinct trends in its properties compared to other elements across and down the periodic table.

Ionization Enthalpy (First)

• Definition: The minimum energy required to remove the most loosely held electron from an isolated gaseous atom in its ground state.
• Value for Rb: 403 kJ/mol
• Trend: Rubidium has a very low first ionization enthalpy. This is because its outermost electron (5s$^1$) is far from the nucleus and experiences significant shielding, making it easy to remove. Ionization enthalpy decreases down Group 1.

Electronegativity

• Definition: The tendency of an atom to attract a shared pair of electrons towards itself in a chemical bond.
• Value for Rb (Pauling Scale): 0.82
• Trend: Rubidium has a very low electronegativity. As an alkali metal, it readily loses electrons and thus has a weak attraction for shared electrons. Electronegativity decreases down Group 1.

Electron Gain Enthalpy

• Definition: The energy change that occurs when an electron is added to an isolated gaseous atom in its ground state.
• Value for Rb: -47 kJ/mol
• Trend: For alkali metals like Rubidium, the electron gain enthalpy is slightly negative, indicating a small amount of energy released when an electron is added to form a gaseous anion ($Rb^-$). However, alkali metals typically prefer to lose electrons rather than gain them due to their low ionization energies.

Atomic Radius

• Definition: The distance from the center of the nucleus to the outermost electron shell.
• Value for Rb (Metallic Radius): 248 pm
• Trend: Rubidium has a large atomic radius. Atomic radius increases down Group 1 due to the addition of new electron shells with increasing atomic number, leading to greater distance of the valence electrons from the nucleus and increased shielding effect.

Key Physical Properties

Rubidium is a soft, silvery-white metallic element that is highly reactive.

• Density: 1.532 g/cm$^3$ (at 20 °C)
• State at Room Temperature (25 °C): Solid (it melts slightly above room temperature)
• Color: Silvery-white, turns tarnished quickly on exposure to air
• Melting Point: 39.3 °C
• Boiling Point: 688 °C