Rubidium is a soft, silvery white metal which is easily oxidized in air. It has only one stable form. Rubidium is the second most electropositive and the second most alkaline element. It also has the lowest ionization potential. Current uses of rubidium are limited but include such applications as vacuum tubes and photoelectric cells. Potential applications include use as a heat transfer medium and as a fuel for ion propulsion engines. The chemical behavior of rubidium is similar to that of potassium; it reacts vigorously with air or water and should be handled and stored in a dry, inert environment.

Physical Properties:

Atomic Weight: 85.467

Density:            

^oC                   g/cc

20                  1.532

39                  1.475

50                   1.47

150                  1.46

220                  1.45

Volume Change on Melting: 2.5% increase

Thermal Properties:

Melting Point: 38.89 ^oC

Boiling Point: 688 ^oC

Coefficient of Thermal Expansion:  

         Linear: 90 μm/m ⋅ K at 20 ^oC; 340 μm/m ⋅ K at 40 to 100 ^oC.

       Volumetric: 3.38 x 10^-4 m/m ⋅ K

Specific Heat: 0.33489 kJ/kg ⋅ K at 0 ^oC; 0.3797 kJ/kg ⋅ K at 50 ^oC.

Enthalpy: Solid: H_T - H298 = 160.16 T + 0.32080 T² - 76,259.  

Liquid: H_T - H298 = 384.47 T - 88,846. T is in K and H_T - H298 is in J/kg.

Entropy: Solid: S = 885.45 log T - 1297.9. T is in K and S is in J/kg.

Latent Heat of Fusion: 25.535 kJ/kg

Latent Heat of Vaporization: 887.46 kJ/kg.

Thermal Conductivity:    

^oC               Phase           W/m ⋅ K

 0             Solid            58.3

 25            Solid            58.2

38.89         Solid            58.1

38.89         Liquid           33.3

Vapor Pressure:           

^oC                    kPa

294            0.1333

387             1.333

519             13.33

569              26.66

628              53.33

Saturated Vapor Pressure: log P = (-4688/T) - 1.76 log T + 15.195, where P is in Pa and
                                        T is in K.

Critical Temperature and Pressure: T_c = 2083 + 20 K and Pc = 14.54 + 0.5 MPa

Electrical Properties:

Electrical Resistivity:        

Temperature              Resistivity,

 ^oC            K                     nΩ ⋅ m

  0         273                115.4

 20        293                 128.4

39.9     312.1                137(a)

39.9     312.1               220(b)(c)

100       373                 274.7(c)

Temperature Coefficient: 0.06 nΩ ⋅ m per K

Electrochemical Equivalent: 

 Valence               Equivalent, mg/C

1                               0.8858

2                               0.4429

3                               0.2214

4                               0.1107

Electrolytic Solution Potential: Versus H₂: -2.924 V at 25 ^oC

Work Function: 2.09 eV (0.335 aJ)

Photoelectric Threshold: 7.3 x 10^-4 C

Magnetic Properties:

Magnetic Susceptibility: Volume: 2.6 x 10^-3 mks at 30 ^oC

Chemical Properties:

General Corrosion Behavior: Rubidium is very active. It ignites in air at room temperature and will react explosively in oxygen. Rubidium can easily dissociate water to form the hydroxide and will react spontaneously with gaseous chlorine and fluorine and explosively with liquid bromine.

Mechanical Properties:

Hardness: 0.3 Mohs

Modulus of Elasticity: 2.35 GPa

Kinematic Liquid Viscosity:      

 ^oC                 mm²/s

 38              0.4573

 39              0.4561

 40              0.4528

 50              0.4267

99.7            0.3359

140.5          0.2904

179             0.2586

220.1          0.2332

Liquid Surface Tension:             

^oC                    N/m

39                   0.0847

100                 0.080

550                 0.051

632                 0.0468

Speed of Sound: At melting point: v = 1260 m/s