Rubidium (Rb), atomic number 37, is a silvery-white alkali metal belonging to Group 1 of the periodic table. It is known for its high chemical reactivity and distinctive physical properties, including low temperature phase transitions. This article explores the specific temperature at which rubidium vaporizes and the chemical principles that explain its boiling point.
The Boiling and Melting Points of Rubidium
Rubidium transitions from a liquid to a gaseous state at approximately 688 degrees Celsius (1,270 degrees Fahrenheit). This is the element’s defined boiling point under standard atmospheric pressure. While this temperature may seem high in everyday terms, it is remarkably low compared to most other metals, such as iron or copper.
The metal also possesses an exceptionally low melting point, transitioning from a solid to a liquid at just 39.3 degrees Celsius (102.7 degrees Fahrenheit). This means rubidium will melt on a particularly hot day or when held in a person’s hand. This low melting point provides context for its overall volatility.
General Characteristics of Rubidium
Rubidium is the fourth element in the alkali metal group, positioned below potassium and above cesium. Like other Group 1 elements, it is a soft, ductile, and silvery-white metal. Its physical properties are linked to its electronic structure, which features a single electron in its outermost shell.
This lone valence electron makes rubidium highly electropositive, meaning it readily gives up this electron in chemical reactions. The metal exhibits extreme reactivity, particularly with water and oxygen. Rubidium must be stored under an inert atmosphere or in mineral oil to prevent spontaneous ignition upon exposure to air.
Rubidium is classified as one of the most volatile metallic elements. Its high density causes it to sink when reacting violently with water, unlike the lighter alkali metals above it in the group.
Explaining the Low Boiling Point
The boiling point of any metal is determined by the energy needed to overcome the forces holding the atoms together in the liquid state. In metals, this force is metallic bonding, which involves a shared “sea” of delocalized valence electrons surrounding a lattice of positive metal ions. The strength of this metallic bond is directly related to the boiling temperature.
Rubidium’s unusually low boiling point is a direct consequence of its atomic structure. The rubidium atom is very large, having five electron shells, which places its single valence electron far from the positively charged nucleus. This large atomic radius significantly weakens the attractive force between the nucleus and the shared electron sea.
Because the metallic bond is weak, only a small amount of thermal energy is required to break the bonds and allow the atoms to escape as a gas. This trend of decreasing boiling points continues down the alkali metal group, with cesium having an even lower boiling point than rubidium.