How are radium and barium chemically similar?

Radium and barium are chemically similar because they are both alkaline-earth metals belonging to Group 2 of the periodic table. Their chemical similarities are so pronounced that separating the two elements is notoriously difficult.12

Key chemical similarities include:

• Oxidation State: Both elements occur exclusively in the +2 oxidation state in their compounds, forming stable $M^{2+}$ ions.132
• Electronic Structure: Both possess two electrons in their outermost orbital, with configurations of [Xe]6$s^2$ for barium and [Rn]7$s^2$ for radium.132
• Reactivity: Both metals are highly reactive, attacking water with the evolution of hydrogen and reacting with air.124
• Compound Properties: Their salts are often isomorphous, and they exhibit similar solubility patterns; for instance, both form insoluble sulfates and carbonates.14
• Preparation: Metallic radium can be prepared using the same electrolytic methods employed for metallic barium.4

Because of these shared characteristics, radium was historically concentrated alongside barium during its discovery, requiring fractional crystallization to separate the two.1