Barium, Beryllium, Radium, and Calcium.
Explanation
All four metal elements (Ba, Be, Ra, and Ca) are found in group 2 of a modern periodic table. By periodicity, they have similar chemical properties.
Shared properties likely include:
- Softness. With only two valence electrons per atom, the four elements form weak metallic bonds. Their pure forms are thus easy to shape and cut.
- High reactivity. Group 2 "alkaline-earth" metals are only two electrons away from ideal gas electron configurations. They lose electrons more readily than most transition metals such as iron and copper do. Group 2 metals tarnish easily in the earth atmosphere, and reacts readily with water to produce hydrogen and a base- hence the name "alkaline."