What is a common property of alkali metals?

Alkali metals, which comprise elements like lithium, sodium, and potassium, are known for their softness and shiny appearance. When you cut alkali metals, they often reveal a shiny surface. This luster is characteristic of metals, although it can quickly oxidize upon exposure to moisture in the air, leading to dullness.

The softness of alkali metals means that they can be easily cut with a knife, which is a defining property that differentiates them from many other types of metals. These metals also possess low melting points and lower densities compared to other metallic elements.

While the other characteristics mentioned in the question relate to various properties of elements or groups, they do not accurately describe alkali metals. For instance, alkali metals are actually highly reactive, especially with water, and they are located in group 1 of the periodic table, not group 18, which is reserved for noble gases that are not typically reactive. Their hardness and density are generally lower than that of other metal groups, making the identification as soft and shiny the distinguishing hallmark of alkali metals.