Why do alkaline-earth metals tarnish when exposed to air?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkaline-earth metal Modern version of the periodic table of the elements (printable).

Alkaline-earth metals, which belong to Group 2 of the periodic table, possess a gray-white luster when freshly cut but tarnish readily when exposed to air.1

This tarnishing occurs because these elements are highly reactive reducing agents that easily lose electrons to become positive ions. When exposed to the atmosphere, they react with oxygen to form metal oxides, which create a layer on the surface of the metal. This reaction is particularly noticeable in the heavier members of the group. Because these metals are highly metallic and chemically active, this surface oxidation happens quickly, dulling their initial shine.213