Britannica Sources used in this answer

Why does calcium develop a yellow coating when exposed to air?

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

A labeled diagram of the periodic table entry for calcium, showing its atomic number 20, symbol Ca, atomic weight 40.078, electron configuration, and classification as an alkaline-earth metal. Icons indicate calcium is a solid with a face-centered cubic crystal structure and strongly basic oxides at room temperature. — calcium The chemical element calcium is the most abundant metallic element in the human body.

When calcium is exposed to air, it reacts slowly with oxygen, water vapour, and nitrogen present in the atmosphere. This chemical process results in the formation of a yellow coating composed of a mixture of oxide, hydroxide, and nitride.1