What element has an atomic number of 40?

The element with atomic number 40 is zirconium. Atomic number counts protons, so every zirconium atom has 40 protons in its nucleus. Its chemical symbol is Zr, and it belongs to group 4 of the periodic table alongside titanium, hafnium, and rutherfordium.

Zirconium was identified in 1789 by German chemist Martin Heinrich Klaproth, who found it in the gemstone mineral zircon. Jöns Jacob Berzelius later isolated it in impure form in 1824. The name ultimately comes from the Persian zargun, meaning “gold-colored,” a reference to zircon’s appearance.

Zirconium is a lustrous, corrosion-resistant transition metal. Its alloys are valuable in nuclear reactors because zirconium absorbs relatively few neutrons, while zirconium dioxide is used in ceramics, refractory materials, and some dental applications. A common mix-up is confusing zirconium with zircon: zirconium is the element, while zircon is a naturally occurring mineral containing it.