Calcium is a silvery-white element that constitutes about 3% of the Earth’s crust, making it the fifth most abundant element overall and the third most abundant metal. In the human body, it is a major component; approximately 99% of the body’s one kilogram of calcium is stored within the bones and teeth. While calcium compounds have been known for millennia, the pure metallic element is highly reactive and does not exist in nature in an uncombined state. The isolation of this pure metal was a significant challenge, a feat achieved in the early 19th century by a pioneering British chemist.
Historical Knowledge of Calcium Compounds
Numerous calcium compounds were widely used by ancient civilizations for practical applications. Lime, or calcium oxide, was produced by heating limestone and utilized extensively by the ancient Romans in the first century under the name calx. This substance formed the basis of their durable mortar and cement for construction.
Another common compound was gypsum, or calcium sulfate, used in the construction of the Great Pyramid of Giza around 2500 BC. Later historical records note the utility of plaster of Paris, a dehydrated form of gypsum, for setting broken bones. Despite this widespread use, scientists were unaware these substances contained a unique metallic element. It was not until 1789 that Antoine Lavoisier suggested that lime might be the oxide of an unknown element.
Sir Humphry Davy and the Isolation of Calcium
The breakthrough in isolating the pure element came in 1808 from the English chemist Sir Humphry Davy at the Royal Institution in London. Davy pioneered the use of electrolysis, employing a voltaic pile to split compounds into their constituent elements. He had previously isolated the highly reactive alkali metals, potassium and sodium, in 1807 using this method.
Davy applied the technique to compounds he suspected contained new elements, though initial attempts to reduce moist lime alone failed. He then electrolyzed a mixture of lime (calcium oxide) and mercuric oxide, designing the setup to capture the newly formed calcium in a mercury electrode. The electric current separated the calcium from the oxygen, combining the free calcium with the mercury to form an amalgam.
Davy then separated the metallic calcium from this amalgam by distillation, boiling off the mercury to leave behind the impure calcium metal. This successful isolation in 1808 confirmed the existence of calcium as a distinct metallic element. Davy also used similar electrolytic methods to isolate other alkaline earth metals, including barium and strontium, around the same time.
The Naming and Elemental Classification of Calcium
Following its isolation, Davy named the new element “calcium,” referencing its ancient origins. The name is derived from the Latin term calx, which translates to “lime” or “chalk,” reflecting the common compounds from which he had separated it.
Calcium is chemically classified as an alkaline earth metal, occupying Group 2 on the periodic table. As an alkaline earth metal, it is highly reactive and readily gives up its two outermost electrons to form a positive ion with a charge of +2. This high reactivity explains why it is never found uncombined in nature, quickly reacting with oxygen or nitrogen in the air. The element’s physical and chemical properties closely resemble those of its heavier group members, strontium and barium.