Alkaline Earth Metals: The Characteristics of Magnesium and Calcium

From the Lecture Series: Understanding the Periodic Table

By Ron B. Davis Jr.Georgetown University

In 1755, Scottish researcher Joseph Black began studying two components of limestone: calcium carbonate (known simply as chalk) and magnesium carbonate (known then as ‘magnesia alba’ or white magnesium). He found that chalk lost less of its mass than magnesia alba when heated to its decomposition temperature, as in the synthesis of quicklime.

Portrait of Joseph Black
The work of Joseph Black on limestone helped discover magnesium and calcium decades later. (Image: James Heath/Public domain)

The Discovery of Magnesium and Calcium 

Knowing what we know today, the reason for the difference in mass (mentioned above) is clear from the periodic table. Calcium’s greater atomic mass means that calcium makes up a greater percentage of its carbonate by mass than magnesium does of its carbonate. So driving off the carbon dioxide produces a greater change in mass for the magnesium carbonate sample.

At this early time, even oxygen had not yet even been discovered, so Joseph Black couldn’t be sure what he had made. Still, his measurements clearly showed that these two substances were different compounds. He called these compounds ‘magnesia’ and ‘lime’.

In 1808, taking a hint from Black’s work, Humphry Davy used his famous electrolysis technique with a voltaic pile to force electrons back onto the metals of magnesia and lime. The newly discovered elements were named magnesium for the magnesia starting material, and calcium for the calcining process.

Where Can We Find Magnesium?

Magnesium is one of the most common elements in our world. An even, and small, number of protons ensures that element-12 is produced in healthy quantities during stellar nucleosynthesis. And its eagerness to bond with silicon helps to concentrate it even more near the earth’s surface.

Crystalised magnesium on a blue background
Dense rock at the bottom of the sea and volcanic rock at the surface of the continents are the first places where magnesium occurs. (Image: CSIRO/Public domain)

It appears most commonly in minerals that form dense rock on sea floors but also in volcanic rocks that are extruded onto the surface of the continents in eruptions. By mass, it is the seventh most abundant element in Earth’s crust.

Like many other elements concentrated on the Earth’s surface, the minerals containing magnesium slowly weather chemically and release their magnesium into the sea. Each liter of seawater, on average, contains about one gram of dissolved magnesium. To put that amount of magnesium into perspective, there are about one billion kilograms of the element dissolved in the world’s oceans.

Similar to the alkali metals, magnesium has a comparatively low first ionization potential. If we check its position on the activity series, we see that magnesium is fairly high on the list, making it no surprise that it is always found in nature in its ionic form.

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Easy to Produce and Store

Magnesium metal itself can be produced, stored, and used without worrying about the entire sample reacting with the air around it. It won’t rapidly tarnish, much less crumble or burst into flames. This is because of magnesium’s tendency to quickly form a hard, passivating oxide outer layer that protects the remaining magnesium metal inside of a sample.

But, if we get magnesium hot enough, its tendency to react with oxygen leads to one of its more famous uses. Though difficult to ignite, magnesium metal itself burns quite vigorously in air once it gets going. As magnesium burns, the magnesium atoms emit a line spectrum that consists of multiple colored lines that combine to produce a strong, white light.

Other Traits of Magnesium

Magnesium is a remarkably low-density metal, weighing in at 1.7 grams per milliliter. It’s about twice the density of its alkali metal neighbor, sodium, but it’s less than two thirds the density of its neighbor to the right, aluminum.

This trend, of course, makes perfect sense, since progressing from sodium to magnesium to aluminum packs a more massive nucleus into an atom with the same valence shell. This means more massive atoms with smaller atomic radii—that’s a greater weight per unit volume.

So magnesium strikes a useful balance. It is much less reactive in air than sodium because of its passivating oxide layer. But magnesium is much less dense than aluminum. These properties combine to make magnesium a popular choice for inclusion in metal alloys that need to be light, sturdy, and corrosion-resistant.

Magnesium is also biologically crucial in many ways. Perhaps most interesting is its role in photosynthesis and the beauty of fall foliage. Magnesium is at the center of a class of molecules called chlorophylls, which are essential compounds used by plants to gather energy from light to perform photosynthesis.

Calcium and Its Higher Tendency to Oxidize

Just below magnesium on the table, calcium also appears only as ions in nature. Those calcium ions make an appearance in bones, in milk, and in many of the minerals around us. But unlike magnesium, you almost certainly would not carry calcium around in your pocket or come into direct contact at all. 

Moonstone, a granite stone containing calcium
Through some geological processes, calcium is dragged from underground to the earth’s surface in the form of rocks such as granite. (Image: James St. John/Public domain)

That’s because calcium’s more distant valence electrons in the fourth energy level are easier to remove, making calcium metal oxidize much more readily. So, working with pure calcium metal requires precautions.

Calcium’s lithophilic nature ensures that geological processes are always dragging calcium from deep underground to the surface in the form of silicon-based rocks like granite. Those rocks will ultimately weather, releasing some of that calcium to seawater, where it is four times as concentrated as magnesium, and is a favorite element for the formation of calcium carbonate shells used by many sea creatures for protection.

And, of course, ‘calcium’ is important in ‘bones’ where it makes up about 40% of the mass of calcium hydroxyapatite, the principal mineral making up bone. In this mineral, calcium is in its +2 oxidation state, happy and stable with a full third valence shell.

Common Questions about Magnesium and Calcium

Q: What are ‘magnesia’ and ‘lime’?

During his studies on chalk and magnesia alba, researcher Joseph Black noted that chalk loses less mass than magnesia alba after heating to decomposition temperature. His measurements indicated that these two substances were different compounds, which he called ‘magnesia’ and ‘lime’.

Q: How were calcium and magnesium discovered?

In 1808, Humphry Davy forced electrons back onto the metals of magnesia and lime using the electrolysis technique with a voltaic pile. These newly discovered elements were named magnesium and calcium.

Q: Why is magnesium easy to produce, store, and use?

Magnesium tends to quickly form a hard oxide layer that protects the remaining magnesium metal inside it, making it relatively easy to store and prepare.

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