With Group 1 covered, whose members are called alkali metals, it’s time to move on to Group 2,
which we refer to as the alkaline earth metals. These first two groups have similar names,
and indeed as we will see momentarily, they have quite similar properties.
This group includes the elements beryllium, magnesium, calcium, strontium, barium, and radium.
These elements all have electron configurations that end in ns2. So similarly to the alkali
metals, due to having only two valence electrons in their outermost shells, the alkaline earth
metals have very low Z effective values, and low ionization energies, which means they are easily
ionized to form 2+ cations. Just like Group 1, ionization energy decreases moving down Group 2,
as does melting point, though these are much higher than for Group 1, and magnesium also
has an anomalously low melting point due to the different type of lattice it possesses.
Additionally, like lithium, both beryllium and magnesium are small enough to make covalent bonds.
All these elements are fairly soft, silvery metals much like the alkali metals, although
beryllium is grayer and somewhat brittle. Now let’s discuss availability. Beryllium is
acquired through the ore bertrandite, which has this formula. This is found primarily in Utah
within the United States, but also in China and Kazakhstan. It is rare and expensive. Magnesium
is found in a variety of ores, such as dolomite, magnesite, brucite, carnallite, and others.
Only a few decades ago, the United States produced around half of the magnesium in the world,
while as of 2019 China has produced about 85%. Calcium is found in the ores calcite,
dolomite, and gypsum. These are widely abundant, primarily in Spain, Iran, and the United States.
Strontium is found in celestine and strontianite, again with China acting as the primary producer.
Barium is found in barite. And finally, radium is found in trace amounts in uranium ores like
uraninite. This is because it is part of the decay chain for radioactive uranium nuclides,
and one ton of uraninite will produce 150 milligrams of radium.
Now, natural sources aside, how are these elements prepared? Calcium, strontium, and barium
are produced by reacting metal chlorides with sodium, yielding pure metal and sodium chloride.
Magnesium is produced by electrolysis of magnesium chloride, calcium chloride, and sodium chloride.
Another method involves reacting magnesium oxide with coke carbon to produce magnesium metal and
carbon monoxide. Beryllium is produced by reacting beryllium chloride with magnesium or calcium.
In terms of reactivity, beryllium makes mainly covalent compounds, like beryllium oxide,
beryllium sulfide, and beryllium fluoride. It can also bond covalently with carbon.
Magnesium has some tendency towards covalency, especially with carbon, such as with the Grignard
reagents we should be familiar with. But it can also make ionic compounds, like magnesium oxide
and magnesium hydroxide. The other members of the group make lattice hydrates with
halide ions, with lower stability and greater solubility as the metal cation gets larger.
Looking at applications, beryllium is strong, resistant to corrosion, and melts at high
temperatures, though it is somewhat toxic. Nevertheless these properties make it quite
valuable, and with 60% of the world’s supply in the United States, it is used largely for
military applications. Beryllium is also used in X-ray tubes as it is transparent to X-rays
and also strong enough to keep a perfect vacuum. Other applications include neutron triggers for
nuclear fission, beryllium-copper alloys for gas valves and oil wells, and a mineral called beryl,
which can take on either an aquamarine or emerald color, and is used to make fake gems.
Magnesium is moderately priced, strong and light, and quite flammable whether
in ribbon or powder form, with the latter form being used in flash powders for pyrotechnics.
It is the first industrially useful structural metal in the periodic table, used in race cars,
airplanes, and bicycles. Magnesium also has a very important role in nature, sitting in the
center of the porphyrin ring in chlorophyll, the pigment molecule which initiates photosynthesis,
a process which all life on Earth depends on, either directly or indirectly.
Calcium is a bit firmer, and is unstable to air, rapidly oxidizing to form calcium oxide.
Calcium can react with water to form calcium oxide and hydrogen gas.
Calcium carbonate is a very important compound which we know of as chalk,
such as what is found on the famous white cliffs of Dover, as well as in seashells and corals.
Blackboard chalk, rather, is calcium sulfate. Calcium is crucial to most animal life, being that
bones contain a rigid hydroxyapatite foam which is a form of hydrated calcium phosphate. Additionally
the movement of calcium ions across the membranes of nerves and muscle cells mediates muscle
contraction, so we literally would not be able to move at all without this important element.
Strontium is used in luminescent paints, which glow for hours after excited with
light in the 200 to 450 nanometer range. It was also used in an early diamond simulant,
prior to the popularization of cubic zirconium. Strontium is used to reduce the brittleness of
aluminum-silicon casting alloys, and it is found in the active ingredient in Sensodyne toothpaste.
Barium reacts with everything in the air, like oxygen, nitrogen, carbon dioxide, and water,
thereby allowing for the removal of air from a particular environment, like vacuum tubes within
electronics. Barium sulfate has application with medical imaging, where patients drink a
suspension of the substance, and this allows for X-ray imaging of the digestive system,
due to this material’s ability to absorb X-rays very well. Barium is also found within
certain superconductors, like this one, which superconducts at liquid nitrogen temperatures,
thereby not requiring the greater difficulty of utilizing liquid helium.
And radium is a naturally radioactive element which forms from the decay of uranium.
All isotopes of radium are radioactive. This element was used in radioluminous
glow-in-the-dark paints in the early 1900s, where a small amount of radium was mixed with
luminescent zinc sulfide. The radiation from the radium was absorbed by the zinc sulfide
which produced a glowing effect, and this was useful for various purposes in World War I.
It slowly became understood that this was not a safe material to use, as so-called “radium girls”
were in the habit of sharpening paint brushes by licking them, causing them to ingest enough radium
to lead to many cases of sickness and death. This brought about new labor laws protecting
workers from undisclosed dangers. And with that we complete a summary
of the alkaline earth metals, which represent Group 2 on the periodic table.