SafekipediaAlkaline earth metal
Adapted from Wikipedia · Discoverer experience
Alkaline earth metals are six chemical elements found in group 2 of the periodic table. These elements are beryllium, magnesium, calcium, strontium, barium, and radium. They share many similar properties: they are all shiny, silvery-white metals that are quite reactive under normal conditions.
These elements have a special electron arrangement where their outer s orbital is full, meaning it has two electrons. They easily lose these two electrons to form positively charged ions with a charge of +2 and an oxidation state of +2. Although helium is usually grouped with noble gases, it is thought to share some traits with beryllium under certain conditions.
All of these elements are found in nature, except for radium, which is created when uranium and thorium decay. Scientists have tried but failed to create element 120, which could potentially be the next member of this group.
Characteristics
The alkaline earth metals are six elements in group 2 of the periodic table. They are shiny, silvery-white, and somewhat reactive metals. These elements include beryllium, magnesium, calcium, strontium, barium, and radium.
These metals share many similar properties. They are soft and have relatively low densities, melting points, and boiling points. They react with certain elements to form new compounds. For example, they react with halogens to make ionic compounds and with water to produce alkaline solutions, though beryllium does not react with water easily. These metals also have low ionization energies, meaning they can lose electrons easily to form ions.
| Metal | M2+ hydration (-MJ/mol) | "MF2" unit hydration (-MJ/mol) | MF2 lattice (-MJ/mol) | Solubility (mol/kL) |
|---|---|---|---|---|
| Be | 2.455 | 3.371 | 3.526 | soluble |
| Mg | 1.922 | 2.838 | 2.978 | 1.2 |
| Ca | 1.577 | 2.493 | 2.651 | 0.2 |
| Sr | 1.415 | 2.331 | 2.513 | 0.8 |
| Ba | 1.361 | 2.277 | 2.373 | 6 |
| Alkaline earth metal | Standard atomic weight (Da) | Melting point (K) | Melting point (°C) | Boiling point (K) | Boiling point (°C) | Density (g/cm3) | Electronegativity (Pauling) | First ionization energy (kJ·mol−1) | Covalent radius (pm) | Flame test color | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Beryllium | 9.012182(3) | 1560 | 1287 | 2744 | 2471 | 1.845 | 1.57 | 899.5 | 105 | White | |
| Magnesium | 24.3050(6) | 923 | 650 | 1363 | 1090 | 1.737 | 1.31 | 737.7 | 150 | Brilliant-white | |
| Calcium | 40.078(4) | 1115 | 842 | 1757 | 1484 | 1.526 | 1.00 | 589.8 | 180 | Brick-red | |
| Strontium | 87.62(1) | 1050 | 777 | 1655 | 1382 | 2.582 | 0.95 | 549.5 | 200 | Crimson | |
| Barium | 137.327(7) | 1000 | 727 | 2170 | 1897 | 3.594 | 0.89 | 502.9 | 215 | Apple-green | |
| Radium | 969 | 696 | 2010 | 1737 | 5.502 | 0.9 | 509.3 | 221 | Crimson red | ||
History
The alkaline earth metals are named after special compounds called alkaline earths. Early scientists called these compounds "earths" because they did not dissolve in water and could resist heat. Later, a scientist named Antoine Lavoisier realized these "earths" were actually compounds made from metals.
The first person to get these metals in a pure form was Humphry Davy in 1808. He used a process called electrolysis to pull the metals from their compounds. This proved that the "earths" were indeed linked to metals.
People have known about some of these metals for a very long time. For example, calcium has been used in building since ancient times. Magnesium was discovered in 1618, strontium in 1790, and the last one, radium, was found in 1898 by Marie and Pierre Curie. They noticed that some materials stayed radioactive even after uranium decayed, leading to the discovery of radium.
Occurrence
Beryllium is found in very small amounts in the Earth’s crust and water. It is one of the rarest elements in seawater but a bit more common in freshwater.
Magnesium and calcium are much more common in the Earth’s crust. They are found in minerals such as carnallite, magnesite, dolomite, chalk, limestone, gypsum, and anhydrite. Strontium is also found in minerals like celestite and strontianite, while barium is often in the mineral barite.
Radium is a decay product of uranium and is found in uranium-bearing ores. Because it changes quickly, the radium we find today comes from the slower decay of uranium.
Production
The six alkaline earth metals can be made in many different ways.
Beryllium is mostly taken from beryllium hydroxide. One way to make it is by mixing beryl with sodium fluorosilicate and soda at high heat. This creates sodium fluoroberyllate, which can be turned into beryllium hydroxide. Another method heats beryl, cools it with water, and then treats it with sulfuric acid to also get beryllium hydroxide. From there, beryllium can be made by processing its compounds.
Strontium is usually taken from the mineral celestite. This can be done by mixing celestite with sodium carbonate, or by using coal in a more complex process.
Barium is made from barite, which is impure barium sulfate. It is turned into barium sulfide using a process with coke. This can then be used to make pure barium sulfate or other compounds like barium nitrate. These are heated to make barium oxide, which can be turned into pure barium. China makes more than half of the world's barium.
Magnesium is often made from magnesite or dolomite. Dolomite is crushed, roasted, and mixed with seawater. This creates magnesium hydroxide, which is then turned into magnesium chloride. Electrolysis of this produces magnesium.
Calcium is found a lot in the Earth's crust as calcium carbonate. China and Russia are the biggest makers of calcium, using a method involving electrolysis of calcium chloride. The U.S. and Canada also make calcium by reducing lime with aluminium at high heat.
Radium was first made by Marie Skłodowska-Curie from materials left after uranium was taken out. These materials mostly contained barium sulfate. Through a careful process, radium was separated. Today, radium is taken from used reactor materials. Pure radium metal can be made by heating it with aluminium in a vacuum.
Main article: Beryllium § Extraction
Main article: Strontium § Production
Main article: Barium § Occurrence and production
Main article: Magnesium § Production
Main article: Calcium § Occurrence and production
Main article: Radium § Production
Applications
Beryllium is used in military and electronic devices. It helps make certain materials strong and able to carry heat. It is also used in special tools that need to stay steady in different temperatures.
Magnesium is used in building strong parts, though it can catch fire easily. It is mixed with other metals to make it stronger and last longer. It also helps make iron and steel, and is used in making titanium.
Calcium helps separate important metals from rocks and is used in making cheese, mortar, and cement.
Strontium and barium have fewer uses. Strontium makes red fireworks and is studied in science. Barium helps keep vacuum tubes clean and is used in oil industries.
Radium used to be used in glow-in-the-dark paints and other products, but it is no longer used because it can make people sick.
Representative reactions of alkaline earth metals
Alkaline earth metals can react in many different ways with other substances. For example, when calcium meets chlorine gas, it forms calcium chloride. This calcium chloride can absorb water from the air and dissolve in it.
These metals also react with oxygen to form oxides. Calcium and magnesium both create oxides when they meet oxygen. They can also react with sulfur and carbon to form other compounds. Some, like beryllium and magnesium, can even react directly with nitrogen.
When these metals meet water, they can produce a hydroxide and hydrogen gas. However, beryllium and magnesium might not react easily because of a protective layer on their surface. In certain conditions, though, magnesium can still react with water vapor.
Alkaline earth metals can also react with acids to form salts and hydrogen gas. Beryllium, for example, reacts with hydrochloric acid to make beryllium chloride and hydrogen gas. Some of these metals can even dissolve in strong bases, showing special chemical properties.
Identification of alkaline earth cations
The flame test can help identify some alkaline earth metals by the color they add to a flame when heated. However, beryllium and magnesium do not change the flame color because they are very small.
When testing in a liquid solution, different alkaline earth metals react in special ways:
- Magnesium ions (Mg2+) form a white solid when mixed with disodium phosphate, ammonium salts, and ammonia.
- Calcium ions (Ca2+) create a white solid when mixed with ammonium oxalate.
- Strontium ions (Sr2+) form a white solid when mixed with soluble sulfate salts.
All of these metal ions also make a white solid when mixed with ammonium carbonate, ammonium chloride, and ammonia.
| Metal | Colour |
|---|---|
| Ca | Brick-red |
| Sr | Crimson red |
| Ba | Green/Yellow |
| Ra | Carmine red |
Compounds of alkaline earth metals
Alkaline earth metals can form different kinds of compounds. One type is called oxides. These oxides are made when certain carbonates are heated. For example, when calcium carbonate is heated, it turns into calcium oxide and carbon dioxide.
These oxides can also be made in labs from hydroxides or nitrates. They show basic properties because they change the color of certain testing liquids. When these oxides mix with water, they create hydroxides.
Another kind of compound is hydroxides, made when oxides mix with water. Most hydroxides also show basic properties, but beryllium hydroxide is different and can react in two ways.
Salts are another type of compound. Calcium and magnesium are found in many natural salts, like in rocks and hard water. Hard water can be softened by using special materials that swap out calcium and magnesium ions for sodium ions.
Biological role and precautions
Magnesium and calcium are very important for all living things. They help in many processes inside cells and are part of our bones.
Strontium is important for sea life, especially corals, which use it to build their hard shells. Strontium and barium have some uses in medicine, like in special X-ray tests. However, too much strontium-90 can be harmful because it is radioactive and acts like calcium, building up in bones and harming bone marrow.
Beryllium and radium are both harmful. Beryllium does not usually get into living things and can be very toxic if it does. Radium is also toxic because it is radioactive and not easily found in nature.
Extensions
The next alkaline earth metal after radium is thought to be element 120, but this might not be correct due to certain scientific effects. Scientists tried to create element 120 in 2007 by combining different materials, but they didn’t find any of this element.
The behavior of element 120 is expected to be more like that of calcium or strontium rather than barium or radium. This is different from what we usually see in patterns of elements, because the energy levels of element 120’s outer electrons are expected to make it less reactive.
It’s not clear what the next alkaline earth metal after element 120 might be. Some ideas suggest it could be element 166, but its properties might place it in a different group instead.
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