Alkaline earth metals: alkaline earth metals periodic table facts

What are the alkaline earth metals?

The alkaline earth metals are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). Every one of the six alkaline earth metals is a true metal: it conducts electricity, has a metallic lustre and gives up its outer pair rather than gaining charge. In the bismuth-209 and nickel-64 fusion reaction at the GSI in Darmstadt, the roentgenium metal is explained step by step.

Together they fill the whole second column of the periodic table, from period 2 down to period 7. Their atomic number rises from 4 for beryllium to 88 for radium, and each starts a new s sub-shell in its period.

None of them occurs free in nature. They are too eager to react, so you meet them in rocks, seawater, shells and living tissue as compounds. Five are stable enough to handle and study in bulk; radium is intensely radioactive, so far less of its chemistry has been measured.

Chemists have also tried to make element 120, which would be the next member of the column, but every attempt so far has failed. If you are studying for a chemistry exam, this group is a good one to learn first, because their trends are clean and easy to predict.

The six group 2 elements at a glance

The table below lists each of the alkaline earth metals with its symbol, position in the periodic table and ground-state electron arrangement. Notice that the last term is always a filled s orbital, written ns², which is what ties these elements together.

Why they are called alkaline earths

Early chemists used the word "earth" for any substance that would not dissolve in water and survived strong heating. The oxides of this group fit that description, and they also turned water basic, so they became the "alkaline" earths. Their old names survive today: beryllia, magnesia, lime, strontia and baria.

Lavoisier suspected in 1789 that these powders were oxides of unknown metals rather than elements, and he was right. When Davy finally split the powders apart, the new metals were mostly named after the oxide they came from, which is why the column still carries a name that sounds more like geology than chemistry.

Properties of alkaline earth metals

The properties of alkaline earth metals change smoothly as you move down the group, because each step adds a full shell. The atoms get bigger, the outer pair sits further from the nucleus, and the metals become softer and more reactive. The covalent radius grows from 105 pm for beryllium to about 221 pm for radium, and that growth in atomic radius drives almost every trend in the sections below. For gadolinite, kolbeckite and extraction as a by-product of other ores, check the entries in scandium uses first of all.

Silvery-white, shiny and soft

Freshly cut, every member of the group is silvery-white with a bright metallic shine. That shine fades fast: calcium, strontium and barium tarnish within minutes in moist air as an oxide and nitride film builds up, which is why lab samples are stored under oil. Beryllium is hard and brittle at room temperature.

The rest are soft enough to cut with a knife, and barium is softer still than lead. Strontium often shows a faint yellow tint once a thin oxide skin forms.

Melting point, boiling point and density

Because each atom contributes a pair to the shared "sea" of charge holding the metal together, these metals have a higher melting point and a higher boiling point than their group 1 neighbours. They are still light for metals: magnesium is the lightest structural metal in common use, and beryllium is lighter again.

That low density is what makes both so valuable in aircraft and portable devices. The trend down the group is not perfectly smooth, as the table shows: magnesium melts lower than calcium, and radium's figures are estimates.

Electrical conductivity and metallic bonding

All six are good conductors. In the solid, the outer electrons are not tied to any single nucleus; they move freely between neighbouring atoms, and that freedom gives the group its good electrical conductivity and its ability to reflect light.

The same metallic bonding explains why these metals can be rolled, drawn into wire or alloyed. Beryllium is a poorer conductor than its neighbours, but among the alkaline earth metals even it beats most non-metals by a wide margin.

Electron configuration and reactivity of group 2

Every member of the column ends its electron configuration with a filled s orbital. Lose that pair and what remains has the same arrangement as the noble gas one row above, a very stable state. Because the outer shell pair is so easy to remove, the chemistry of the column is dominated by one charge, one type of bond and a reactivity that climbs steadily from top to bottom.

Valence electrons and the +2 oxidation state

Each atom has two valence electrons, and in nearly every compound it gives both away. The result is a fixed oxidation state of two and doubly charged ions such as Mg²⁺ and Ca²⁺, often called divalent cations.

In water the small, highly charged ions pull surrounding molecules tightly around themselves, which affects how well each compound dissolves. A singly charged state would leave a lone, unpaired electron, so the alkaline earth metals never show one in ordinary compounds.

Ionization energy and electronegativity down the group

The energy needed to remove the first electron falls from 899.5 kJ/mol for beryllium to 502.9 kJ/mol for barium. The second step costs roughly double the first but is still low enough that the doubly charged ion is the normal outcome.

Electronegativity drops in step, from 1.57 to about 0.9, and electron affinity is close to zero across the column: the alkaline earth metals have almost no pull on extra charge. Put together, the trends say the heavier the element, the more readily it gives up its outer pair.

Reactivity with water and alkaline solutions

Reactivity rises down the column, and water shows it clearly. Beryllium does not react with water or steam even near red heat.

Magnesium reacts only slowly with cold water but burns in steam. Calcium, strontium and barium react at room temperature, fizzing as hydrogen gas is released:

The products are hydroxides that give alkaline solutions with a pH well above 7. This is the "alkaline" half of the group's name in action. In practice, the heavier alkaline earth metals react far more vigorously than the lighter alkaline earth metals, which are protected by a tight oxide film that keeps water away from the metal underneath.

How alkaline earth metals compare with alkali metals

The alkali metals in group 1 have one outer electron; the alkaline earth metals have two. Removing a pair costs more energy than removing one, so the second column is less eager to react: sodium explodes on water while magnesium barely bubbles.

The alkaline earth metals are also harder, denser and melt at higher temperatures, because a pair per atom binds the metal more strongly. Both groups form ionic compounds that dissolve to give alkaline liquids, but the second column's bases are weaker and many of its sulfates and carbonates are far less soluble.

Alkaline earth metal compounds and reactions

Each alkaline earth metal forms the same simple set of compounds: oxide, hydroxide, halides, hydride, nitride and the salts of common acids. The chemical reactions that make them follow the same pattern for each member, with beryllium as the frequent exception.

Oxides and hydroxides

The alkaline earth metal oxides form when the metals burn in air, or when their carbonates are heated. Heating crushed calcium carbonate rock to around 900 °C is how industry makes calcium oxide, better known as quicklime:

Quicklime reacts with water so strongly that it gives off heat, forming slaked lime, Ca(OH)₂. Magnesium oxide is a white, high-melting ceramic used in furnace linings, while beryllium oxide combines electrical insulation with unusually high thermal conductivity. Strontium oxide and barium oxide react with water the same way quicklime does, only faster.

Beryllium is the odd one out among the alkaline earth metals: its hydroxide is amphoteric, dissolving in both acids and concentrated alkali. Calcium carbonate itself is the most common compound of the group on Earth, and any metal carbonate from this column decomposes this way on strong heating, needing higher temperatures the further down you go.

Halides, hydrides and nitrides

All six react with the halogens to form the alkaline earth metal halides, such as calcium chloride. These are ionic crystals, with one exception: beryllium chloride, bromide and iodide are covalent, because the tiny Be²⁺ ion distorts the electron cloud of its partner so strongly that the electrons end up shared.

Heated in hydrogen, the metals form a saline hydride, for example CaH₂, that releases hydrogen gas on contact with water. Only beryllium and magnesium combine directly with nitrogen to give a nitride:

In organic chemistry, magnesium reacts with alkyl halides in dry ether to make Grignard reagents, one of the most widely used tools for building carbon-carbon bonds.

Sulfates, carbonates and solubility

Compounds of the alkaline earth metals follow two opposite solubility trends. The hydroxide becomes more soluble down the group, while each sulfate becomes less soluble: magnesium sulfate dissolves easily, but barium sulfate is so insoluble that it is safe to swallow for X-ray imaging even though soluble barium compounds are poisonous. Adding a sulfate solution to a sample that contains strontium or barium ions produces a white precipitate, which is a simple first test for these two metals.

Identifying alkaline earth cations with a flame test

Heat a salt in a Bunsen flame and the energy lifts electrons into higher levels; as they fall back, they emit light at wavelengths that are characteristic of the metal. This makes the flame test a fast way to identify alkaline earth cations. Beryllium and magnesium give no visible colour because their electrons are held too tightly for the emitted light to fall in the visible range, so you need a chemical test for them instead.

Telling brick-red calcium from crimson strontium by eye takes practice; a cobalt-blue glass or a cheap spectroscope separates them reliably. The apple-green barium flame is distinctive enough that you rarely confuse it with anything else in the group.

Where the alkaline earths occur and how they are produced

Magnesium and calcium are among the most abundant elements in the earth's crust, ranking eighth and fifth. Calcium sits in limestone, chalk, gypsum and anhydrite; magnesium in dolomite, magnesite and seawater.

Strontium comes mainly from celestite and strontianite, barium from barite, and beryllium from beryl, whose green gem variety is emerald. These minerals and ores are the only practical sources, since none of the metals is found native.

Getting the pure metals out usually means electrolysis of a molten chloride, the same route Humphry Davy used in 1808. Magnesium is produced from seawater or dolomite via magnesium chloride; calcium is made either by electrolysis or by reducing quicklime with aluminum at high temperature; and barium is made by reducing its oxide with aluminum. Beryllium is first converted from beryl to its hydroxide, then to a halide that can be reduced or electrolysed.

Isotopes and radioactivity

The first five members have stable isotopes, 19 in all, while radium has none. Because these elements all have an even number of protons, they tend to have more stable isotopes than their neighbours. The best-known unstable ones matter for health and energy.

Strontium-90, with a half-life of 28.9 years, is a fission product made in nuclear reactors running on uranium fuel. Radium-226 forms in the natural decay chain of uranium-238, which is why every uranium ore holds a trace of radium. Beryllium-9 releases a neutron when struck by alpha particles, the reaction used to discover the neutron in 1932.

Radioactive isotopes of this group behave like calcium in the body, so they collect in bone. These "bone seekers" are dangerous when absorbed by accident, but the same targeting lets a radioactive radium compound treat cancer that has spread to bone, delivering its radioactivity where it is needed most.

Why radioactive alkaline earth metals build up in bone

Your skeleton is mostly calcium phosphate, and the body cannot easily tell calcium apart from its heavier relatives. Strontium and radium ions slip into the same crystal sites, so the radioactive alkaline earth metals end up locked inside bone for years.

Stable strontium does the same thing harmlessly, which is why it is studied as a bone-strengthening treatment. This shared behaviour is a direct result of the alkaline earth metals all forming ions of the same charge and similar size.

Uses and applications of the alkaline earth elements

The alkaline earth elements are far more than textbook entries. Their applications run through nearly every heavy industry, and magnesium and calcium alone are produced in millions of tonnes a year.

Which element gets chosen depends on what the job needs most: low weight, cheap bulk supply, a strong flame colour or radioactivity. The sections below group the main applications by element.

Beryllium and magnesium in aerospace and alloys

Light weight and stiffness make beryllium and magnesium natural choices for aerospace parts, satellite structures and space-telescope mirrors. Magnesium is alloyed with aluminum, zinc and manganese for car wheels, gearbox housings and laptop shells, while copper-beryllium alloys make springs and non-sparking tools. The drawback of magnesium is flammability: fine chips burn with an intense white light, which is also why it appears in flares.

Calcium in construction, cement and steel

Limestone and the lime made from it are the backbone of construction: they go into cement, mortar, plaster and glass. In steelmaking, calcium and magnesium deoxidize molten metal and strip out sulfur, improving steel quality. Calcium metal also works as a reducing agent, freeing metals such as uranium and thorium from their compounds.

Strontium, barium and radium in medicine and electronics

Strontium and barium compounds colour fireworks red and green, the best-known of their everyday applications. In medicine, a barium sulfate drink outlines the gut on an X-ray, and radium-223 treats bone metastases.

In electronics, beryllium oxide carries heat away from high-power components and barium is used as a getter that removes stray gas from vacuum tubes. Radium was once painted onto watch dials as luminous paint, until the workers who applied it fell ill.

Biological role and health risks

Two of the alkaline earth metals, calcium and magnesium, are essential to life. Calcium builds bones and teeth and controls muscle contraction; magnesium is needed by hundreds of enzymes and sits at the centre of every chlorophyll molecule, so plants cannot photosynthesise without it.

Both are key minerals in nutrition, and both turn up as food additives and in antacids. The rest of the group is a health concern: beryllium dust is toxic and can cause chronic beryllium disease, soluble barium salts are poisonous, and radium is dangerous because of its radiation.

History and discovery of the beryllium group

The beryllium group was known through its compounds long before its metals. Lime mortar dates back thousands of years, emerald and beryl were prized in ancient Egypt, and Epsom salt was found in an English spring in 1618. In 1808 Humphry Davy used electrolysis to isolate magnesium, calcium, strontium and barium in a single year.

Beryllium followed in 1828, separated by Friedrich Wöhler and Antoine Bussy by reacting its chloride with potassium. Marie Curie and Pierre Curie discovered radium in uraninite in 1898, and Marie Curie isolated the pure metal by electrolysis in 1910.

Element 120 and the search for a seventh member

If it is ever made, this element would sit below radium in the periodic table and become the seventh of the alkaline earth metals. Calculations suggest relativistic effects would make it less reactive than radium, breaking the downward trend. Several labs have fired calcium, titanium and chromium beams at heavy targets without a confirmed atom so far.