The Alkaline Earth Metals: Occurrence, Extraction and Physical Properties – Trends in Chemical Properties of Group 2 Elements and Their Compounds (Senior 4 Chemistry) | Ganzaa

Trends in Chemical Properties of Group 2 Elements and Their Compounds

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Senior 4 · Chemistry

Trends in Chemical Properties of Group 2 Elements and Their Compounds

5 lessons

Lesson 1 of 5

The Alkaline Earth Metals: Occurrence, Extraction and Physical Properties

25 min

From chlorophyll to cement

At the centre of every chlorophyll molecule in a tea leaf sits one magnesium ion. The walls of your classroom are probably held together by cement, made by roasting calcium carbonate, and finished with plaster made from calcium sulphate. A patient who swallows a white paste of barium sulphate before an X-ray lets a doctor see the outline of the stomach. Magnesium, calcium and barium are all members of Group 2, the alkaline earth metals. Like the alkali metals of Unit 6 they are too reactive to occur free, but they hold their outer electrons more tightly, and that one difference runs through the whole unit.

Definition

Alkaline earth metals: the elements of Group 2, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). Each atom has two electrons in its outer s orbital, $n s^{2}$ , and in compounds each forms a 2+ ion, $\mathrm{M^{2+}}$ , with the configuration of the noble gas before it. Example: $\mathrm{Ca}$ is $[\mathrm{Ar}]\,4s^2$ and $\mathrm{Ca^{2+}}$ is $[\mathrm{Ar}]$ . The name comes from their oxides, once called "earths", which give alkaline solutions with water.

Where they occur

All the Group 2 metals occur only in compounds, mostly as carbonates, sulphates and silicates in rocks. Calcium and magnesium are both among the eight most abundant elements in the Earth's crust.

Table 7.1: Important minerals of the Group 2 elements

Element	Mineral or source	Formula
Beryllium	beryl (emerald is a green variety)	$\mathrm{Be_3Al_2Si_6O_{18}}$
Magnesium	magnesite; dolomite; dissolved $\mathrm{Mg^{2+}}$ in sea water	$\mathrm{MgCO_3}$ ; $\mathrm{CaMg(CO_3)_2}$
Calcium	limestone, chalk and marble; gypsum; fluorite	$\mathrm{CaCO_3}$ ; $\mathrm{CaSO_4{\cdot}2H_2O}$ ; $\mathrm{CaF_2}$
Strontium	celestine; strontianite	$\mathrm{SrSO_4}$ ; $\mathrm{SrCO_3}$
Barium	baryte; witherite	$\mathrm{BaSO_4}$ ; $\mathrm{BaCO_3}$
Radium	traces in uranium ores; radioactive	formed by the decay of uranium

How the metals are extracted

The $\mathrm{M^{2+}}$ ions hold on to their electrons too strongly for carbon to reduce the oxides cheaply, so the metals are made either by electrolysis of a molten chloride or by reduction with a more reactive element.

Magnesium from sea water (the Dow process): $\mathrm{Mg^{2+}}$ is precipitated as $\mathrm{Mg(OH)_2}$ by adding slaked lime, the hydroxide is turned into $\mathrm{MgCl_2}$ with hydrochloric acid, and the molten chloride is electrolysed: $\text{cathode: } \mathrm{Mg^{2+} + 2e^- \rightarrow Mg} \qquad \text{anode: } \mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}$
Magnesium from dolomite (the Pidgeon process, the main method in China): roasted dolomite is heated with ferrosilicon at about 1200 °C under vacuum. Silicon reduces magnesium oxide, and magnesium vapour condenses as crystals (photo below). $\mathrm{2MgO(s) + Si(s) \rightarrow 2Mg(g) + SiO_2(s)}$
Calcium by electrolysis of molten $\mathrm{CaCl_2}$ , or by heating lime with aluminium; beryllium by reducing $\mathrm{BeF_2}$ with magnesium.

Physical properties down the group

Table 7.2: The alkaline earth metals

Property	Be	Mg	Ca	Sr	Ba
Atomic number	4	12	20	38	56
Outer configuration	$2 s^{2}$	$3 s^{2}$	$4 s^{2}$	$5 s^{2}$	$6 s^{2}$
Metallic radius / pm	112	160	197	215	222
Ionic radius of $\mathrm{M^{2+}}$ / pm	45	72	100	118	135
First ionisation energy / kJ mol⁻¹	899.5	737.7	589.8	549.5	502.9
Second ionisation energy / kJ mol⁻¹	1757.1	1450.7	1145.4	1064.2	965.2
Electronegativity (Pauling)	1.57	1.31	1.00	0.95	0.89
Melting point / °C	1287	650	842	777	727
Density / g cm⁻³	1.85	1.74	1.54	2.64	3.62

Radius increases down the group: each element has one more occupied shell. Each $\mathrm{M^{2+}}$ ion is much smaller than its atom, because the whole outer shell has gone.
Ionisation energies decrease: the outer s electrons are further from the nucleus and better shielded by inner shells. The extra protons are outweighed by distance and shielding (Figure 7.1).
Electronegativity decreases, so compounds become more ionic down the group. Beryllium (1.57) is electronegative enough to form mainly covalent bonds (Lesson 4).

Figure 7.1: First and second ionisation energies of the Group 2 elements. Data: CRC Handbook of Chemistry and Physics, as compiled in Wikipedia's ionisation energy data page.

Melting points: higher than Group 1, and not a smooth trend

Each Group 2 atom gives two electrons to the delocalised "sea", and its ion is smaller and doubly charged, so metallic bonding is much stronger than in Group 1. Every Group 2 metal melts hundreds of degrees above its Group 1 neighbour (Figure 7.2). Down the group the melting points fall overall, as the ions get larger, but not smoothly: magnesium melts lower than calcium. The metals crystallise in different structures (Be and Mg hexagonal close-packed, Ca and Sr face-centred cubic, Ba body-centred cubic), so the packing changes from element to element and a simple trend is not expected. Density also first falls and then rises, because atomic mass grows faster than atomic volume from calcium onward.

Figure 7.2: Melting points of the Group 1 and Group 2 metals in Periods 2 to 6. Data: CRC Handbook of Chemistry and Physics, as compiled in Wikipedia's melting point data page.

A bead of beryllium metal. Beryllium is hard, light (1.85 g cm⁻³) and grey; it is not handled in school laboratories, because its dust and compounds are very toxic when breathed in. Photo: Hi-Res Images of Chemical Elements, CC BY 3.0, via Wikimedia Commons

Worked example 1: why 2+ and never 1+ or 3+?

The first three ionisation energies of magnesium are 737.7, 1450.7 and 7732.7 kJ mol⁻¹. Explain why magnesium forms $\mathrm{Mg^{2+}}$ in its compounds, and not $\mathrm{Mg^{+}}$ or $\mathrm{Mg^{3+}}$ .

Solution. Magnesium is $1s^2\,2s^2\,2p^6\,3s^2$ .

Not $\mathrm{Mg^{3+}}$ : the third electron would come from the full 2p subshell, close to the nucleus and poorly shielded. It costs $\frac{7732.7}{1450.7} \approx 5.3$ times the second ionisation energy. No lattice or hydration energy can repay this.
Not $\mathrm{Mg^{+}}$ : removing the second 3s electron does cost about twice the first, but the small, doubly charged $\mathrm{Mg^{2+}}$ ion attracts anions and water molecules so strongly that the extra lattice or hydration energy released more than repays it. So $\mathrm{MgCl_2}$ forms, not $\mathrm{MgCl}$ .

Worked example 2: energy to make gaseous ions

Calculate the energy needed to turn 1.00 g of gaseous magnesium atoms into gaseous $\mathrm{Mg^{2+}}$ ions. $A_r(\mathrm{Mg}) = 24.31$ .

Solution. For one mole: $\mathrm{Mg(g) \rightarrow Mg^{2+}(g) + 2e^-} \qquad \Delta H = IE_1 + IE_2 = 737.7 + 1450.7 = 2188.4\ \mathrm{kJ\,mol^{-1}}$ Amount of magnesium: $n = \frac{1.00\ \mathrm{g}}{24.31\ \mathrm{g\,mol^{-1}}} = 0.0411\ \mathrm{mol}$ Energy: $E = 0.0411\ \mathrm{mol} \times 2188.4\ \mathrm{kJ\,mol^{-1}} = 90.0\ \mathrm{kJ}$ Answer: 90.0 kJ (3 significant figures, as in the mass given).

Warning

Common mistakes. (1) Saying Group 2 metals form 2+ ions "because they want a full shell": the reason is energetic, as in Worked example 1. (2) Expecting melting point and density to change smoothly down the group: both are irregular, because the crystal structures differ. (3) Forgetting that the ion is much smaller than the atom.

Safety. Beryllium and its compounds are not used in school laboratories: the dust is toxic and can cause a lung disease. Soluble barium compounds, such as barium chloride, are poisonous if swallowed. Calcium reacts with water and must be kept dry in a closed container. Wear eye protection whenever these metals are handled.

Tip

Group 2 in one line: two outer s electrons, held more tightly than in Group 1, and less tightly at each step down. Bigger atom → lower ionisation energy → more reactive metal and more ionic compounds.

Chemistry Student: Group 2 Metals, Properties and Trends (9 min). Watch how atomic radius, first ionisation energy and melting point change down the group, and the explanation for each in terms of shells, shielding and metallic bonding.

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Practice

Write the full electron configuration of calcium and of the calcium ion.
Name the main mineral of barium and give its formula.
Describe and explain the trend in first ionisation energy from beryllium to barium.
Explain why the melting point of magnesium (650 °C) is much higher than that of sodium (98 °C).
Explain why carbon is not used to extract magnesium from magnesium oxide, and name two methods that are used.
Use Table 7.2 to calculate the energy needed to convert 0.500 g of gaseous calcium atoms into gaseous $\mathrm{Ca^{2+}}$ ions. $A_r(\mathrm{Ca}) = 40.08$ .
Radium is below barium. Predict, with reasons, its first ionisation energy (more or less than 502.9 kJ mol⁻¹?) and the charge on its ion.

Show answersHide answers

Ca: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2$ ; $\mathrm{Ca^{2+}}$ : $1s^2\,2s^2\,2p^6\,3s^2\,3p^6$ .
Baryte, $\mathrm{BaSO_4}$ .
It decreases (899.5 to 502.9 kJ mol⁻¹): the outer 2s to 6s electron is further from the nucleus at each step, and more inner shells shield it, which outweighs the larger nuclear charge.
Magnesium gives two electrons per atom to the electron sea, and $\mathrm{Mg^{2+}}$ (72 pm, 2+) is smaller and more highly charged than $\mathrm{Na^{+}}$ (102 pm, 1+), so metallic bonding is much stronger.
Magnesium binds oxygen more strongly than carbon does at ordinary furnace temperatures, so carbon cannot reduce MgO cheaply. Magnesium is made by electrolysis of molten $\mathrm{MgCl_2}$ (Dow process) or by reducing MgO with silicon (Pidgeon process).
$I E_{1} + I E_{2} = 589.8 + 1145.4 = 1735.2$ kJ mol⁻¹; $n = \frac{0.500}{40.08} = 0.01248$ mol; energy $= 0.01248 \times 1735.2 = 21.6$ kJ.
Less than 502.9 kJ mol⁻¹, because the outer 7s electrons are further from the nucleus and more shielded (the measured value is 509 kJ mol⁻¹, slightly higher, an effect of the very large nuclear charge that this simple model leaves out); the ion is $\mathrm{Ra^{2+}}$ , because it also has two outer s electrons.

Quick check

Why does magnesium form Mg²⁺ ions but never Mg³⁺ ions in its compounds?

Key points

Group 2 (Be to Ba) atoms are $n s^{2}$ and form $\mathrm{M^{2+}}$ ions; the metals occur only in compounds such as limestone, dolomite, gypsum, celestine and baryte.
Magnesium is extracted by electrolysis of molten $\mathrm{MgCl_2}$ or by reducing MgO with silicon; calcium by electrolysis of molten $\mathrm{CaCl_2}$ .
Down the group: atomic and ionic radii increase; ionisation energies and electronegativity decrease. Melting points and densities are irregular.
Group 2 metals melt far higher than Group 1 metals: two delocalised electrons per atom and a smaller, 2+ ion.

Sources and credits

Image: Calcium unter Argon Schutzgasatmosphäre.jpg by Matthias Zepper, via Wikimedia Commons, public domain.
Image: Magnesium crystals.jpg by Warut Roonguthai, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Image: Beryllium (Be).jpg by Hi-Res Images of Chemical Elements, via Wikimedia Commons, licensed under CC BY 3.0.
Data: ionisation energies, melting points, densities and electronegativities from the CRC Handbook of Chemistry and Physics, via Wikipedia's data pages for ionisation energies, melting points and densities; metallic radii from the atomic radii data page; ionic radii are Shannon's six-coordinate values.
Video: Group 2 Metals - Properties and Trends, Chemistry Student, embedded from YouTube.
The text, charts, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1From chlorophyll to cement

2Where they occur

3How the metals are extracted

4Physical properties down the group

5Melting points: higher than Group 1, and not a smooth trend

From chlorophyll to cement

Where they occur

How the metals are extracted

Physical properties down the group

Melting points: higher than Group 1, and not a smooth trend