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Properties of Alkaline Earth Metals

 

The Alkaline Earth Metals are also known as the Group IIA elements of the periodic table. They shown certain properties which separate them from other elements.

The Alkaline Earth Metals are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They are called alkaline earth metals because:

1. They react with water to give alkaline solutions.

2. Their oxides resemble typical earths, in that they are hard to reduce. They all have two electrons in the outermost shell of their atoms, and all show the oxidation number of +2 in their compounds (i.e., they lose 2 electrons to be isoelectronic with the noble gases).

M → M2+ + 2e-

Properties - Beryllium and magnesium have a tendency to form covalent as well as ionic compounds, but the other members of the group form ionic compounds almost exclusively. - Beryllium does not react with water, even at high temperatures; magnesium reacts only with boiling water and steam; the others are very reactive with cold water.

Ca + 2H2O → (Ca2+ + 2OH-) + H2

- They are less reactive than the alkali metals, but are too reactive to be found free in nature.

- They are of low densities, although on the average they are more dense than the alkali metals (see table below). - When exposed to air, they are oxidized (except Be and Mg). Beryllium and magnesium form a thin oxide film which protects them against further oxidation. - Barium is the only member of this group that forms a peroxide when heated in oxygen.

Ba + O2 → BaO2

The other metals form normal oxides when heated in oxygen.

2Ca + O2 → CaO

- When heated in an atmosphere of nitrogen, the metals form nitrides (notice that lithium is the only alkali metal that does this)

3Mg + N2 → Mg3N2

Therefore, when heated in air, the metals form a mixture of oxides and nitrides.

- They react with sulphur to form sulphides.

- They react with halogens to form halides:

Mg + S → MgS

Mg + Cl2 → MgCl2

- When heated in hydrogen, Ca, Sr and Ba form ionic hydrides, MH2, which readily react with water, liberating hydrogen gas: Ca + H2 → CaH2

CaH2 + 2H2O → Ca(OH)2 + 2H2

Magnesium combines with hydrogen only when heated at high pressures; and beryllium hydride must be prepared by indirect means.

 

Element

Density (g/ml) at 20 oC

Melting Point oC

Boiling Point oC

Be

1.86

1283

1500

Mg

1.74

650

1120

Ca

1.55

850

1490

Sr

2.54

770

1384

Ba

3.50

704

1638

   

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