Alkaline earth metal
The name of the alkaline earth metals [e ː ˌ ka rtʔal ː ˌ lime talə ] denotes the elements of the second main group of the periodic table. The name is derived from the two adjacent main groups, the alkali metals and earth metals, from. It includes the stable elements beryllium (Be), magnesium (Mg ), calcium (Ca), strontium (Sr) and barium (Ba) at. The sixth member of the group, radium ( Ra), is a radioactive intermediate natural decay series.
- 2.1 Water Hardness
- 3.1 Beryllium- precious stones
Properties
Physical Properties
With increasing atomic number atomic mass grow, atomic radius and ionic radius.
The lowest density of calcium has 1550 kg / m³. It rises up, and especially toward the bottom, with radium at 5500 kg / m³ reaches the maximum value.
The Mohs hardness is beryllium with 5.5 in the middle. The other elements of the second main group have low hardness, which decrease with increasing atomic number.
The first three alkaline earth metals, in particular beryllium and calcium, are very good conductors of electricity. Although the other items in this major group are by no means poor conductors, the difference is considerable.
The first ionization energy decreases with increasing atomic number of 9,322 electron volts in beryllium at 5.212 eV for barium from. Radium has 5.279 eV again a slightly higher value.
The electronegativity of 1.5 drops to 1.0 with calcium beryllium and remains at approximately the other elements at this level.
Electron configuration
The electronic configuration is [ X] ys ². The X indicates the period are used, in which the element is found for the electron configuration of a period of higher standing noble gas, and for the y must.
For the individual elements are the electron configurations:
- Beryllium: [ He] 2s ²
- Magnesium: [ Ne ] 3s ²
- Calcium: [ Ar] 4s ²
- Strontium: [ Kr ] 5s ²
- Barium: [ Xe] 6s ²
- Radium: [ Rn] 7s ²
The oxidation state 2, since the two electrons can be easily discharged into the outer shell. Me2 ions have noble gas configuration.
Chemical reactions
In the following equations which Me is an element from the second main group ( alkaline earth metals ).
- Reaction with oxygen:
- Reaction with hydrogen:
- Reaction with water:
- Reaction with halogens ( chlorine on the example ):
On the reaction behavior which increases with the atomic number reactivity is easily observed:
- Resistance in air: Beryllium is stable at room temperature in dry air, since it is covered by a passivating oxide layer.
- Magnesium is also passivated in air, thin strip and foil, however, can easily ignite.
- Calcium, strontium, barium and radium run in dry air to quickly and ignite spontaneously in finely divided form.
- Beryllium does not react without a catalyst with hydrogen.
- Magnesium reacts only at high pressure.
- The other alkaline earth metals react with an atmospheric pressure of hydrogen.
- Beryllium is recognized as aluminum in water.
- Magnesium is also recognized, however, the passivation layer dissolves in hot water.
- The other alkaline earth metals react vigorously at room temperature with water.
Compounds
Beryllium is the only alkaline earth predominantly covalent compounds. The remaining elements of the second main group are found almost exclusively as Me2 ions. The table provides a rough overview of the most important connections is:
Other:
- In Zintl phases, the associated anions form a remarkable lattice.
- Grignard compounds are magnesium compounds of the form R- MgX. R stands for an organic radical and X is a halogen. You will find in organic synthesis using.
- Calcium carbide ( CaC 2 ) is a lattice with ions Ca2 - and ( | C ≡ C | ) 2 - ion (NaCl ) structure. The connection is required for three major processes: Ethinherstellung by hydrolysis:
- Desulfurization of crude steel
- Azotierung to calcium cyanamide
Water hardness
For the hardness of the water dissolved calcium and magnesium ions are primarily responsible. For example, the water-soluble calcium bicarbonate (Ca ( HCO3 ) 2) goes in the heat on in the poorly soluble compound calcium carbonate ( CaCO3), which is also known as " scale ":
By the escape of carbon dioxide from the solution, the reverse reaction is prevented, and the scale is deposited in cooking pots, etc. from.
Another characteristic of alkaline earth metal ions, but especially of Ca2 and Mg2 , to form insoluble compounds with soap is. Soaps are chemically considered salts, they are composed of cations and anions. The anions are always higher fatty acids, and cations as alkali metal ions are usually used. The alkaline earth metal ions replace these to form insoluble compounds, which are summarized under the term " lime soap ".
Occurrence
The earth's crust is made up 4.16% of alkaline earth metals. These are distributed as follows among the elements:
- 67 % of calcium
- 31% magnesium
- 1.4% barium
- 0.7% strontium ( the values are rounded off, thus resulting in a total of slightly more than 100%)
- Traces of beryllium and even smaller amounts of radium
Calcite (CaCO3)
Celestine ( SrSO4 )
Barite ( BaSO4 )
The alkaline earth metals never appear solid and are usually bound as silicate, carbonate or sulfate.
Beryllium- precious stones
Although beryllium is very rare, it is represented in 30 different minerals. The most famous include:
Aquamarine
Emerald
Chrysoberyl
Proof
The detection of the alkaline earth metals is primarily due to spectral analysis of the characteristic spectral lines. Wet chemical methods such as precipitation as carbonates, sulfates or hydroxides are now used only for demonstration purposes.
Safety
In air, only beryllium and magnesium are resistant. The other elements of this main group must be kept under paraffin oil or inert gas. The storage under alcohol is only possible with beryllium, magnesium, and calcium, as already barium it splits hydrogen and reacts to the alcoholate.
In finely divided form, magnesium is highly flammable; Calcium, strontium and barium powder may ignite in air itself. Burning earth metals must not be extinguished with water!
The alkaline earth metals are strong reducing agents that are even capable of releasing alkali metals from your connections. These reactions are highly exothermic; under certain circumstances it can even cause an explosion.
Beryllium is a toxic lung, the mechanism of action is still largely unknown. Also its compounds are carcinogenic.
Barium compounds are highly toxic if they are soluble in water. 1 gram can already be fatal.
Radium is extremely harmful due to its radioactivity, but still shifted to 1931 with radium water was sold under the trade name Radithor to drink. The number of injured or perished, who had taken such as steel magnate Eben Byers Radithor to himself, is unknown.