Alkaline-Earth Metals

Alkaline-earth metalsare the elements of group 2 (formerly IIA) of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The elements are sometimes referred to as the 'alkaline earths', although strictly the 'earths' are the oxides of the elements. All have a characteristic electron configuration that is a noble-gas structurewith two outer s-electrons. They are typical metals (in the chemical sense) and readily lose both outer electrons to form stable M²⁺ ions; i.e. they are strong reducing agents. All are reactive, with the reactivity increasing down the group. There is a decrease in both first and second ionization energies down the group. Although there is a significant difference between the first and second ionization energies of each element, compounds containing univalent ions are not known. This is because the divalent ions have a smaller size and larger charge, leading to higher hydration energies (in solution) or lattice energies (in solids). Consequently, the overall energy change favours the formation of divalent compounds. The third ionization energies are much higher than the second ionization energies, and trivalent compounds (containing M³⁺) are unknown.

Beryllium, the first member of the group, has anomalous properties because of the small size of the ion: its atomic radius (0.112 nm) is much less than that of magnesium (0.16 nm). From magnesium to radium there is a fairly regular increase in atomic and ionic radius. Other regular changes take place in moving down the group from magnesium. Thus, the density, melting and boiling points increase. Beryllium, on the other hand, has higher boiling and melting points than calcium and its density lies between those of calcium and strontium. The standard electrode potentials are negative and show a regular small decrease from magnesium to barium. In some ways beryllium resembles aluminium.

All the metals are rather less reactive than the alkali metals. They react with water and oxygen (beryllium and magnesium form a protective surface film) and can be made to react with chlorine, bromine, sulphur, and hydrogen. The oxides and hydroxides of the metals show the increasing ionic character in moving down the group. Beryllium hydroxide is amphoteric. Magnesium hydroxide is a very slightly soluble in water and weakly basic, calcium hydroxide is sparingly soluble and distinctly basic. Strontium and barium hydroxides are quite soluble and basic. The hydroxides decompose on heating to give the oxide and water:

M (OH)₂(s)®MO (s) +H₂O (g).

The carbonates also decompose on heating to the oxide and carbon dioxide:

MCO₃(s) ® MO (s) + CO₂(g).

The nitrates decompose to give the oxide:

2M(NO₃)₂(s) ® 2MO (s) + 4NO₂(g) + O₂(g).

As with the alkali metals, the stability of the salts of oxo acids increases down the group. In general, the salts of the alkaline-earth elements are soluble if the anion has a single charge (e.g. nitrates, chlorides). Most salts with a doubly charged anion (e.g. carbonates, sulphates) are insoluble. The solubilities of salts of a particular acid tend to decrease down the group.

<== попередня сторінка	\|	наступна сторінка ==>
Practice	\|	Exercise 5.

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.005 сек.