(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 conÜguration
that is a noble-gas structure with two
outer s-electrons. They are typical
metals (in the chemical sense) and
readily lose both outer electrons to
form stable M
2+ ions; i.e. they are
strong reducing agents. All are reactive, with the reactivity increasing
down the group. There is a decrease
in both Ürst and second ionization
energies down the group. Although
there is a signiÜcant difference between the Ürst 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 M
3+ )
are unknown.
Beryllium, the Ürst 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 and melting and boiling
points all 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 (see
diagonal relationship).
All the metals are rather less reactive than the alkali metals. They
react with water and oxygen (beryllium and magnesium form a protective surface Ülm) 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 only very slightly soluble
in water and is 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) + H 2 O(g)
The carbonates also decompose on
heating to the oxide and carbon dioxide:
MCO 3 (s) → MO(s) + CO 2 (g)
The nitrates decompose to give the
oxide:
2M(NO 3 ) 2 (s) → 2MO(s) + 4NO 2 (g) +
O 2 (g)
As with the *alkali metals, the stability of salts of oxo acids increases
down the group. In general, 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. (Solubilities of hydroxides increase for larger cations.)
alkaloid One of a group of nitrogenous organic compounds, mostly derived from plants, and having diverse
pharmacological properties. They are
19
alkaloid
a
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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 conÜguration
that is a noble-gas structure with two
outer s-electrons. They are typical
metals (in the chemical sense) and
readily lose both outer electrons to
form stable M
2+ ions; i.e. they are
strong reducing agents. All are reactive, with the reactivity increasing
down the group. There is a decrease
in both Ürst and second ionization
energies down the group. Although
there is a signiÜcant difference between the Ürst 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 M
3+ )
are unknown.
Beryllium, the Ürst 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 and melting and boiling
points all 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 (see
diagonal relationship).
All the metals are rather less reactive than the alkali metals. They
react with water and oxygen (beryllium and magnesium form a protective surface Ülm) 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 only very slightly soluble
in water and is 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) + H 2 O(g)
The carbonates also decompose on
heating to the oxide and carbon dioxide:
MCO 3 (s) → MO(s) + CO 2 (g)
The nitrates decompose to give the
oxide:
2M(NO 3 ) 2 (s) → 2MO(s) + 4NO 2 (g) +
O 2 (g)
As with the *alkali metals, the stability of salts of oxo acids increases
down the group. In general, 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. (Solubilities of hydroxides increase for larger cations.)
alkaloid One of a group of nitrogenous organic compounds, mostly derived from plants, and having diverse
pharmacological properties. They are
19
alkaloid
a
www.AzShimi.ir www.AzShimi.com