(formerly IIA) of the *periodic table:
beryllium (Be), magnesium (Mg), cal-
cium (Ca), strontium (Sr), and barium
(Ba). The elements are sometimes re-
ferred to as the ‘alkaline earths’, al-
though 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 M2+ions; i.e. they are
strong reducing agents. All are reac-
tive, with the reactivity increasing
down the group. There is a decrease
in bothÜrst and second ionization
energies down the group. Although
there is a signiÜcant difference be-
tween theÜrst and second ionization
energies of each element, com-
pounds containing univalent ions are
not known. This is because the diva-
lent ions have a smaller size and
larger charge, leading to higher
hydration energies (in solution) or
lattice energies (in solids). Conse-
quently, the overall energy change
favours the formation of divalent
compounds. The third ionization
energies are much higher than the
second ionization energies, and tri-
valent compounds (containing M3+)
are unknown.
Beryllium, theÜrst member of the
group, has anomalous properties be-
cause 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 elec-
trode 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 reac-
tive than the alkali metals. They
react with water and oxygen (beryl-
lium and magnesium form a protec-
tive surfaceÜlm) and can be made to
react with chlorine, bromine, sul-
phur, and hydrogen. The oxides and
hydroxides of the metals show the
increasing ionic character in moving
down the group: beryllium hydrox-
ide is amphoteric, magnesium hy-
droxide is only very slightly soluble
in water and is weakly basic, calcium
hydroxide is sparingly soluble and
distinctly basic, strontium and bar-
ium 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 diox-
ide:
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 stabil-
ity of salts of oxo acids increases
down the group. In general, salts of
the alkaline-earth elements are solu-
ble if the anion has a single charge
(e.g. nitrates, chlorides). Most salts
with a doubly charged anion (e.g. car-
bonates, sulphates) are insoluble. The
solubilities of salts of a particular
acid tend to decrease down the
group. (Solubilities of hydroxides in-
crease for larger cations.)alkaloidOne of a group of nitroge-
nous organic compounds, mostly de-
rived from plants, and having diverse
pharmacological properties. They are19 alkaloid
a