I once again welcome you to MSB lecture series
on main group chemistry.
Let me begin my talk today on group 2 elements.
Group 2 elements are known as alkaline earth
elements.
This group consists of beryllium, magnesium,
calcium, strontium, barium and radium The
electronic configuration of alkaline earth
elements is ns2, i.e. valence electrons; that
means, we have 2 electrons in the valence
shell.
As a result the group oxidation state is +2
and as expected, being the first element,
berylliumdiffers from other elements owing
to it is smaller size.
In fact, beryllium resembles more of aluminum
than magnesium and calcium and that is about
diagonal relationship between beryllium and
aluminum.
I will discuss those aspects later.
Atomic and ionic radii are smaller than those
of group one elements.
Of course, here one electron is added to the
same shell and nuclear charge is also increased.
As a result one can expect the atomic and
ionic radii of alkaline earth metals smaller
than the corresponding group one elements.
The alkaline earth metals are named after
their oxides.
The alkaline earths whose names were berilia,
magnesia, lime, strontia, and barita, respectively,
for beryllium, magnesium, calcium, strontium
and barium.
These oxides are basic when combined with
water; that means, these oxides when they
are reacted with water they give hydroxides,
the basic solution.
As a result they are called as basic oxides.
Earth in these elements is referredearlier,
essentially for non metallic substances.
Earlier the term ‘earth’ was added since
they were insoluble in water and resistant
to heating.
And of course, these properties are shared
by these oxides and hence they are called
alkaline earth metals and they readily form
M2+ ions since the group oxidation state is
+2 and as expected M2+ions are smaller than
M. Atomic and ionic radii increases down the
group as the size increases.
This is the periodic trend; we are familiar
with, and ionization enthalpy decreases down
the group.
First ionization enthalpies are higher than
those of the corresponding group 1 elements.
This is again following the same expected
periodic trends.
And second ionization enthalpies are higher
than the first ionization enthalpies for all
alkaline earth metals.
You can see here, the ionization energies
of group 2 elements are shown here.
For example, you can see first ionization
energy is much higher and the second ionization
energy is shown here and overall the ionization
energy of all alkaline earth metals are shown
in this plot.
You can see here, the first ionization energy
or ionization enthalpy or of all alkaline
earth metals are much higher than those of
alkali metals or group one elements.
why?
So, then hydration enthalpy, the hydration
enthalpy decreases down the group due to the
increase in the ionic size, as the ionic size
increases down the group charge-to-size ratio
also decreases as a result what happens hydration
enthalpy that depends heavily on the charge-to-size
ratio decreases due to the increase in the
ionic size and decrease in the charge-to-size
ratio.
As a result one can expect the trend in this
fashion.
Due to the larger size, group 2 elements are
more extensively hydrated compared to group
1 elements.
For example, if you see magnesium chloride,
is MgCl2.6H2O; that means, it has 6 water
molecules.
Similarly calcium chloride can take up to
6 water molecules.
If we compare these things with sodium chloride
or potassium chloride they do not form hydrates.
So, that is the difference between group 1
and group 2 elements.
Similar to group 1 metals, group 2 metals
are also silvery white in color, soft, but
harder than group 1 elements.
The hardness relatively more compared to group
1 elements because of the availability of
2 electrons for metal—metal bonding.
Strong electropositive property is shown by
group 2 elements and this trend increases
down the group..
All these properties are displayed here, whatever
I said, so far.
Let us look into the properties of group 2
elements and the trends.
Whatever I said, can be clearly seen here.
Metallic radius as expected, is increasing
from beryllium to radium and ionic radius
also should increase and it is increasing
with beryllium showing 27 pm.
Whereas, radium showing 170 pm.
First ionization energy is decreasing as the
valence electrons are moving farther from
the nucleus.
Ionization of those electrons would be rather
easy that you can see here that is also reflected
in the reduction potential here.
Reduction potential, you can see here the
trend.
The density is given.
Melting point also decreases down the group
and enthalpy also decreases.
All these properties are according to the
periodic trends, by looking into the trends
seen along rows and also down the group.
Let me tell you little about alkaline earth
metals and who discovered.
As I had mentioned, all alkaline earth metals
are silvery white in color.
So, you can see here beryllium, magnesium
turnings, I have shown calcium, strontium
little gold color is there, freshly sublimed
one, and barium, and radium is radioactive.
Beryllium was discovered by Louis Nicolas
Vauquelin in 1798 and magnesium was discovered
by Joseph Black in 1755 and Humphry Davy in
fact, her has contributed significantly to
the understanding and growth of chemistry,
especially alkali metals and alkaline earth
metals.
In fact, he is responsible for discovering
and isolating in the pure form many elements
that I will be discussing in the next slide.
He is responsible for discovering calcium,
barium and strontium.
Strontium was collectively, together discovered
by Crawford and Cruickshank in 1790 and later
it was isolated in its pure form by Humphry
Davy and Marie Curie and Pierre Curie they
discovered radium in 1898.
And of course, as I mentioned, Humphry Davy’s
contribution is remarkable.
He just lived for 51 years and he is known
for the electrolysis and also giving the method
of isolation of and discovery of elements
such as aluminum, sodium, potassium, calcium,
magnesium, barium, boron and also he is well
known for his safety lamp also known as Davy
lamp for people who work in the mines.
So, let us look into the occurrence of alkaline
earth elements.
Beryllium occurs naturally as the mineral
beryl, important ore of beryllium or important
source of beryllium with composition Be3Al2(SiO3)6
and magnesium is the eighth the most abundant
element in the Earth’s crust.
And the third most abundant element dissolved
in sea water.
It is commercially extracted from sea water
and the mineral dolomite is nothing but a
combination of calcium carbonate and magnesium
carbonate in 1:1 ratio and calcium is the
5th most abundant element in the earths’
crust, but only the 7th most common in sea
water due to the low solubility of calcium
carbonate.
And of course, calcium is a major component
of biominerals such as shells and coral and
calcium, strontium, and barium, all are extracted
by electrolysis of their corresponding molten
chlorides.
Radium can be extracted from uranium bearing
minerals such as pitch blend.
All its isotopes are radioactive and beryllium
is extracted by heating beryl with sodium
hexafluorosilicate that is Na2SiF6.
Magnesium is the only group 2 element extracted
on an industrial scale and magnesium, calcium,
strontium and barium can be extracted from
the respective molten chlorides very similar
to Downs process employed in the extraction
and isolation of sodium from sodium chloride.
Calcium is extracted by electrolysis of the
molten chloride which is itself obtained as
a byproduct of Solvay process for the production
of sodium carbonate and strontium is extracted
by electrolysis of molten strontium chloride
or by reduction of strontium oxide using aluminum
very similar to thermite process, and barium
is extracted by electrolysis of the molten
chloride or by reduction of barium oxide with
aluminum.
That means, essentially we have 2 methods
at our disposal to extract strontium either
from molten halides or from the corresponding
oxides.
So, when it comes to radium all isotopes of
radium are radioactive they undergo alpha,
beta and gamma decay with half-life that vary
anywhere between 42 minutes to 1599 years
for some of their radioactive isotopes.
Radium was discovered by Pierre Curie and
Marie Curie in 1898 after painstaking extraction
from the uranium ore Pitchblende and in fact,
pitchblende is a complex mineral containing
many elements in it besides uranium.
It is a source of many trace elements and
also some of the radioactive isotopes of some
of the heavier elements and it contains approximately
1 gram of radium in 10 tons of ore.
In fact, Curies together took nearly 3 years
to isolate 0.1 gram of radium chloride from
pitchblende.
So, let me give the extraction method of beryllium
starting from Beryl.
So, as I said beryl is nothing, but Be3Al2(SiO3)6.
So, first it has to be treated with sodium
hexafluorosilicate.
So, it gives Na2BeF4.
So, it acts as a fluorinating agent and then
this is treated with sodium hydroxide, leads
to the formation of sodium fluoride and Be(OH)2
will be separated, beryllium hydroxide.
And on treatment of beryllium hydroxide with
this salt gives beryllium tetrafluoride.
This one on heating gives beryllium fluoride.
So, this beryllium fluoride on reduction with
magnesium gives pure beryllium through the
formation of magnesium fluoride.
So, this is the method used for the extraction
of beryllium starting from beryl.
So, let me give the extraction method used
in the case of magnesium.
As I said in case of magnesium, one can use
molten magnesium chloride, and; that means,
preparation of magnesium chloride involves
the treatment of magnesium hydroxide
with HCl, essentially a neutralization process,
giving the salt and water.
So, in molten electrolysis at cathode, at
Anode reduction of chloride takes place and
this is for magnesium.
The similar method in case of strontium and
barium as well.
In case if we have oxide, one can use aluminum.
Of course, here to initiate the formation
of aluminum oxide that is highly exothermic,
first reaction needs triggering.
So, this method one can conveniently use in
case of barium as well.
So, other method of molten will be similar
to this reaction.
Now let us look into the flame test.
We are familiar with flame test in case of
alkali metals.
Similarly in case of alkaline earth metals,
calcium imparts brick red color, whereas strontium
imparts crimson red and barium imparts characteristic
apple green color to the flame and with this
let me begin the discussion about the chemical
reactivity of group 2 elements.
Of course, when we are looking to the reactivity,
we follow this sequence.
Let us look into first, the reaction with
hydrogen to form hydrides and then with oxygen
to form oxides and maybe with other chalcogens
to form the corresponding chalcogenides.
And the interaction with water and then halogens
and nitrogen and other reagents and then its
reducing nature and also behavior of alkaline
earth metals in liquid ammonia.
So, this is the flame test you can see here.
So, calcium, strontium and barium all I have
shown.
In fact, because of these vibrant colors they
are also used in fireworks.
So, before I proceed to consider the reactivity
of alkaline earth metals, let us look into
the difference in the properties of beryllium
from the rest of the elements.
That means, the anomalous properties of beryllium
should be looked into and let me discuss those
properties.
Generally beryllium forms covalent compound
such as beryllium halides BeCl2, BeBr2, BeI2
and also with hydrogen it forms covalent hydride
BeH2.
They have a tendency to form complexes because
of larger hydration enthalpy with the formation
of molecular compounds such as beryllium acetate.
Hydrolysis of beryllium salts in aqueous solution
leads to the formation of species such as
[Be(H2O)3(OH)]+ acidic solutions.
Hydrated beryllium salts tend to decompose
by hydrolysis reactions where beryllium oxo-
and hydroxo- salts are formed rather than
by the simple loss of water.
So, that is the specialty of beryllium.
The oxide and other chalcogenides of beryllium
adopt structures with more directional coordination
structures; that means, essentially they form
some coordination compounds very similar to
transition metal coordination compounds having
preferably 4 coordination.
So, beryllium forms many stable organometallic
compounds as well, including dimethyl beryllium,
diethyl beryllium and di-t-butyl beryllium
and also it forms beryllocene similar to ferrocene.
That means, (bis-cyclopentadienyl)beryllium
compound.
And of course, since I am devoting a few lectures
for the organometallic chemistry of main group
elements, that time I will be elaborating
more about the synthesis and stability and
structural aspects.
Both beryllium and aluminum form covalent
hydrides and halides.
The analogous compounds of other group 2 elements
are essentially ionic in nature.
The oxides of beryllium and aluminum are amphoteric,
whereas the oxides of the rest of the group
elements are again basic.
This trend is also very similar to what we
saw in case of lithium.
In the presence of excess of hydroxide ions
beryllium
and aluminum form tetrahydroxy species like
[Be(OH)4]2-.
Of course, aluminum forms similar soluble
hydroxides.
No equivalent chemistry is observed for whereas,
magnesium does not show any of these properties
and both elements form structures based on
linked tetrahedra.
So, beryllium forms structures built from
[BeX4]n- tetrahedra and aluminum also forms
numerous aluminates and aluminosilicates containing
[AlO4]n- units.
So, this shows the similarities in the reactions
and properties of beryllium with aluminum
and both elements form carbides that contain
C4- ion.
Of course, in carbides these ions will be
there and which on hydrolysis are reaction
with water produce methane.
The other group 2 carbides contain essentially
C22- ion and produce ethylene, this essentially
gives ethylene or acetylene, of course, those
things I will discuss later, when the corresponding
carbides are treated with water.
Alkyl compounds of beryllium and aluminum
are electron deficient compounds that contain
bridges; that means, essentially this is nothing,
but 3c‒2e bonds.
So, this is quite common in case of alkyl
beryllium and aluminum compounds and they
undergo association to form one dimensional
chain.
So, I will be elaborating later about the
more details about this one, let us now continue
with the chemical reactivity of group 2 elements.
Smaller size compared to alkaline metals you
should remember alkaline earth metals have
smaller size compared to alkali metals.
And they are strongly hydrated and they have
high lattice energy compared to group 1 elements
and in fact, beryllium has a different chemistry
compared to its heavier congeners.
Free Be2+ ion does not exist.
All its compounds are covalent or contain
solvated ions.
Such as [Be(H2O)4]2+; that means, it is very
difficult to isolate free Be2+ ion.
Many of the salts are less soluble in water.
K2SO4 is soluble.
Many of the group 2 salts are less soluble
in water K2SO4 or Na2SO 4 if you take they
are soluble in water, whereas CaSO4 or SrSO4
are insoluble.
In fact, that trend increases down the group.
In fact, most of the barium salts are insoluble
and beryllium is a rare element and its important
ore or source is beryl Be3Al2Si6O18.
Of course, let me stop here.
In my next lecture I will be continuing the
chemistry of main group 2 elements and begin
my lecture with group 2 hydrides.
Have a pleasant inorganic chemistry reading
until then.
Thank you.
