CH226.7
•The alkali metals
Watch the movie on
http://www.youtube.com/watch?v=cqeVEFFzz7E
Alkali metals
Li, lithium (in 1817, Berzelius named the element "lithos", from the Greek λιθoς (lithos,
"stone"), to reflect its discovery in a mineral).
Na, sodium (the name is derived from the English soda and the symbol comes from the
Latin natrium, which means the same. Soda may mean sodium carbonate (washing soda,
Na2CO3) or sodium bicarbonate (baking soda, NaHCO3)).
K, potassium (the symbol is derived from kalium, the Mediaeval Latin for potash, or the
Arabic word qali; meaning alkali. Potash is an impure form of potassium carbonate,
K2CO3). Potassium and sodium were first isolated by H. Davy in 1807.
Rb, rubidium, discovered in 1861 (the name is from the Latin rubidius, meaning ruby).
Cs, cesium, discovered in 1860 (the name comes from the Latin caesius, meaning sky
blue, in reference to the color its compounds impart to a flame).
Fr, francium, discovered in 1939 (this highly radioactive metal is named after France).
The alkali metals are members of group 1 and have the electronic configuration ns1.
Pictures on the following slides are from: http://www.theodoregray.com/PeriodicTable/
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Lithium
Lithium has a silvery appearance. Li reacts with N2, O2, and
moisture in air and turns black. Reported industrial production is
about 30,000 tones a year. More could be produced for nuclear
weapons. Lithium salts are used in greases, batteries, glass,
glass ceramics, and in alloys with aluminum and magnesium, to
greatly improve their strength and make them lighter.
The “hydrogen bomb” is made of 6Li2H (lithium-6 deuteride) in the core of 235U. When
the bomb detonates, it releases neutrons from the fission of 235U. These are absorbed
by 6Li to give 4He and 3H. Then 3H fuses with 2H to form more 4He and neutrons. The
consequence of this is the release of much energy in a fraction of a second with the
explosive force of millions of tones of TNT.
There is some biological need for lithium in humans (1 mg/day).
Lithium is moderately toxic by ingestion. Recent (2016) research
suggests that lithium carbonate is the best treatment for bipolar
disorder (also known as manic depression and is characterized
by manic highs and bouts of depression). Bipolar disorder carries
one of the highest rates of suicide of any mental illness.
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Sodium
A soft metal that tarnishes within minutes of being exposed to air,
and which reacts vigorously with water. World production of Na
metal is ca. 100,000 tones. Liquid sodium metal is used in heat
exchangers in some nuclear reactors. Some “breeder” nuclear
reactors are cooled with a massive amount (e.g., 6,000 tones) of
liquid Na. Compounds of Na: NaCl and Na2CO3 have many
applications in the production of Cl2, NaOH, glass, and in the food
industry.
Our bodies contain about 100 grams of sodium as Na+ which is mainly found in the
fluids outside cells. Blood needs a lot of sodium ions to regulate blood pressure.
Sodium’s most important function is the movement of electrical impulses along the
nerve fibers (in conjunction with potassium).
The link between salt intake and the health is controversial. It was suspected that
excess salt could be responsible for high blood pressure. However, in 1996, a report
from the Mount Sinai Hospital, Ontario found that salt could pose a threat only to
people who already had high blood pressure.
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Potassium
Potassium is a soft white metal which is silvery when cut. It is
one of the most reactive and electropositive of metals, and as it
oxidizes rapidly in air, it must be preserved in a mineral oil such
as kerosene. Its reaction with water is vigorous - it catches fire
spontaneously and decomposes with the evolution of hydrogen.
Potassium is an essential element for almost all living things. The K+ ions concentrate
inside cells and have many functions. Potassium deficiency can cause muscular
weakness, which can affect the heart muscle, causing irregular beat and even cardiac
arrest. On the other hand, an excess of potassium depresses the central nervous
system, paralyzes it, and the heart muscles stop beating. Injection of a concentrated
solution of KCl is a form of capital punishment in the USA.
Most industrially produced compounds of potassium (95%) go into fertilizers. About
200 tones of potassium metal is produced every year. Potassium superoxide, KO2 is
formed when K burns in air. This material is kept in mines, submarines, and space
vehicles in order to regenerate the oxygen:
4KO2 + 2CO2  2K2CO3 + 3O2
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Rb, Cs, Fr.
Rubidium and cesium are solids at room temperature, but
liquids just a bit higher. Cesium actually melts in your hand at
28º, while rubidium melts at 39 °C. Cesium also has a much
prettier color, exactly the color of gold. Both metals would
produce an explosion on contact with moist air.
Rb and Cs have no known biological role and their salts are not
considered to be toxic. The amount of Rb produced is small, and
the metal is used for research purposes only. The metal costs
$20,000 per kg, and is more expensive than gold. 28% of
rubidium is 87Rb, which is weakly radioactive (it is a β-emitter
with a half-life of 50 billion years).
World production of cesium compounds is a mere 20 tones per year and this comes
mainly from Bernic Lake, Manitoba. Cesium is little used outside chemistry labs: CsF
and Cs2CO3 are useful bases. Cesium “thrusters” are used to steer satellites. Cs is
ionized in a vacuum chamber and then the ions are accelerated through an electric
field and ejected through a nozzle, thereby giving a counter-thrust to the satellite.
Francium is toxic due to its radioactivity. No weighable quantity of the metal has ever
been made since the longest lived isotope 223Fr has a half life of only 22 minutes. 6
Flame tests
Metal
Color
lithium
red, crimson
sodium
yellow
potassium
lilac
rubidium
red
cesium
blue-violet
Colored flame from
burning lithium
Colored flame from
burning sodium
For technical reasons, some colors can be difficult to reproduce.
Colored flame from
burning potassium
Colored flame from
burning rubidium
Colored flame from
burning cesium
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Properties of the alkali metals
•The atoms become larger down the group
•The strength of M-M bonding decreases from Li to Cs
•The ionization energies decrease from Li to Cs
Li
Na
K
Rb
Cs
3
11
19
37
55
Ionic radius, Å
0.76
1.02
1.38
1.49
1.70
Melting point, °C
180
98
63
39
28.5
Standard reduction potential,
E°(M+/M)
-3.04
-2.71
-2.93
-2.98
-3.03
Enthalpy of atomization,
ΔHa°(298 K) / kcal/mol
35
24
19
18
16
First ionization energy, IE1 / kcal/mol
123
118.5
100
96
90
Standard enthalpy of hydration of M+,
ΔhydH°(298 K) / kcal/mol
-124
-97
-77
-71
-65
Atomic number, Z
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Standard reduction potentials
Values of E° are similar for the alkali metals, because the atomization, ionization, and
hydration enthalpies decrease down the group.
M(s) → M+(aq) + e-,
ΔH°
ΔH° = ΔH°(atomization) + ΔH°(ionization) + ΔH°(hydration)
(all energies are in kcal/mol)
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