The alkali metals exhibit many of the physical properties common to metals, although
their densities are lower than those of other metals. Alkali metals have one electron in
their outer shell, which is loosely bound. This gives them the largest atomic radii of
the elements in their respective periods. Their low ionization energies result in their
metallic properties and high reactivities. It is easy to remove valence electrons of an
alkali metal to form a univalent cation. Alkali metals have low electronegativities.
They react readily with nonmetals, particularly halogens.
Physical Properties
1. They are grey and soft, and can be easily cut with a knife to expose a shiny
surface which dulls on oxidation.
2. They have low densities - Li, Na and K are less dense than water. They have
low standard enthalpies of melting and vaporization. They show relatively
weak metallic bonding as only one electron is available from each atom.
3. Good conductors of heat and electricity
4. They have low standard enthalpies of melting and vaporization. They show
relatively weak metallic bonding as only one electron is available from each
atom.
5. Alkali metals colour flames. When the element is placed in a flame the heat
provides sufficient energy to promote the outermost electron to a higher energy
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level. On returning to ground level, energy is emitted and this energy has a
wavelength in the visible region
Li
red
Na yellow
K
lilac Rb red Cs blue
6. The ionic radii of the alkali metals are all much smaller than the corresponding
atomic radii. This is because the atom contains one electron in an s level relatively
far from the nucleus in a new quantum shell, and when it is removed to form
the ion the remaining electrons are in levels closer to the nucleus. In addition,
the increased effective nuclear charge attracts the electrons towards the nucleus
and decreases the size of the ion.
Chemical Properties
1. Alkali metals react with water, oxygen, and other chemicals.
2. Alkali metals are never found as free elements in nature (i.e. - sodium metal
does not exist as Na metal in nature, sodium exists in compounds such as
NaCl, Na CO , etc...). The reactivity increases on descending the Group from
2
3
lithium to cesium. There is a closer similarity between the elements of this
Group than in any other Group of the periodic table.
3. Alkali metal compounds are water soluble and are present in seawater and
salt deposits.
4. All alkali metals have one valence electron which form ions with a positive
(1+) charge
Storage: since alkali metals react quickly with oxygen, they are stored under
mineral oil or kerosene
Chemical Properties
The alkali metals are strong reducing agents. The standard electrode potentials all lie
between -2.7V and -3.0V, indicating a strong tendency to form cations in solution.
They can reduce oxygen, chlorine, ammonia and hydrogen. The reaction with oxygen
tarnishes the metals in air, so they are stored under oil. They cannot be stored under
water because they react with it to produce hydrogen and alkali hydroxides:
2M(s) + 2H O(l) ➔ 2M+(aq) + 2OH-(aq) + H (g)
2
2
eg 2Na(s) + 2H O(I) ➔ 2NaOH(aq) + H (g)
2
2
This reaction illustrates the increasing reactivity on descending the Group. Li reacts
steadily with water, with effervescence; sodium reacts more violently and can burn
with an orange flame; K ignites on contact with water and burns with a lilac flame;
Cs sinks in water, and the rapid generation of hydrogen gas under water produces a
shock wave that can shatter a glass container.
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Na dissolves in liquid ammonia to give a deep blue solution of sodium cations and
solvated electrons. This solution is used as a reducing agent. At higher concentrations
the colour of the solution changes to bronze and it conducts electricity like a metal.
The chemistry of Li shows some anomalies, as the cation Li+ is so small it polarises
anions and so introduces a covalent character to its compounds. Li has a diagonal
relationship with magnesium.
Oxides
The alkali metals form ionic solid oxides of compostion M O when burnt in air.
2
However, Na also forms the peroxide Na O as the main product, and K forms the
2
2
superoxide KO , also as the main product.
2
Hydroxides
Alkali metal hydroxides are white ionic crystalline solids of formula MOH, and are
soluble in water. They are all deliquescent except LiOH. The aqueous solutions are all
strongly alkaline (hence the name of this Group) and therefore dangerous to handle.
They neutralise acids to form salts, eg:
NaOH(aq) + HCI(aq) ➔ NaCl(aq) + H O(l)
2
[In general OH-(aq) + H+(aq) ➔ H O(l)]
2
Halides
Alkali metal halides are white ionic crystalline solids. They are all soluble in water
except LiF, which has a very high lattice enthalpy arising from the strong electrostatic
interaction of the small Li+ and F- ions.
Oxidation States and lonisation Energies
Alkali metals have oxidation states of 0 and +1. All the common compounds are
based on the M+ ion. This is because the first ionisation energy of these elements is
low, and the second ionisation energy much higher. The outermost electron is well
shielded from the attraction of the nucleus by filled inner electron levels and so is
relatively easy to remove. The next electron is much more difficult to remove as it is
part of a full level and is also closer to the nucleus.
The first ionisation energy decreases down the Group because the outermost electron
is progressively further from the nucleus and so is easier to remove.
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1st
2nd
3rd
Li
513.3
7298.0