Alkali and alkaline earth metals

33 Pages·2017·0.68 MB·English

The s-block elements:
Alkali and alkaline earth metals
The s-block elements:alkali and alkaline earth metals
Alkali metals
oThe alkali family is found in the first column of
the periodic table include; Li, Na, K, Rb, Cs, Fr.
o Metallic elements.
o Among the alkali metals;
 sodium and potassium are abundant and
 lithium, rubidium and caesium have much
lower abundances.
 Francium is highly radioactive; its longest-
lived isotope 223Fr has a half-life of only 21
minutes.
The s-block elements:alkali and alkaline earth metals
Alkali metals
oAtoms of the alkali metals have a single
electron in valence shell (1 valence
electron) and electron configuration of 1s1.
oSoft metals (easily cut with a knife).
oThey are the most reactive metals.
oThey react violently with water.
oAlkali metals are never found as free
elements in nature. They are always
bonded with another element.
The s-block elements:alkali and alkaline earth metals
Alkali metals
The metals are all very reactive* when pure.
– strongly reducing (very powerful reducing agents)
 In nature, these elements only ever appear as their +1 cations.
 Their cations (with the exception of lithium) have low charge densities.
– often leads to good solubility and an ability to stabilize anions that can not
be formed along with any other cation.
Cations occur in wide variety of important chemicals.
 Form ionic compounds with highly electronegative (nonmetals) found in
Groups 6 (O, S, Se and 7 (F, Cl, Br, I, At) , as well as P and N from Group 5.
*Most alkali metals are stored under oil. But lithium is so light that it floats on oil. So
a coat of petroleum jelly is applied to lithium before it is stored.
The s-block elements:alkali and alkaline earth metals
Physical Properties
• Lowest ionization energies and electronegativities in periodic table
- easily ionized to form ions with +1 charge/+1 oxidation state.
The s-block elements:alkali and alkaline earth metals
Physical Properties
• They are good conductors of electricity.
Their ability to conduct electricity is due to the availability of their outer electrons.
• Low densities.
• Low melting points and boiling points.
Due to the presence of a single valence electron.
The s-block elements:alkali and alkaline earth metals
Flame tests
o Alkali metal ions are identied by flame tests.
o In flame ions in excited states return to their ground states  emit light.
o The emission spectrum depends
on a number of factors;
- the flame temperature;
- but is most strongly is the
energy levels of the emitter, leading
to characteristic colours.
Element Li Na K Rb Cs
Flame colour crimson yellow lilac purple blue
The s-block elements:alkali and alkaline earth metals
Production of alkali metals
o Lithium sodium and potassium metal are produced by electrolysis of
the molten chlorides.
o Overall reactions:
LiCl  Li + ½ Cl
(l) (l) 2(g)
NaCl  Na + ½ Cl
(l) (l) 2(g)
The s-block elements:alkali and alkaline earth metals
Chemical reactions
Reactivity towards Bronsted acids
The alkali metals react even with weak Bronsted acids such as water
– The rate of reaction depends upon the acidity of the medium
2Na + 2H O  2NaOH + H
2 2
2Na + 2EtOH  2NaOEt +H
2
2Na + 2NH  2NaNH +H
3 2 2
– very slow reaction in absence of catalyst
The s-block elements:alkali and alkaline earth metals
Oxides of the alkali metals
oAlkali metal on combustion in Oxygen
M + O → depends on the alkali metal
2
Li O (oxide)
2 The oxidation state of O?
Na O (peroxide)
2 2
KO , RbO , CsO (superoxide)
2 2 2
o On combustion in excess of air, lithium forms mainly the oxide, Li O (plus
2
some peroxide Li O ), sodium forms the peroxide, Na O (and some
2 2 2 2
superoxide NaO ) whilst potassium, rubidium and caesium form the
2
superoxides, MO .
2
oThe increasing stability of the peroxide or superoxide, as the size of the
metal ion increases, is due to the stabilisation of large anions by larger
cations through lattice energy effects.