All S-blocks elements are highly reactive except Be and Mg; why?
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Answer:
292 C:\ChemistryXI\Unit-10\Unit-10-Lay-3(reprint).pmd Reprint 27.7.6
292 CHEMISTRY
Lithium and beryllium, the first elements
of Group 1 and Group 2 respectively exhibit
some properties which are different from those
of the other members of the respective group.
In these anomalous properties they resemble
the second element of the following group.
Thus, lithium shows similarities to magnesium
and beryllium to aluminium in many of their
properties. This type of diagonal similarity is
commonly referred to as diagonal relationship
in the periodic table. The diagonal relationship
is due to the similarity in ionic sizes and /or
charge/radius ratio of the elements.
Monovalent sodium and potassium ions and
divalent magnesium and calcium ions are
found in large proportions in biological fluids.
These ions perform important biological
functions such as maintenance of ion balance
and nerve impulse conduction.
10.1 GROUP 1 ELEMENTS: ALKALI
METALS
The alkali metals show regular trends in their
physical and chemical properties with the
increasing atomic number. The atomic,
physical and chemical properties of alkali
metals are discussed below.
10.1.1 Electronic Configuration
All the alkali metals have one valence electron,
ns1
(Table 10.1) outside the noble gas core.
The loosely held s-electron in the outermost
valence shell of these elements makes them the
most electropositive metals. They readily lose
electron to give monovalent M+
ions. Hence they
are never found in free state in nature.
increase in atomic number, the atom becomes
larger. The monovalent ions (M+
) are smaller
than the parent atom. The atomic and ionic
radii of alkali metals increase on moving down
the group i.e., they increase in size while going
from Li to Cs.
10.1.3 Ionization Enthalpy
The ionization enthalpies of the alkali metals
are considerably low and decrease down the
group from Li to Cs. This is because the effect
of increasing size outweighs the increasing
nuclear charge, and the outermost electron is
very well screened from the nuclear charge.
10.1.4 Hydration Enthalpy
The hydration enthalpies of alkali metal ions
decrease with increase in ionic sizes.
Li+
> Na+ > K+
> Rb+
> Cs+
Li+
has maximum degree of hydration and
for this reason lithium salts are mostly
hydrated, e.g., LiCl· 2H2O
10.1.5 Physical Properties
All the alkali metals are silvery white, soft and
light metals. Because of the large size, these
elements have low density which increases
down the group from Li to Cs. However,
potassium is lighter than sodium. The melting
and boiling points of the alkali metals are low
indicating weak metallic bonding due to the
presence of only a single valence electron in
them. The alkali metals and their salts impart
characteristic colour to an oxidizing flame. This
is because the heat from the flame excites the
outermost orbital electron to a higher energy
level. When the excited electron comes back to
the ground state, there is emission of radiation
in the visible region as given below:
Alkali metals can therefore, be detected by
the respective flame tests and can be
determined by flame photometry or atomic
absorption spectroscopy. These elements when
irradiated with light, the light energy absorbed
may be sufficient to make an atom lose electron.
Element Symbol Electronic configuration
Lithium Li 1s22s1
Sodium Na 1s22s22p63s1
Potassium K 1s22s22p63s23p64s1
Rubidium Rb 1s22s22p63s23p63d104s24p65s1
Caesium Cs 1s22s22p63s23p63d104s2
4p64d105s25p66s1 or [Xe] 6s1
Francium Fr [Rn]7s1
10.1.2 Atomic and Ionic Radii
The alkali metal atoms have the largest sizes
in a particular period of the periodic table. With
Metal Li Na K Rb Cs
Colour Crimson Yellow Violet Red Blue
red violet
© NCERT
λ/nm 670.8 589.2 766.5 780.0 455.5
not to be republished