S- block elements

1. Introduction:-

(a) Elements of IA and IIA group of the periodic table are called s-block elements.
(b) For these elements outer s-orbital in the process of filling.
(c) IA [ns1] group elements are called alkali metals and IIA [ns2] group elements are called alkaline
earth metals.


2. General Characteristic:-

(a) They are good conductors of heat and electricity.
(b) They are malleable and ductile.
(c) Exhibit group valency of 1 and 2 for IA and IIA groups respectively.
(d) They are prepared by the electrolysis of their fused salts.
(e) They are very reactive as their last shell contains 1 or 2 electrons which can be given off easily (low
ionization potential).
(f) They form colourless compounds except chromates, dichromates etc.
(g) Their cations are diamagnetic.
(h) They form ionic compounds (except Li and Be).
(i) Their solutions in liquid ammonia are good conductor of electricity and are good reductant.
(j) Oxides are basic in nature.

3-Oxidation State :

These metals exhibit + 1 oxidation state, difference
of their second and third ionisation potentials
is more than 16 eV. Therefore, their + 1 oxidation
state is more stable.

4- Density :
Density =Atomic weight/Volume = M/V
Atomic weight increase from Li to Cs in the group
and volume also increase, but increase in atomic
weight is more a compared to volume. Therefore,
density
increases from Li to Cs.
Exception :
Density of Na is more than that of K.
Density :
Li < K < Na < Rb < Cs

5- Tendency of forming ionic Bond :
One electron is present in the outermost shell of
these metals. They form cation by the loss this
electron, i.e., they form ionic bond in their compounds.

6- Standard Electrode Potential or Standard
Oxidation Potential :
The measure of the tendency of donating electrons
of a metal in water is called its electrode potential.
If concentration of metal ions is unity, then it is
called standard electrode potential.

7-Colourless and Diamagnetic Ions :
The property of an ion as being colourless or
coloured, depends on the number of unpaired
electrons present in the ion. If unpaired electrons
are more in an ion, then these electrons get excited
by the atmospheric energy and show colour on
coming back to the ground state.
Intensity of the colour ∝ Number of unpaired
electrons
The ions which have unpaired electrons, show
magnetic properties. Whereas, the ions having
paired electrons nullify the magnetic fields of each
other. Such ions are called diamagnetic ions.

8- Flame Test :
Alkali metals have large size. When they are
heated in the flame of Bunsen burner, the electrons
present in the valence shell move from lower
energy level to higher energy level by absorption
of heat from the flame (ns1 or ns2 nºp). When
they come back to the ground state, they emit the
extra energy in the form of visible light to provide
colour to the flame. Elements and their respective
colours imparted to the flame are given below.
Element Li             Na               K           Rb                 Ca
Colour Red  Golden yellow   Violet     Red violet         Blue

9- Photoelectric effect :
Size of Cs is large and one electron is present in
its outermost shell. Due to this, electron of
outermost shell gets excited by absorption of visible
light. Therefore, Cs shows photoelectric effect.
This is the reason that it is used in the cells.