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s-Block elements


									Variation in Physical Properties
      of s-Block elements
 Variation in
 • atomic / ionic radius.
 • ionization enthalpy
 • hydration enthalpy
 • melting point.
               s-Block elements
Group Group
  I     II
      1        2
                   • They are called s-block
[X]ns [X]ns
                     elements because their
 Li       Be         outermost electron(s) is /are in
                     the s-orbitals.
 Na       Mg
                   • They are all metals (alkali
 K        Ca         metals, alkaline earth metals).
                   • They are very reactive.
 Rb       Sr
                   • Most of them have
 Cs       Ba         characteristic flame colour.
Variation in atomic and ionic radii
Trends in atomic/ionic radius
Li   Be

Na   Mg

K    Ca

Rb   Sr
               increase of e- shells
Cs   Ba

           increase of effective nuclear charge
      Trends in atomic/ionic radius
1) Ionic radii of Gp I and II elements is always
smaller than its atomic radii.
Reason: The cation formed M+ or M2+ has one
electron shell less than the atom, thus size is
2) Atomic and ionic radii increase down a
Reason: There is one more shell of inner core
e- than its group predecessor.
 Comparing atomic radii of Group
   I and II in the same period

Atomic radius of Group I > Group II.
• Group II atoms have higher effective
  nuclear charges, and the outer s-electrons do
  not screen each other effectively,  Group
  II atoms have higher effective nuclear
  charges, the e- are pulled stronger inward.

S-Block elements have relatively
large atomic radii amongst the
elements of the same period
because of their
small effective nuclear charge
Variation in Ionization Enthalpy
Variation in Ionization Enthalpy
Trends in ionization enthalpy
Li   Be

Na   Mg

K    Ca
Rb   Sr         atomic radius increases
                  outer e- are further out
Cs   Ba           and better shielded from the nucleus

           nuclear charge / ENC increases
               Trends in I.E.
1) Both Gp I and II metals have low first and
second ionization enthalpy respectively
Reson: Their outer s-electrons are well shielded
 from the nucleus by inner shells of electrons.

2) I.E. decrease as both groups are descended
Reason: As the atomic radius increases down
  both groups, the outer electrons are further out
  and better shielded from the nucleus.
• S-block metals have generally low first I.E.
  among the elements on the same period.
         Hydration enthalpy
• Hydration enthalpy is the energy released
  when one mole of aqueous ions is formed
  from its ions in gaseous state .
• e.g. Mn+(g) + aq       Mn+(aq)       H = -ve
• or Yn-(g) + aq        Yn-(aq)        H = -ve
Variation in Hydration Enthalpy of Gp I
    metal ions (always exothermic)
Variation in Hydration Enthalpy of Gp II
    metal ions (always exothermic)
     Trends in hydration enthalpy
Li+   Be2+

Na+   Mg2+

K+    Ca2+
Rb+    Sr2+      ion gets larger,
                  charge density falls
Cs+   Ba2+
               ion gets higher charge but smaller size,
                charge density increases
 1) On going down both Groups I and II, the
 hydration enthalpies of the cations decrease in
 magnitude (less –ve).
Reason: As the ions get larger down the group, the
  charge density of cations decrease, thus the
  electrostatic interaction between the polar water
  molecules and ions get less.
2) Group II cations have higher in magnitude
  (more –ve) hydration enthalpies than group I
Reason: Gp II ions have higher charge and smaller
  size, thus their charge density is higher.
Variation in Melting Point
     Trends in Melting Point
Li   Be

Na   Mg

K    Ca
Rb   Sr          ionic radius increases
                  e- cloud becomes more diffuse
Cs   Ba           weaker attraction between e-
                  and the ion
         2 e- per atom participate
          in metallic bond for Group II,
          1 e- for Group I metals
1) The melting points of the elements are found
  to decrease generally down each group.
Reason: As go down each group, the ionic size
  increases, thus the metallic bond strength
2) The of group II metals are generally
  higher than group I metals in the same period.
Reason:Greater no. of valence electrons and
  smaller ionic size of gp II metals increase the
  metallic bond strength.
  Changes in Group II are irregular
 Reason: It is partly due to differences in
 crystal structure of the metals

Group I element   Structure   Group II element Structure
      Li            b.c.c.          Be           h.c.p.
      Na            b.c.c.          Mg           h.c.p.
      K             b.c.c.          Ca           f.c.c.
      Rb            b.c.c.           Sr          f.c.c.
      Cs            b.c.c.          Ba           b.c.c.
          Flame Tests
  for Group I and II compounds
• When salts of Group I and II elements are
  brought into a hot flame, some M+/M2+ gain
  e- momentarily to form atoms.
• The electrons of this atoms in the outermost
  shell of M may also be excited to a higher
  energy level in the flame.
• When the electron drops back to its original
  position it gives out the energy it obtained
  during excitation.
          Flame Tests
  for Group I and II compounds

• The energy is released in the form of
  E = h
• It appears as visible light giving the
  characteristic flame colourations.
      Characteristic flame colours of
       Group I and II compounds

Group I   Flame colour   Group II        Flame colour
  Li        crimson        Be       no characteristic colour
  Na         yellow        Mg       no characteristic colour
  K           lilac        Ca              brick red
  Rb           red         Sr               crimson
  Cs          blue         Ba             apple green
     How to carry out flame tests?
• Pt wire is first cleaned of any impurities by
dipping it into conc. HCl solution
•heating it in a non-luminous Bunsen flames till no
flame colour is detected
•the clean wire is then dipped in to a fresh clean
portion of conc HCl again, and then into a small
sample of powdered compound whose flame colour
is to be determined.
Lilac flame of potassium
Golden yellow flame of sodium
Brick-red flame of calcium
Apple green flame of barium

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