The polarizing power of a cation can be estimated from the ratio of its charge to ionic radius Zr

1
Polarizing Power and Polarizability

• The polarizing power of a cation can be estimated
  from the ratio of its charge to ionic radius
  (Z/r)
• Consider Al3 which is highly polarizing
  
• Ionic radius 0.39 Å in CN 4. Charge 3
• Z/r 7.7. This is known as the charge density
  or ionic potential

2

• SECOND ROW ANOMALY
• Li, Be, B, C, N, O, F
• These shows atypical behaviour from the heavier
  elements in their respective groups.
• REASON FOR THE ANOMALY
• Their atoms are small thus higher Z/r and higher
  polarizing power
• Their electrons are tightly held and so not
  readily ionized or polarized like the heavier
  members of the groups
• They have no d orbitals available for bonding

3
Anomalous behaviour in terms of1. Reactivity of
metals and metalloids

• These elements are less reactive toward water
  than their heavier congeners - Higher ionization
  energies
• In group 1
• Li reacts slowly with water at 25?C
• Na reacts violently and K in flames
• 2M(s) 2H2O(l) ? 2M(aq) 2OH- H2(g)
• In conditions of plenty oxygen, only Li forms
  Li2O. Other metals form peroxides and
  superoxides Large Lattice and Bond Energy
• Li reacts directly with N2 to form Li3N
• 6Li(s) N2 (g) ? 2Li3N(s). No other alkali
  metal reacts with N2
• LiF, LiOH and Li2CO3 are less soluble that the
  corresponding Na and K compounds large lattice
  energies

4
Anomalous behaviour in terms of1. Reactivity of
metals and metalloids

• SIMILAR DIFFERENCES EXIST AND ARE MORE
  PRONOUNCED BETWEEN Be AND THE OTHER METALS IN
  GROUP 2
• CONSIDER
• All Group 2 metals except Be react with water
  
• M(s) 2H2O(l) ? M 2(aq) 2OH-(aq) H2 (g)
• Be only reacts with air above 600?C when finely
  powdered
• BeO that is formed is amphoteric (other Group 2
  oxides are basic)
• Of Group 2 elements only Be reacts with NaOH or
  KOH to liberate H2 and form Be(OH)42-
• Crystalline Boron is chemically inert
  unaffected by boiling HCl and only slowly
  oxidized by hot concentrated HNO3 when finely
  powdered.

5
Anomalous behaviour in terms of 2. Metallic
Character

• Row 2 elements are less metallic than their
  heavier congeners
• Li and Be are metals but less conducting due to
  their high IEs (electrons are close to nucleus)
• To ionize B to B3 requires a large input of
  energy so it usually adopts a covalent polymeric
  structure (semi-metal)
• C is a non-metal and a poor conductor of
  electricity (except graphite)
• Other members of Group 14 are metals and
  metalloids
• The other elements become increasingly metallic
  as the group is descended due to decrease in
  ionization energies

6
Anomalous behaviour in terms of 3. Covalent
Character

• Li and Be2 are small and have large polarizing
  abilities.
• Their compounds are more covalent than those of
  the heavier elements in their groups
• e.g. all Be2 compounds are covalent or contain
  solvated beryllium ions Be(H2O)42
• BeCl2 is covalent while MCl2 (M Mg-Ba) are
  ionic
• The conductivity of fused beryllium chloride is
  only 1/1000 that of sodium chloride under the
  same conditions

7
Anomalous behaviour in terms of 3. Covalent
Character

• B prefers to share its electrons in covalent
  bonds rather than transfer them to another
  element
• B compounds usually resemble those of nonmetals
  (such as Si) in properties and reactions.
• eg. B2O3 acidic,
• the hydrides or boranes BnHm are volatile
  molecular compounds
• The halides are highly reactive, volatile,
  covalent compounds eg. BF3 is a gaseous molecular
  halide while AlF3 is a typical high melting
  point ionic solid.

8
Anomalous behaviour in terms of 4. Lower
Coordination Number

• Second row elements form compounds with the
  element in CN 4 while heavier elements have CNs
  of 5 and 6
• Due to unavailability of d orbitals for bonding
  more atoms
• only 2s, 2px, 2py and 2pz (4 valence orbitals) so
  they form a maximum of 4 covalent bonds
• by contrast third row elements can use d
  orbitals to form more than 4 bonds
• consider the reactivity of SiCl4 (third row Si)
  and CCl4 (second row C) with water
• Also consider while nitrogen only forms NCl3
  phosphorous forms both PCl3 and PCl5

9
Anomalous behaviour in terms of 5. Multiple Bonds
Consider
The 3p orbitals of the corresponding third row
elements Si, P, and S are more diffuse and the
longer bond distances for these larger atoms
result in poor ? overlap.
10
Anomalous behaviour in terms of 5. Multiple Bonds

• So C, N, O form multiple bonds (double and
  triple)
• In group 14 C forms CC double bonds but SiSi
  double bonds (Si in third row) are uncommon.
• Similarly in group 15 elemental N2 contains N?N
  triple bond but white phosphorous contains
  tetrahedral P4 molecules each P atom forming 3
  single instead of 1 triple bond.
• In group 16 O2 contains OO double bonds but in
  S8 (elemental sulfur) each S forms 2 single
  rather that one double bond.
• Compare the structure of CO2 and SiO2

11
Anomalous behaviour in terms of 6. Oxidizing
ability oxygen and fluorine

• This is due to their high electron affinities and
  electronegativities
• They tend to form strong ionic bonds with other
  elements
• Oxygen and Fluorine even react with noble gases
  to form compounds such as XeO3, XeO4, XeF6 and
  XeF4

O2(g) 4H(aq) 4e- 2H2O(l) Eº 1.23V
F2(g) 2e- 2F-(aq) Eº 2.87V