MODULE 5 Periodic Properties by ishita sanyal

Periodic properties

1. Module 5
Periodic Properties
by
Dr. Ishita Sanyal
Assistant Professor
Chemistry Department, TMSL

2. Effective Nuclear Charge
The energy of outer electron is determined by
Effective Nuclear Charge (Z
eff) which is less than Z
(nuclear charge) by an amount shielding constant
(S) of the intervening core of electrons.
Z
eff = Z -S

3. Slater’s rule
The electronic configuration of the element is first written in the
following order:
(1s);(2s,2p);(3s,3p);(3d);(4s,4p);(4d);(5s,5p);(5d) so on…..
Consider a particular electron in an ns or np orbital:
Electrons beyond the group considered does not contribute to
shielding constant
Each of the other electrons in the (ns, np) group contributes S = 0.35
Each of the electrons in the (n-1) shell contributes S = 0.85
Each of the electrons in the (n-2) or lower shells contributes S = 1.00
Consider a particular electron in an nd or nf orbital:
Each of the other electrons in the (nd, nf) group contributes S = 0.35
Each of the electrons in a lower group than the one being
considered contributes S = 1.00

4. Calculation of Z
eff
Calculate Z
effexperienced by (i) 5s electron, (ii) 4d electron in Ag atom
(Z = 47).
Electronic configuration:(1s
2
)(2s
2
2p
6
)(3s
2
3p
6
)(3d
10
)(4s
2
4p
6
)(4d
10
)(5s
1
)
(i) For 5s elecrtron, S = (0x0.35)+(18x0.85)+(28x1.00) = 43.30
Z
eff = 47 –43.3 = 3.7
(ii) For 4d electron, S = (9X0.35)+(36X1.00) = 39.15
Z
eff= 47 –39.15 = 7.85

5. Ionization energy
Definition:An amount of energy required to remove the most loosely
bound electron from an isolated gaseous atom
Factors on which ionization energy depends:
Nuclear charge
Size of atom
Screening effect of the inner electrons
Penetration effect of the electrons
Electronic configuration
Periodic Trends:
IE increases from left to right along a period
IE decreases down a group

6. Anomalies in periodic trend
On going from Group 2 to Group 3, IE decreases
Group 2, valence shell configuration: ns
2
(filled shell, stable)
Group 3, valence shell configuration: ns
2
np
1
Example: Be to B
Mg to Al
On going from group 15 to group16, IE decreases
Group 15, valence shell configuration ns
2
np
3
(half filled, stable)
Group 16, valence shell configuration ns
2
np
4
Example: N to O
P to S

7. Electron Affinity
Definition:An amount of energy released when a neutral atom in
gaseous state acquires an extra electron to form a negatively
charged ion
Factors on which electron affinity depends:
Nuclear charge
Size of atom
Electronic configuration
Periodic Trends:
EA increases from left to right along a period uptohalogen group
EA decreases down a group

8. Anomalies in periodic trend
On going from Group 1 to Group 2, EA decreases
Group 1, valence shell configuration: ns
1
Group 2, valence shell configuration: ns
2
(filled shell, stable)
Example: Li to Be; Na to Mg
On going from group 14 to group15, EA decreases
Group 14, valence shell configuration ns
2
np
2
Group 15, valence shell configuration ns
2
np
3
(half filled, stable)
Example: C to N; Si to P
On going down a Group from 2
nd
period to 3
rd
period, EA
increases
Due to the small size of group 2 elements the incoming electron face
more electron-electron repulsion. So electron affinity is less.
Example: F to Cl; O to S; N to P

9. Electronegativity
Definition:Power of an atom in a molecule to attract electrons to itself
Factors on which electronegativitydepends:
Oxidation state
Hybridization
Charge
Periodic Trends:
Electronegativityincreases from left to right along a period upto
halogen group
Electronegativitydecreases down a group

10. Polarization
What is Polarization?
When a small highly charged cation comes
close to an anion it distort the electron cloud
of the large anion in a manner that it
increases the electron density between the
nuclei. This phenomenon is termed as
polarization.
This increases covalent character in ionic
compounds.

11. Polarizability and Polarizing
Power
Polarizing power: The power of an ion to
distort the other ion is called polarizing power.
Polarizability: The tendency of an anion to
distort is called polarizability.

12. Example of polarization
The large iodide ion by itself is perfectly
symmetrical. However, when a small
positively charged lithium ion comes close to
the iodide ion, the electron cloud on the anion
is pulled towards the positive lithium ion. The
iodide is said to be polarised and the process
is polarization.

13. Fajan’s rule
Small and highly charged cations:
Small highly charged cations have higher
polarizing effects on anions than large, lower
charged cations.
This is because very small cations with higher
charge have higher charge density which
tend to distort the electron cloud around
anion more.

14. Example:
Na
+
andCa
2+
havealmostsimilarionic
radiibutCaCl
2(m.p.=772
0
C)has
highercovalentcharacterthanNaCl
(m.p.=800
0
C).AsCa
2+
hashigher
polarizingpowerCaCl
2hasmore
covalentcharacterandsoitsmelting
pointislessthanmoreioniccompound
NaCl.

15. Fajan’s Rule
Large highly charged anions:
Covalentcharacterinducedinionic
compoundduetopolarizationofthe
anionincreaseswithanincreaseinsize
andchargeontheanion.
Theelectroncloudonthelargesizedanion
islessstronglyheldbyitsnucleusand
henceitiseasilypolarizedbythe
approachingcation.

16. Example:
ForagivencationLi
+
,asthesizeofthe
anionincreasesfromF
-
toI
-
,thecovalent
characterincreasesandconsequently
meltingpointdecreases.
Meltingpointsofcompounds:
LiF=870
0
C,LiCl=613
0
C
LiBr=574
0
C,LiI=446
0
C

17. Fajan’s Rule
Cationconfiguration:
Cationswithns
2
np
6
configurationcause
lessdistortionthanthosewithns
2
np
6
nd
10
configuration.
Thisisbecaused-electronsshieldthe
nucleuspoorlywiththeconsequencethat
suchacationwillexertahighernuclear
chargeontheinteractinganionandwill
causemorepolarization.

18. Example:
Cu
+
(...3s
2
3p
6
3d
10
)andNa
+
(...2s
2
2p
6
)
havesimilarionicradiibutCu
+
ismore
polarizingandinduceshighercovalent
characterthanNa
+
.
Meltingpoints:
CuCl=430
0
C
NaCl=800
0
C

19. Hard Soft Acid Base

20. Usanovich concept of acid:
An acid is a substance that reacts with bases to form
salts, gives up cationsor takes up anions or electrons.
Hard acid:
(i)Cationsof smaller size or lighter elements
(ii)Cationsof higher charge
(iii)Not easily polarizable
(iv)Molecules or ions with lesser no. of valence
electrons.
Example: Alkali metal ions, alkaline earth metal ions, B,
Al, Si ions, lighter transition metal ions such as Ti
4+
,
Cr
3+
, Fe
3+
, Co
2+
etc.

21. Soft Acid
Soft acid:
(i)Cationsof larger size or heavier elements
(ii)Cationsof lower charge
(iii)Easily polarisable
(iv)Molecules or ions with larger no. of valence electrons
Example: Heavy transition metal ions (2
nd
and 3
rd
row)
such as Hg
2+
, Pd
2+
, Cd
2+
etc. and those in lower
oxidation state such as Cu
+
, Ag
+
, Hg
+
.

22. Usanovich concept of Base
According to Usanovich concept ligands
come in the category of bases.
Hard base:
(i) Donor atoms of higher electronegativity
(ii) Donor atoms of low polarisability
Example: Hard bases are molecules or ions that
have donor atoms N, O, F like H
2O, NH
3, F
-
,
ROH.

23. Soft Base
Soft bases have
(i) donor atoms of lower electronegativity
(ii) donor atoms of higher polarisability
Example: Molecules or ions where the donor
atomis P, As, S, Se etc. like R
3P, R
2S, I
-
.

24. HSAB Principle
According to Pearson
Hard acids prefer to bond to hard bases
Soft acids prefer to bond to soft bases
Hard acids will show the following preference of bonding
F
-
> Cl
-
> Br
-
> I
-
R
2O > R
2S
Soft acids will show the opposite trend. They preferentially
bonds with
I
-
> Br
-
> Cl
-
>F
-
R
2S > R
2O

25. Application of HSAB Principle
Stability of complexes
This principle explains the special stability of
complexes formed by hard-hard and soft-soft
interactions.
Example: The complex between a soft acid Ag
+
and a soft base I
-
[AgI
2]
-
is more stable than
one between Ag
+
and hard base F
-
[AgF
2]
-

26. Course of reaction:
Reaction course can be predicted by this
principle. Reaction between LiI and CsF will
always proceed to give LiF and CsI.
LiI + CsF LiF + CsI