Chapter 3

Uniqueness principleDiagonal effectInert-pair effect

TKT2023 noorshida mohd ali

PERIODIC TRENDS OF THE ELEMENTS

The way certain properties (chemical and physical) of elements vary according to their location on the periodic table

Some variations in:

Effective nuclear charge increases

Eff

ecti

ve n

ucl

ear

char

ge

decr

ease

s

Atomic radius decreases

Ion

ic r

adiu

s in

crea

se

Ionic radius decreases

Ato

mic

rad

ius

incr

ease

Ion

izat

ion

en

erg

y de

crea

se

Ionization energy increases

Electron affinity increases

Ele

ctro

n a

ffin

ity

decr

ease

s

Electronegativity increases

Ele

ctro

neg

ativ

ity

decr

ease

s


Uniqueness principle

The chemistry of the second-period elements (Li, Be, B, C, N, O, F, Ne) are significantly different from

other elements in their respective groups

Li Be B C N O F Ne


Li Be B C N O F Ne


Second element in each group (Na, Mg, Al, Si, P, S, Cl, Ar) are more representative

Na Mg Al Si P S Cl Ar


Li Be B C N O F Ne


Why the first elements of the groups are different from their congeners (elements of the same group)?

a) The small size of the elements leading to a high polarizing power and a high degree of covalent character in their compoundsb) The greater probability of bonds (p-p)c) The lack of availability of the d orbitals


small size of the first elements

• Size effect leads to :

1) Smaller electron affinities - ability to accept one or more electrons

Electrons added to these small atom, experience more electron repulsions

2) Larger charge densities

3) Enhanced degrees of covalent character in their compounds

Electron affinity is the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion

X (g) + ē X-(g)


The first electron affinities of the halogens


What about fluorine?

> It is a very small atom.

> Incoming electron quite close to the nucleus.

> The existing electron density is very high.

> The extra repulsion is particularly great and

lessens the attraction from the nucleus

enough to lower the electron affinity below that

of chlorine.


lithium behaves differently than others

small size

high charge density of cation allow it to polarize nearby anion – allows a large degree of covalency in its bond and less ionic

less soluble in water and more soluble in polar organic solvents

GROUP 1 (ALKALINE METALS) : Li, Na, K, Rb, Cs, Fr


1) Small lithium cation get very close to larger, more diffuse or filled electron cloud of chloride ion.

2) Electron cloud of chloride ion is distorted or polarized by lithium ion

3) This distortion makes overlap between two ions. Orbital overlap and sharing electron between two species – characteristic of a covalent bond overlap between the valence orbitals in Li+ (empty 2s) and Cl- (filled 3p) is increased


Weakness of the fluorine – fluorine bond

-owing to the small atomic size – closeness/ nearness in F2 compared to Cl2

-lone pairs of electrons on adjacent fluorine atoms repel each other (increase repulsion )

-weakening the F-F bond in F2 molecule

Bond energies increase TKT2023 noorshida mohd ali

increased bonding in the first element• Due to small size, increase bond formation among themselves and with other elements (capable of forming strong double and triple bond)

bonds involve parallel overlap between for example two p orbitals ( bonding can occur using d orbitals and antibonding molecular orbitals of some molecules) • Utilizing bond than bond :

• Parallel orbital overlap or bonding is more effective in smaller first elements for example carbon than larger congeners, silicon

C C C O N N C C O O C O C C


lack of availability of d orbitals in the first elements

• The lighter elements lack availability of d orbitals and therefore cannot form compounds with expanded octets

CF4ClF3

SiF62-


• Similarities between pairs of elements in different groups and periods of the periodic table

Phenomenon happens :

1) small ionic size

2) closeness of the charge densities of their cations (charge density = charge of an ion divided by its radius : charge nm-1)

3) electronegativity

Diagonal effect

Li Be

Mg

B

Al Si

1 2 13 14

DIAGONAL RELATIONSHIPS exists between the chemistry of the first member of a group and the second member of the next group


Li Be

Mg

B

Al Si

1 2 13 14

Charge of ionIonic radius, ÅCharge densityElectronegativity

Charge of ionIonic radius, ÅCharge densityElectronegativity

+10.731.41.0

+20.414.91.5

+11.130.880.9

Na

C

+20.712.81.2

+30.25122.0

+30.535.71.5

+40.29142.5

+40.40101.8

Diagonal effect TKT2023 noorshida mohd ali

•High charge density of Al3+ ion (5.7) and Be2+ ion (4.9)

•Same electronegativity (1.5)

•Be-X and Al-X : covalent character

•Its small size & high charge density of cation Be2+ and Al3+ allow them to polarize the electron cloud of anion (X atom in M-X bond) to give additional covalent character

AlH3 resembles BeH2 in its properties (example of the diagonal relationship)

Li Be

Mg

B

Al Si

1 2 13 14

4.9

2.8 5.7

Diagonal effect •Ionic radius of Be2+ (0.41Å) is more similar to Al3+ (0.53Å) than Mg2+ (0.71Å)

0.41Å

0.71Å 0.53Å


Oxidation state : +4 CCl4, CO2, SiCl4, SiO2, SnO2

CO easily oxidized to CO2 (+2 to +4)

Convert tin(II) to tin(IV) – more stable. Sn2+ ions in solution suitable as reducing agents

Reversed situation for lead

C Si Ge Sn Pb

+4

Inert-pair effect

C Si Ge Sn Pb

+2Moving down a group, there are more and more examples oxidation state +2 : CO, SnCl2, PbO, Pb2+


Ion

izat

ion

en

erg

y de

crea

se Ionization energy increases

ATOMIC RADIUSThe increase in size of atom is accompanied by a decrease of ionization energy

-going down a group, size of the atom increases -the outer electrons lie farther away from the nucleus -attractive pull from the nucleus on the outer electrons decreases -easier to pull out an electron from the outer shell of the atom

Inert-pair effect


IONIZATION ENERGY

Slight increase between tin and lead

Large increase between tin and lead


Inert-pair effect

• All four elements (Al, Ga, In, Tl) give trivalent compounds but the univalent state becomes increasingly important for Ga, In and Tl

• Valence ns2 electrons of metallic elements : In, Tl, Sn, Pb, Sb, Bi and Po

are less reactive than expected.

• Inert ns2 pairs mean oxidation state is 2 less than the expected group valence for the heavier elements of groups 13, 14, 15 and 16.

As an example in group 13 the +1 oxidation state of Tl is the most stable and TlIII compounds comparatively rare. The stability increases in the following sequence:

AlI < GaI < InI < TlI


http://en.wikipedia.org/wiki/Group_13_element





Inert-pair effect

• Valence electrons in an s orbital are more tightly bound are of higher energy than electrons in p orbitals and therefore less likely to be involved in bonding.

• Get closer to the bottom of the group (heavier elements – increasing tendency s2 pair not to be used in the bonding (left unchanged / inert pair)

• Electrons closer to the nucleus – difficult to remove the heavier the element the greater this effect


Inert-pair effect

• Two major reasons for this effect:

a) larger than normal effective nuclear charges (higher than expected ionization energies for Ga, In, Tl)

4s, 5s and 6s electrons experience larger effective nuclear charge than expected – they are more difficult to ionize

b) lower bond energies (as expected)

due to increase in atomic size and bond distance


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Chapter 3