INORGANIC CHEMISTRY (AS)

The Periodic table

Groups II , VII , V and VI

Transition elements

THE PERIODIC TABLE (AS)

Trends in physical and chemical properties

Across a period from left to right

Periodic table

1869 : proposed by Mendeleev

63 elements arranged according to increasing atomic mass

1913 : modern periodic table proposed by Moseley

108/109 elements arranged according to increasing atomic number

Structure of the Table

Elements are arranged according to their atomic no.

Period = the horizontal row

Group = the vertical column

Divided into sections:

s-block (Groups I and II) : outermost subshell s

p-block (Groups III to VIII):outermost subshell p

d-block with transition elements : partially filled d subshell

Diagonal relationship

First element in any group usually shows unusual behaviour unlike the rest of the elements in the same group

More similar to the second element in the neighbouring group

Example :

Group I II III

Li Be B

Na Mg Al

Rxn Typical Group I element

Li

(Group I)

Mg

(Group II)

Heat carbonate

No reaction

Li2O + CO2 MgO + CO2

Heat nitrate

MNO2 + O2 Li2O + O2 + NO2

MgO + O2 + NO2

Trends in physical properties ( Physical periodicity )

1. Atomic and ionic radii

a. Atomic radius :

i) Size of atoms = atomic (covalent) radii

= half internuclear distance between 2 atoms of the same element joined by a single bond

Eg Cl - Cl

x

Size of Cl = x/2

Note :atomic radius for other elements

(1)Metals metallic radius of the atom in the metal

(2)Noble gases -atomic/ VDW radius is half the average distance between adjacent non-bonded atoms -unusually large value

ii) Atomic size decreases from left to right due to increase in nuclear charge

iii) Decrease in size becomes smaller with increasing atomic number due to increased repulsion between electrons

Eg Li Be B C

Radius 0.123 0.089 0.080 0.077

Decrease 0.034 0.009 0.003

Period 2

Atomic number

Atomic

radius

Li

Be

F

Period 3 ( data from Data Booklet )

Atomic number

Atomic

radius

Ar

Unusually large value

(VDW radius)

b. Ionic radius

i) Positive ions :

Atom - electrons Positive ion

(p = e) ( p > e)

In ion , nuclear charge has greater pull on remaining electrons

Or sometimes positive ion has one shell less than atom

Size of positive ion smaller than neutral atom

ii) Negative ions :

Atom + electrons Negative ion

(p = e) ( p e)

In ion , nuclear charge has weaker pull on the electrons

Also increased repulsion between electrons

Size of negative ion bigger than neutral atom

c. Eg : Period 3

i) Positive ions Na+ Mg2+ Al3+ Si4+

Ionic radius 0.095 0.065 0.050 0.041

( nm)

All ions are isoelectronic (10 e ), [Ne]

From Na to Si, nuclear charge increases

Attractive force on outer electrons increases

Decrease in ionic radii

ii)Negative ions P3- S2- Cl-

Ionic radius/nm 0.212 0.184 0.181

All ions are isoelectronic ,18 e , [Ar]

From P to Cl, nuclear charge increases

Attractive force on outer electrons increases

Decrease in ionic radii

iii)Radii of negative ions are larger than that of positive ions

Reasons :

Extra shell of electrons in negative ions

( 18 e vs 10 e ) (*)

Increased repulsion between larger no of electrons

iv)Graph

Atomic

number

Ionic

radius

Na+ Si4+ P3- Cl-

2. Melting* and boiling points :

Depends on strength of the bonds and on the structure in the solid state

Eg Period 3 ( Na to Ar )

Na Mg Al Si P S Cl Ar

Giant metallic structure

Simple molecular structure

Giant molecular structure

a. Na , Mg , Al :

Giant metallic structure

Strong metallic bonds high m.p

M.p increases from Na to Al as metallic bonds becomes stronger

Reasons : Increasing no of delocalised electrons (*)

Increasing nuclear charge

Decreasing atomic radius

b. Si :

Giant molecular structure

Numerous strong covalent bonds present

Highest melting point

c. P to Ar :

Simple molecular structure

Weak VDW forces low m.p

Strength of VDW forces no of electrons

P4 S8 Cl2 Ar

no of electrons most least

VDW force strongest weakest

m.p / 0C 44 119 -101 -189

highest lowest

Note:

Structure of P4 : tetrahedral

Sulphur, S8

Atomic number

Melting

Point / 0 C

Na

Mg Al

Si

P

S

Cl Ar

3. Electrical conductivity : In period 3 (from Na to Ar) , elements

change from metal to non metal a. Na , Mg , Al : Metals with metallic structures consisting of

cations in a sea of delocalised electrons Mobile electrons Good conductors/high

conductivity Conductivity increases as no of delocalised

electrons increases from Na to Al

b. Si :

Semi metal , semi conductor / moderate conductivity

c. P to Ar :

Non metals , exists as simple discrete molecules

All electrons paired in covalent bonds

No free electrons or ions non conductors/ low conductivity

Electrical conductivity

Atomic number

Na

Al

Si

4. Ionisation energy ( first I.E ) :

a. Generally , the I.E of elements increase across a period due to

i) increase in nuclear charge (*)

ii)slight decrease in atomic radius

Note : However there are anomalies :

(1)between elements in Groups II and III , and

(2)between elements in Groups V and VI

b. In any period , the elements in Group VIII ( inert or noble gases ) being stable have the highest first I.E

Atomic number

First I.E

Na

Mg

Al

Si

P

S

Cl

Ar

5. Electronegativity :

a. Definition : electronegativity of an element is a measure of its attraction for bonding electrons

b. Electronegativities increases from left to right across a period due to

increase in nuclear charge

c. Electronegativity decreases down a group

due to increase in shielding effect

Changes in Physical Properties of Elements in a Period

Property Na Mg Al Si P S Cl Ar

Proton # 11 12 13 14 15 16 17 18

Ionisation energy (kJ/mol)

500 740 580 790 1010 1000 1260 1520

Covalent bond radius (nm)

0.156

0.136 0.125 0.117 0.110 0.104 0.099 0.192

Electronegativity 1.0 1.25 1.45 1.74 .05 2.45 2.85

Structure GiantMetal

-lic

Giant molec-ular

Simple

Molec-ular

Density (gcm-3) 0.97 1.74 2.70 2.33 1.82 2.07 gas Gas

Melting pt (oC)

Boiling pt (oC)

98

low

890

651

high

1117

660

high

2447

1410

high

2355

44

low

280

119

low

445

-101

low

-35

-189

low

-186

Electrical conductivity

high high high moderate

low low low low