Chemical periodicity part 3

Learning Outcome : Interpret the variation in melting point and electrical conductivity in terms of presence of simple molecular ,giant molecular and metallic bonding in elements

Bonding and structure of elements
Ø Metallic Bonds
Ø Giant Metallic Structure
>Na, Mg, Al
Ø Covalent bonds
Ø Giant covalent structure
>Si
Ø Intramolecular : Covalent bonds
Ø Intermolecular: Van Der Waals forces
Ø Simple molecular structure
>P, S, Cl

Trend of Melting Point
· Na, Mg ,Al
- High Melting Point
Since it is a :
>Giant Metallic structure
» High energy is required to overcome the strong attractive forces between the metal cations and delocalized electrons

>Melting point: Na< MgBecause:
-decrease in cation radius
- increase in no of delocalized electrons
»increase in charge/size ratio, increase in the metallic bond strength

· Si
-Has the highest melting point
>Giant molecular structure
»large amount of energy required to break many strong covalent bonds in the giant molecular structure

· P, S, Cl
-Low melting point
>exists as simple molecular structures
>Intermolecular forces : Van der Waals forces
>Low thermal energy is required to overcome the weak Van der Waals forces
»The melting point of these elements is dependant on the size the electron cloud
»The higher the molecular mass(Mr), the larger the electron cloud,the stronger the VDW forces. Hence,the higher the melting point.

Trend of Electrical Conductitvity
· Na, Mg, Al
-Good electrical conductors
»they are metals with delocalized electrons
-Degree of electrical conductivity :
Al >Mg >Na
»As more valence electrons are contributed to the ‘sea of electrons’,electrical conductivity increases
· Si
-Low electrical conductivity under ordinary conditions
»Conductivity increases with an increase in temperature
»At a higher temperature ,electrons are excited to higher energy levels. Silicon is a semi- conductor .
· P ,S, Cl
- Non-conductor of electricity because absence of delocalized electrons or mobile ions.