Electronegativity and polarity

Polar and non-polar bonds:

1)  Non-Polar bonds:

  • As the 2 atoms are identical, they will have the same number of protons.

  • This means that the electrons are being 'pulled' equally by both of the hydrogen atoms.

  • We say that the H - H bond is non-polar

  • The same is true for any diatomic molecule where the atoms are identical:



2)  Polar bonds:

Hydrogen chloride:



                        is used to mean ' a little bit of '

                    Di   -   ' 2 '

                    Pole    -    poles (positive and negative)

Polar molecules:



How is electronegativity measured?

Pauling definition of electronegativity:-

                 The electronegativity of an atom represents the power of an atom in a molecule to attract electrons to itself.

Electronegativity and bonding type    Explanation

1)    Covalent


2)    Polar covalent


3)    Ionic


Covalent to ionic:

Polarity Qu

Questions 1-2 P61

Intermolecular forces

Strengths of bonds and forces:

Ionic - When ionic compounds melt/boil, the forces of attraction are overcome and the ions separate.

NaCl(s)  Na+(g) + Cl-(g)


Molecular - When molecular compounds melt/boil, the covalent bonds remain intact.    animation

H2O(l) H2O(g)


    1)    Van der Waals' forces

    2)    Permanent dipole - dipole forces

    3)    Hydrogen bonding

Permanent dipole - dipole interactions:

Van der Waals' forces (induced dipole - dipole interactions)

What causes Van der Waals' forces:



  • Imagine an atom to be like a large spherical jelly with a golf ball at the center.

  • The golf ball is the nucleus, the jelly is the cloud of electrons whizzing about this.

  • The net average field will be zero because the (+)ve nucleus field will be exactly balanced by the electron cloud.

  • Atoms vibrate, at any instant the cloud is likely to be slightly off center.  This creates an instantaneous dipole.

  • If we have another atom next to it, this atom will be affected by the instantaneous dipole.

  • This will induce a dipole in the neighbouring atom.

  • The 2 dipoles attract one another – producing an attractive interaction.

            Hydrocarbons        Diatomic elements



Polarity Qu

Questions 1-2 P63

Hydrogen Bonding:

Water is peculiar

Why are hydrogen bonds so strong?

  • Maximum bond strength is when the bond angle O-H-O is 180o.

  • The strength of a hydrogen bond is typically ~30Kj.

  • Compare this with the strength of a covalent bond ~300Kj.  A Hydrogen bond is ~1/10th a covalent bond.

  • Similarly with VDW attraction ~3Kj.



1)    Within a molecule, the hydrogen must be highly polarised (very positive)


2)    Within a molecule, the atom joined to the hydrogen must be very electronegative, O,N,F.


3)    Within a molecule, the atom joined to the hydrogen must also have a lone pair of electrons.

Ice is less dense than water:




Hydrogen bonding in biological molecules:




Questions P65  1-2 / P73  6,8P74  2,5


Metallic bonding and structure



  • The model consists of metal ions surrounded by ‘Mobile sea of electrons’.

  • Attraction occurs between the ions and the delocalised electrons

  • The sea of electrons explains its electrical conductivity and its thermal conductivity.

  • The sea of electrons bonds the metal ions tightly into the lattice.

  • This explains its high melting point.

  • Since strong forces of attraction exists even in the liquid phase, metals tend to have a wide temperature range over which they remain liquid.

  • The giant metallic lattice is often referred to as


Properties of giant metallic lattices:

1)  High melting and boiling points -

2)  Good electrical conductors -

  • Mobile electrons will be attracted to a positive terminal.
  • Electrons are replaced from the negative terminal.

3)  Malleability and ductile -


Q1-3  P 67

Structure of ionic compounds

Giant ionic lattices

  • Each sodium ion is surrounded by 6 chloride ions.

  • Each chloride ion is surrounded by 6 sodium ions.

  • This continues in all directions and is describes as a Giant Ionic Lattiice


Properties of ionic compounds


1)  High melting and boiling points:

2)  Electrical conductivity:

Video clip


Where does the energy come from?


Video clip    Video clip

Qu 1-3  P69

Structures of covalent compounds

A)    Simple molecular lattice

B)    Giant covalent lattice

A)    Simple molecular structures:


Properties of simple molecular structures:

1)    Low melting and boiling points:


2)    Electrical conductivity:

3)    Solubility:

B)    Giant covalent structures

Diamond Graphite


Properties of giant covalent structures

1)    High melting and boiling points:

2)    Electrical conductivity:

3)    Solubility:

4)    Hardness:



Practical        Fill in


Bond & St


Review all


Questions 1-2 P71