1.7.4 Electronegativity

• Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself
• The electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical
• This phenomenon arises from the ability of the positive nucleus to attract the negatively charged electrons, in the outer shells, towards itself
• The Pauling scale is used to assign a value of electronegativity for each atom

First three rows of the periodic table showing electronegativity values

• Fluorine is the most electronegative atom on the Periodic Table, with a value of 4.0 on the Pauling Scale
• It is best at attracting electrons towards itself when covalently bonded to another atom

Electron distribution in the C-F bond of fluoromethane

Ionic and covalent bonding

• Elements with large differences in electronegativity tend to form ionic bonds
• Atoms of elements with similar electronegativity tend to form covalent bonds
• Intermediate differences in electronegativity between covalently bonded atoms lead to polarity in the bond
  • As a rule, an electronegativity difference of 2 or more on the Pauling scale between atoms leads to the formation of an ionic bond
  • A difference of less than 2 between atoms leads to covalent bond formation

• When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar

The two chlorine atoms have the same electronegativities so the bonding electrons are shared equally between the two atoms

• The difference in electronegativities will dictate the type of bond that is formed
• When the electronegativities are very different (difference of more than 1.7) then ions will be formed and the bond will be ionic
• When two atoms in a covalent bond have a difference in electronegativities of 0.3 to 1.7 a covalent bond is formed and the bond will be polar
  • The electrons will be drawn towards the more electronegative atom
• As a result of this: 
  • The negative charge centre and positive charge centre do not coincide with each other
  • This means that the electron distribution is asymmetric
  • The less electronegative atom gets a partial charge of δ+ (delta positive)
  • The more electronegative atom gets a partial charge of δ- (delta negative)

• The greater the difference in electronegativity the more polar the bond becomes

Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive

Dipole moment

• The dipole moment is a measure of how polar a bond is
• The direction of the dipole moment is shown by the following sign in which the arrow points to the partially negatively charged end of the dipole:

 

The sign shows the direction of the dipole moment and the arrow points to the delta negative end of the dipole

Assigning polarity to molecules

• To determine whether a molecule with more than two different atoms is polar, the following things have to be taken into consideration:
  • The polarity of each bond
  • How the bonds are arranged in the molecule

• Some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such way that the individual dipole moments cancel each other out

There are four polar covalent bonds in CH₃Cl which do not cancel each other out causing CH₃Cl to be a polar molecule; the overall dipole is towards the electronegative chlorine atom

Though CCl₄ has four polar covalent bonds, the individual dipole moments cancel each other out causing CCl₄ to be a nonpolar molecule