Electronegativity and polarity

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Electronegativity and polarity

Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
A permanent dipole is a small charge difference across a bond that results a difference in the electronegativities of the bonded atoms.
A polar molecule has an overall dipole, when you take into account any dipoles across the bonds.

Polar and non-polar bonds
In a molecule of hydrogen, H2, the two bonding atoms are identical. Each hydrogen atom has an equal share of the pair of electrons in the bond, resulting in a perfect 100% covalent bond.
The nucleus of each bonded atom is attracted to the bonding electron pair.
We say that a H-H bond is non-polar - the electrons in the bond are evenly distributed between the atoms that make up the bond.

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Electronegativity and polarity

If the bonding atoms are different, one of the atoms is likely to attract the bonding electrons more. The bonding atom with a greater attraction for the electron pair is said to be more electronegative than the other atom.  
In a molecule of HCl, the two bonding atoms are different. 

  • The Cl atom is more electronegative than the H atom. 
  • The Cl atom has a greater attraction for the bonding pair of electrons than the H atom.
  • The bonding electrons are closer to the Cl atom than the H atom 

There is now a small charge difference across the H-Cl bond. 
The charge difference is called a permanent dipole and is shown by:

  • a small positive charge on the H atom, delta positive +
  • a small negative charge on the Cl atom, delta negative - 

We now have a polar covalent bond.

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Electronegativity and polarity

Electronegativity measures the attraction of a bonded atom for the pair of electrons in a covalent bond. 

  • As we go up the periodic table, electronegativity increases, atoms get smaller and there is less shielding by electons in inner shells.
  • As we go across the periodic table, electronegtivity increases. Nuclear charge increases but the shielding stays the same, the number of inner main levels remain the same and the atoms become smaller. Fluorine being the most electronegative atom.
  • Electronegativity decreases down a group as the number of shells increases, so shielding increases and the electrons are less strongly attracted to the atom.

Electronegativity and bonding type

  • For a bond between atoms of two different elements with a small difference in electronegativity, the more electronegative atom will have slightly more than its fair shair of the bonded electrons.
  • The result is a polar covalent bond.
  • For a bond between atoms of two different elements with a large difference in electronegativty, the more electronegative atom will effectively have captured both the bonding electrons.
  • The result is an ionic bond
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