Electronegativity: The ability of an atom to attract electrons in a covalent bond.
• Periodic Table:
– Across the periodic table, the nuclear charge increases and the atomic radius decreases.
– Electronegativity increase up the periodic table and across the periodic table.
– Therefore fluorine in the top most right hand corner is most electronegative.
– Potassium in bottom most left hand corner is least electronegative.
– Noble gases not included as they don’t tend to form compounds.
• Pauling Scale: The larger the Pauling electronegativity values, the more electronegative.
• Bond Types: Can work out difference between the electronegative values of two elements to work
out the bond type. 0 electronegativity difference is covalent. 0 to 1.8 is polar covalent. 1.8+ is
ionic. Means there is a spectrum of bonding with polar covalent in the middle.
• Non- Polar Bond/ Covalent: The bonded electron pair is shared equally between bonded atoms.
For example when the same element or the bonded atoms have similar electronegativity e.g. C
and H form non- polar bonds.
• Polar Covalent Bond: It is a bond with a permanent dipole, having positive (δ+) and negative (δ-)
partial charges on the bonded atoms. The more electronegative atom, δ- atom, will have a greater
attraction for the bonded pair of electrons between the two atom.
• Permanent Dipole: It is a separation in electrical charge so that one atom of a polar covalent bond
has a small positive charge and the other atom has a small negative charge.
• Polar Molecule: It is a molecule with an overall dipole. One atom must be more electronegative
than another to be polar bond, before being polar molecule.
Water is a polar molecule as…
– O is more electronegative than H or there is a difference in electronegativity between O and
H so OH bonds are polar (have a permanent dipole).
– The molecule is –state shape- so unsymmetrical (as has lone pairs) so two dipoles do not
cancel out.
– Overall the oxygen end of molecule has δ- charge and hydrogen end has δ+ charge.
• CO2 is a non-polar molecule although it has polar bonds…
– O is more electronegative than C or there is a difference in electronegativity between O and
C so C=O bonds are polar (have permanent dipole).
– The molecule is- state shape- so symmetrical so the two dipoles act in opposite directions and
the dipoles cancel out.
• All alkanes/ alkenes non- polar.
• H2S and SF6 both have polar bonds. However only H2S polar molecule as unsymmetrical.