Study notes for Unit 2 Topic 4 - Electronegativity and Polarisation for Chemistry Edexcel A Level 2015. Focusing on electronegativity and its trends, polar vs non-polar molecules, how dipoles arise, and what makes a compound covalent or ionic.
Chemistry Notes Topic 2 - Electronegativity and Polarisation
Electronegativity:
● The ability of an atom to attract the bonding electrons in a covalent bond.
● Usually measured using the Pauling scale - the higher the value, the more
electronegative the substance.
● More electronegative substances have higher nuclear charges (more protons) and
smaller atomic radii, therefore electronegativity increases over periods, and
decreases down the group.
Differences in electronegativity and bond polarity:
(Polar = a structure which has an uneven distribution of electron density. Non-polar =
a structure in which electron density is more or less evenly distributed is called
nonpolar).
● The difference in electronegativity changes where the electrons sit between the two
nuclei in a covalent bond.
● If the atoms have equal/very similar electronegativities (for example covalent bonds
in diatomic gases) then the electrons are equally attracted to both nuclei so sit
roughly midway between them, and the covalent bonds are non-polar.
● So, only bonds between atoms of a single element can be purely covalent as they
have an electronegativity difference of zero.
● If the bond is between two atoms with different electronegativities, then the bonding
electrons will be pulled towards the more electronegative atom. This causes the
electrons to be spread unevenly, so there will be a charge across the bond and this
bond is said to be polar.
● In a polar bond, the difference between the two electronegativities causes a dipole. A
dipole is a difference in charge between the two atoms caused by a shift in
electron density in the bond.
● The greater the electronegativity difference = the greater the shift in electron density
= the more polar the bond.
For example, HCl is polar. H and Cl have different
electronegativities. Chlorine is more electronegative
and so pulls the electrons towards it. This causes
uneven distribution of electrons and the polar bond
gives the whole molecule a permanent dipole.
Covalent and ionic bonds:
● Like few compounds are purely covalent, very few compounds are completely ionic.
● In reality most compounds are between the two extremes - they usually have both
ionic and covalent properties.
● You therefore use electronegativity difference to predict what type of bonding will
occur.
● The higher the difference in electronegativity, the more ionic in character the
bonding becomes.
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