CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.3.3 Electronegativity & Bonding

Electronegativity: Predicting Bond Formation

  • The differences in Pauling electronegativity values can be used to predict whether a bond is covalent or ionic in character

Electronegativity & covalent bonds

  • Single covalent bonds are formed by sharing a pair of electrons between two atoms
  • In diatomic molecules the electron density is shared equally between the two atoms
    • Eg. H2, O2 and Cl2
  • Both atoms will have the same electronegativity value and have an equal attraction for the bonding pair of electrons leading to formation of a covalent bond
  • The equal distribution leads to a non-polar molecule

Chemical Bonding Covalent Bonds in Diatomic Molecules, downloadable AS & A Level Chemistry revision notes

The electronegativity values are equal resulting in the formation of a nonpolar covalent bond

Electronegativity & ionic bonds

  • When atoms of different electronegativities form a molecule, the shared electrons are not equally distributed in the bond
  • The more electronegative atom (the atom with the higher value on the Pauling scale) will draw the bonding pair of electrons towards itself
  • A molecule with partial charges forms as a result
  • The more electronegative atom will have a partial negative charge (delta negative, δ)
  • The less electronegative atom will have a partial positive charge (delta positive, δ+)
  • This leads to a polar covalent molecule

Chemical Bonding Polar Covalent Bonds, downloadable AS & A Level Chemistry revision notes

The electronegativity values are not equal – polar bond forms

  • If there is a large difference in electronegativity of the two atoms in a molecule, the least electronegative atom’s electron will transfer to the other atom
  • This in turn leads to an ionic bond – one atom transfers its electron and the other gains that electron
    • The cation is a positively charged species which has lost (an) electron(s)
    • The anion is a negatively charged species which has gained (an) electron(s)

Chemical Bonding Ionic Bonds_1, downloadable AS & A Level Chemistry revision notes

Large differences in electronegativity values lead to the formation of ionic bonds

Exam Tip

You can use the Pauling scale to decide whether a bond is polar or nonpolar:

Chemical Bonding Table_4, downloadable AS & A Level Chemistry revision notes

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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