CIE A Level Chemistry复习笔记1.3.3 Electronegativity & Bonding

• 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

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

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)

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