In a non-polar covalent molecule, the electrons are localized and not mobile. This is because the electronegativity of the atoms involved is similar, so the electrons are shared equally between the atoms. This creates a strong bond between the atoms, and the electrons are not able to move freely.

In contrast, in a polar covalent molecule, the electrons are delocalized and mobile. This is because the electronegativity of the atoms involved is different, so the electrons are not shared equally between the atoms. This creates a weaker bond between the atoms, and the electrons are able to move more freely.