Nonpolar Covalent Bonds:

* Electrons are shared equally: The atoms involved in the bond have similar electronegativity (the ability to attract electrons). This means neither atom pulls the shared electrons closer to itself.

* No dipole moment: Since the electrons are distributed evenly, there's no separation of charge, and therefore no dipole moment (a measure of the separation of positive and negative charges).

Polar Covalent Bonds:

* Electrons are shared unequally: The atoms involved in the bond have different electronegativities. The atom with higher electronegativity pulls the shared electrons closer, creating a partial negative charge on that atom and a partial positive charge on the other atom.

* Dipole moment: The uneven distribution of electrons creates a separation of charge, resulting in a dipole moment. This means the molecule has a positive end and a negative end.

Examples:

* Nonpolar: A molecule of oxygen (O2) has a nonpolar covalent bond because the two oxygen atoms have the same electronegativity.

* Polar: A molecule of water (H2O) has polar covalent bonds because oxygen is more electronegative than hydrogen, creating a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.

Key takeaway: The equal or unequal sharing of electrons in a covalent bond determines whether it's nonpolar or polar, with significant consequences for the molecule's overall properties.