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A polar bond occurs when electrons are shared unequally between two atoms, leading to an unequal distribution of electron density. This happens when atoms involved in the bond have different electronegativity. Electronegativity is the measure of an atom's ability to attract electrons. The greater the difference in electronegativity between the two atoms involved in a bond, the more polar the bond becomes. For example, in a water molecule (H2O), oxygen (O) is more electronegative than hydrogen (H). So, the electrons in the O-H bonds are attracted more towards the oxygen atom, leading to a polar bond with an unequal distribution of electron density.

A molecule can be classified as either polar or non-polar based on the presence of polar bonds and the overall distribution of electron density within the molecule. A polar molecule has polar bonds and an asymmetrical arrangement of atoms, which results in an uneven distribution of electron density. This causes the molecule to have a net dipole moment - a measure of the overall polarity of the molecule. On the other hand, a non-polar molecule can have either non-polar bonds or polar bonds, but the key difference is that it must have a symmetrical arrangement of atoms, resulting in a balanced distribution of electron density. This leads to no net dipole moment in the molecule.

Example 1: Water (H2O) is a polar molecule. It has two polar O-H bonds, and the molecule has a bent shape, resulting in an uneven distribution of electron density and a net dipole moment. Example 2: Carbon dioxide (CO2) is a non-polar molecule. It has two polar C=O bonds, but the molecule is linear and symmetrical, leading to a cancellation of the bond polarities and resulting in no net dipole moment. In summary, a polar bond is caused by an unequal sharing of electrons between two atoms with different electronegativities, while a polar molecule is a molecule with polar bonds and an asymmetrical arrangement of atoms, leading to an overall net dipole moment. A non-polar molecule can have polar bonds, but its symmetrical arrangement results in a cancellation of the bond polarities, leading to no net dipole moment.