Question

How can a molecule with polar bonds be nonpolar? Give an example.

Short answer

Polar bonds can cancel each other out in a symmetrical molecule, like CO₂, making it nonpolar.

Step-by-step solution

Understanding Polar Bonds

Polar bonds occur when there is a difference in electronegativity between two atoms, leading to an unequal sharing of electrons. One atom attracts the bonding electrons more than the other, creating a dipole (positive and negative ends).

Analyzing Molecular Shape

A molecule can have polar bonds but be nonpolar overall if it has a symmetrical shape. The molecular geometry can cause the dipoles to cancel each other out, resulting in no net dipole moment.

Identifying an Example

Carbon dioxide (CO₂) is a classic example. It is a linear molecule with two polar C=O bonds. However, because it is symmetrical, the dipoles cancel each other out.

Calculating the Dipole Moment

In CO₂, the individual dipoles of the C=O bonds are equal in magnitude and opposite in direction, laying on a straight line. Therefore, their vector sum is zero, leaving the molecule with no net dipole moment.

Key concepts

Polar Bonds

In chemistry, a bond is considered polar when two atoms share electrons unequally. This unequal sharing occurs because one atom has a stronger pull on the electron pair than the other. This usually happens between atoms of different elements. The differing abilities to attract electrons lead to the formation of a dipole, a kind of molecular magnet, with one end having a slight positive charge and the other a slight negative charge.
For example, in a water molecule (H₂O), the oxygen atom is more electronegative than the hydrogen atoms. It attracts the shared electrons more strongly, resulting in a partial negative charge on the oxygen and a partial positive charge on the hydrogens. This is what we call a polar bond. Polar bonds are vital as they contribute to the overall properties and behaviors of molecules in various chemical reactions and interactions.

Electronegativity

Electronegativity is a key concept for understanding chemical bonds. It refers to an atom's ability to attract and hold electrons. The higher the electronegativity, the stronger the atom attracts electrons.
Periodic trends show that electronegativity generally increases from left to right across a period and decreases down a group. This means that elements like fluorine, oxygen, and nitrogen are highly electronegative.
Electronegativity differences lead to the formation of polar bonds. When there's a significant difference in the electronegativity between two atoms, electrons spend more time closer to the more electronegative atom. This shift leads to partial charges, essential in polar bond formation. Knowing electronegativity values helps predict if a bond will be polar or nonpolar.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms within a molecule. This geometry plays a crucial role in the overall polarity of a molecule. Even if a molecule contains polar bonds, the spatial arrangement of these bonds can determine the molecule's overall polarity.
Symmetrical shapes, like linear, tetrahedral, or trigonal planar, can lead to the cancellation of dipole moments. This happens because the individual dipoles point in opposite directions and have the same magnitude. For example, carbon dioxide (CO₂) is a linear molecule. Despite having two polar bonds, the molecule's symmetry results in the cancellation of the bond dipoles, making CO₂ nonpolar overall.

Dipole Moment

A dipole moment measures the polarity of a molecule. It's a vector quantity, having both magnitude and direction, representing the separation of charges within the molecule.
The presence of a dipole moment indicates that a molecule is polar, while a dipole moment of zero means that the molecule is nonpolar. This occurs when the polar bonds' dipoles cancel each other out.
In carbon dioxide, the dipole moments of the polar C=O bonds are equal and opposite. Since the molecule is linear, these dipoles act along the same line, cancelling each other and resulting in no overall dipole moment. Understanding dipole moments helps in predicting molecular interactions, solubility, and the reactivity of substances.