Chemical bonds are the fundamental forces that hold atoms together, forming molecules. These bonds arise from how atoms interact with their electrons. While atoms often share electrons for stability, this sharing is not always equal, leading to bond polarity. This unequal electron distribution forms the basis for many properties observed in different substances.
Understanding Electronegativity
Electronegativity is an atom’s ability to attract shared electrons towards itself within a chemical bond. Different elements possess varying electronegativity values, which quantify this electron-attracting power.
Electronegativity values exhibit predictable trends across the periodic table. Electronegativity increases from left to right across a period, due to an increasing nuclear charge that pulls electrons more strongly. It decreases down a group because atomic size increases, placing the valence electrons further from the nucleus and reducing the nuclear attraction. Fluorine, located in the upper right of the periodic table, is the most electronegative element, while cesium and francium, in the bottom left, are among the least electronegative. The difference in electronegativity between two bonded atoms is a key factor in determining the nature of their bond.
Classifying Bond Types
Chemical bonds are classified into distinct types based on the degree of electron sharing or transfer, primarily determined by the electronegativity difference. A nonpolar covalent bond forms when electrons are shared equally between two atoms. This occurs when the bonded atoms are identical or have a very small difference in their electronegativity. For instance, in diatomic molecules like oxygen (O₂) or chlorine (Cl₂), the electrons are shared equally.
When there is a moderate difference in electronegativity between two bonded atoms, a polar covalent bond forms. Electrons are shared unequally, being drawn closer to the more electronegative atom. This unequal distribution creates partial positive (δ+) and partial negative (δ-) charges on the respective atoms, establishing a dipole moment within the bond. The atom with the higher electronegativity acquires the partial negative charge, while the less electronegative atom becomes partially positive.
In cases where the electronegativity difference between atoms is very large, a complete transfer of electrons can occur, leading to an ionic bond. This usually happens between a metal and a nonmetal. The atom that loses electrons becomes a positively charged ion (cation), and the atom that gains electrons becomes a negatively charged ion (anion). These oppositely charged ions are then held together by strong electrostatic attraction.
Distinguishing Molecular Polarity
Understanding bond polarity is important, but it is crucial to distinguish it from overall molecular polarity. A molecule may contain individual polar bonds, but its overall polarity depends on both these bonds and its three-dimensional geometry.
If a molecule contains polar bonds, but its shape is symmetrical, the individual bond dipoles can cancel each other out. This results in a net dipole moment of zero, making the overall molecule nonpolar despite having polar bonds. For example, carbon dioxide (CO₂) has polar carbon-oxygen bonds, but its linear geometry causes the bond dipoles to oppose each other perfectly, resulting in a nonpolar molecule.
If a molecule contains polar bonds and its geometry is asymmetrical, the bond dipoles do not cancel. The molecule will then have a net dipole moment, making it a polar molecule. Water (H₂O) provides a common example; it has polar oxygen-hydrogen bonds and a bent molecular shape, which prevents the bond dipoles from canceling, leading to an overall polar molecule.
Impact on Substance Properties
Molecular polarity significantly influences the physical and chemical properties of substances. One notable effect is on solubility, often summarized by the principle “like dissolves like.” Polar substances dissolve well in polar solvents, such as water, because their partial charges allow for strong attractive interactions. Similarly, nonpolar substances dissolve in nonpolar solvents. Polar and nonpolar substances generally do not mix, like oil and water, as the lack of compatible interactions prevents them from forming a solution.
Molecular polarity also affects melting and boiling points. Polar molecules have higher melting and boiling points than nonpolar molecules of similar size. This is because polar molecules exhibit stronger intermolecular forces, such as dipole-dipole interactions, due to their partial positive and negative ends. More energy is required to overcome these stronger attractive forces to transition from a solid to a liquid, or from a liquid to a gas.