When atoms combine, they form chemical bonds that hold them together, classified broadly into two main categories: ionic and covalent. Understanding a molecule’s bond type reveals much about its physical properties, such as its melting point, boiling point, and solubility. To determine where a common molecule like carbon dioxide (\(\text{CO}_2\)) fits, we must examine how its atoms interact.
The Fundamental Difference Between Ionic and Covalent Bonds
The distinction between chemical bond types rests on the mechanism of electron interaction between the atoms involved. An ionic bond involves the complete transfer of one or more valence electrons from one atom to another. This typically occurs between a metal and a nonmetal, resulting in the formation of oppositely charged ions. These ions, such as those in table salt (NaCl), are then held together by a strong electrostatic attraction.
In contrast, a covalent bond involves the sharing of electrons between atoms, which usually happens between two nonmetals. Instead of transferring electrons, the atoms pool their valence electrons to achieve a stable configuration. This shared pair of electrons orbits both nuclei, effectively gluing the atoms together.
Electronegativity: The Tool for Classifying Chemical Bonds
Chemists use electronegativity to determine the nature of electron sharing or transfer in a bond. Electronegativity is defined as an atom’s ability to attract a shared pair of electrons to itself when it is part of a compound. This value is assigned to most elements on a scale ranging from approximately 0.7 to 4.0 (Fluorine).
The bond type is determined by calculating the difference in electronegativity (\(\Delta\text{EN}\)) between the two bonded atoms. A difference generally less than 0.4 indicates a nonpolar covalent bond, where electrons are shared almost equally. A moderate difference, typically between 0.4 and 1.7, signifies a polar covalent bond, where sharing is unequal. When the difference is greater than 1.7, the unequal sharing results in a complete transfer, forming an ionic bond.
Why Carbon Dioxide is Covalent (And What Kind of Covalent)
To classify the bonds in carbon dioxide, we must apply the electronegativity tool to the carbon and oxygen atoms. Carbon has an electronegativity value of approximately 2.55, and oxygen is significantly more attractive to electrons with a value of about 3.44. Calculating the difference yields a \(\Delta\text{EN}\) of \(0.89\) (3.44 minus 2.55).
Since the calculated difference of 0.89 falls within the range of 0.4 to 1.7, the individual carbon-oxygen bond is classified as polar covalent, not ionic. This confirms that the atoms in \(\text{CO}_2\) are sharing electrons, which is expected since both are nonmetals. The molecule’s structure consists of one carbon atom double-bonded to two oxygen atoms (\(\text{O=C=O}\)).
In this structure, the shared electrons are pulled closer to the more electronegative oxygen atoms, creating a partial negative charge \((\delta-)\) on each oxygen and a partial positive charge \((\delta+)\) on the central carbon. This charge separation in a bond is known as a bond dipole. However, the overall \(\text{CO}_2\) molecule is nonpolar because of its linear geometry, where the two oxygen atoms are positioned exactly 180 degrees apart from the central carbon atom.
The two C=O bond dipoles are equal in magnitude but point in opposite directions, causing them to precisely cancel each other out. This symmetrical arrangement results in a zero net dipole moment for the entire molecule, classifying carbon dioxide as a nonpolar molecule despite containing polar covalent bonds.