Chemical bonds hold atoms together to form molecules and compounds. Atoms combine to achieve stability, often by arranging their valence electrons into stable configurations. For non-metal atoms, this is achieved through covalent bonding, where electrons are mutually shared. The distribution of this shared electron density determines the properties of the resulting molecule and is key to understanding when electrons are shared equally.
Defining Nonpolar Covalent Bonds
Electrons are shared equally between two atoms in a specific type of chemical linkage called a nonpolar covalent bond. This equal distribution means the shared electron cloud is symmetrical, sitting perfectly centered between the two atomic nuclei.
Because the electron density is evenly spread, the bond does not develop any separation of charge across its length. The atoms involved in the bond maintain an electrically neutral state, meaning there are no distinct positive or negative ends. This lack of charge separation is the reason the bond is described as “nonpolar.”
The purest examples of equal sharing occur in diatomic molecules, where two identical atoms bond together. In a molecule like elemental hydrogen (\(\text{H}_2\)), oxygen (\(\text{O}_2\)), or nitrogen (\(\text{N}_2\)), both atoms have the exact same pull on the shared electrons. This perfect symmetry guarantees the bonding electrons are shared with complete equality, resulting in a bond that is entirely nonpolar.
Electronegativity: The Determining Factor
The property that dictates the distribution of shared electrons is called electronegativity, which is a measure of an atom’s inherent attraction for electrons within a chemical bond. Every atom attempts to pull the shared electron pair toward its own nucleus with a specific strength. This attraction is influenced by factors like the atom’s nuclear charge and the distance of the valence electrons from the nucleus.
To determine if electrons are shared equally, chemists calculate the difference in electronegativity values between the two bonded atoms. The Pauling scale is the most common system used to assign a numerical value to this attractive force.
Equal sharing is guaranteed only when the difference in electronegativity between the two atoms is precisely zero. This condition is met when two atoms of the same element bond, such as two chlorine atoms in a \(\text{Cl}_2\) molecule. If the numerical difference is zero, the electron cloud is perfectly symmetrical, creating a pure nonpolar covalent bond.
In practice, a bond is still categorized as nonpolar covalent even if there is a tiny, measurable difference in electron attraction. Most chemists consider a bond to be nonpolar if the electronegativity difference is very small, typically less than 0.4. For instance, the carbon-hydrogen (\(\text{C}-\text{H}\)) bond has a difference of about 0.4, making it nearly nonpolar and a common feature in many organic compounds.
The Spectrum of Bonding: Unequal Sharing and Transfer
Chemical bonding exists on a continuum, with equal electron sharing representing just one extreme. Moving away from the condition of equal attraction, the sharing of electrons becomes increasingly unequal, leading to other bond types. This shift is directly proportional to the increasing difference in the electronegativity values between the two bonded atoms.
When the electronegativity difference is moderate, generally between 0.5 and 1.7, the bond is classified as a polar covalent bond. Here, the more attractive atom pulls the shared electrons closer to its side, creating an uneven distribution of charge known as a dipole. The atom with the stronger pull gains a partial negative charge (\(\delta-\)), while the less attractive atom acquires a partial positive charge (\(\delta+\)).
At the opposite end of the bonding spectrum, when the electronegativity difference becomes very large, the sharing of electrons essentially stops. With a difference greater than about 1.7, the more attractive atom pulls the electron completely away from the other atom. This complete transfer results in the formation of charged particles called ions, which are held together by electrostatic forces in what is known as an ionic bond. This spectrum demonstrates that the equal sharing of electrons is a distinct point on a gradient of attraction.