Covalent bonds are a fundamental chemical linkage, responsible for creating the vast majority of molecules. This bond forms when atoms share a pair of electrons between them. This mechanism creates simple molecules like water (H2O) and complex, life-sustaining structures such as DNA and proteins. Understanding the types of elements that participate in this sharing is central to understanding chemical structure and molecular behavior.
The Defining Characteristic: Sharing Valence Electrons
Atoms participate in covalent bonds to achieve a more stable, lower-energy electron configuration in their outermost shell, known as the valence shell. This drive for stability often means atoms seek to attain a full outer shell, typically eight electrons, a concept known as the octet rule.
Instead of one atom completely transferring an electron to another, the atoms pull the electron pair into the space between their nuclei, where it is simultaneously attracted to both positive centers. The sharing arrangement occurs when the difference in electron-attracting power between the two atoms is not significant enough to allow for a complete electron transfer.
Identifying the Participants: Nonmetals and Hydrogen
The types of elements that primarily form covalent bonds are nonmetals and hydrogen. Nonmetals are found on the right side of the periodic table and have a high tendency to attract electrons to themselves. When two nonmetal atoms interact, neither atom has a strong enough advantage to completely remove electrons from the other, making sharing the logical outcome.
Hydrogen is a special case because it only requires two electrons to fill its valence shell, mimicking the stability of the noble gas helium. This need for a single additional electron makes it prone to forming a single covalent bond with other atoms, including carbon, oxygen, and nitrogen, as seen in methane (CH4) and water.
Metalloids, such as silicon and boron, also participate in covalent structures due to their properties falling between those of metals and nonmetals. Silicon, for instance, forms an extensive network of covalent bonds in its crystal structure, similar to carbon in diamond. The defining feature of all these participants is their moderate to high electronegativity, which prevents the easy donation of electrons characteristic of metals.
The Spectrum of Covalent Bonds
While the fundamental act of a covalent bond is electron sharing, the quality of that sharing can vary, leading to a spectrum of bond types. This variation is determined by the difference in electronegativity between the two bonded atoms. Electronegativity is the measure of an atom’s ability to attract the shared electrons toward itself.
A nonpolar covalent bond occurs when the electron pair is shared equally between the two atoms. This equal sharing happens when the atoms are identical, such as in diatomic molecules like oxygen gas (O2), or when they have a very small difference in electronegativity (typically less than 0.4 on the Pauling scale). In these cases, the electron density is symmetrically distributed around both nuclei.
In contrast, a polar covalent bond involves an unequal sharing of the electron pair. This occurs when the two atoms have an intermediate difference in electronegativity, generally falling between 0.4 and 1.7. The atom with the higher electronegativity pulls the shared electrons closer to its nucleus, which gives it a slight negative charge. This leaves the other atom with a corresponding slight positive charge, creating a molecular dipole, as seen in the bonds within a water molecule.