What is a Covalent Bond?
What causes atoms to bond with one another? The answer is that all atoms “want” to obtain a stable electron configuration, which usually means eight electrons in their outermost electron shell. To do this, atoms can take electrons from another atom, give electrons to another atom, or share electrons with another atom.
Covalent bonds are the kind of bond where atoms share an electron pair to obtain a full outer shell. This is sometimes also called a molecular bond. Not all atoms are capable of forming covalent bonds. To form a covalent bond, two atoms must be nonmetals and have close electronegativity values.
An electronegativity value is a measure of how strongly an atom attracts electrons, or how badly it “wants” to fill its outer shell. If one atom has very low electronegativity and the other has very high electronegativity, they won’t be good at sharing electrons because one “wants” the electrons so much more. An electronegativity difference of greater than 1.7 makes the formation of covalent bonds impossible. As you may have guessed, covalent bonds form very readily between the same type of atom because their electronegativity values are identical.
Even among covalent bonds, some bonding partners are far better at equally sharing their electrons than others. When we draw electron diagrams we tend to portray the electrons as stationary, but in reality electrons are in a constantly moving cloud orbiting their respective nuclei. When electronegativity values are different, but not so different as to exclude a covalent bond, the electrons spend more time near the more electronegative atom.
This is what happens with a polar covalent bond. To give an example, let’s focus on a water molecule, or H20. Oxygen has a greater electronegativity value than hydrogen does. The difference is not so great that oxygen isn’t capable of sharing, but the electrons are more strongly attracted to the oxygen atom than either of the hydrogen atoms. Since the electrons spend more time near the oxygen side of the molecule, that side ends up being slightly more negatively charged while the two hydrogen atoms are slightly more positively charged. This polar dynamic is part of what gives water its unique properties.
In a nonpolar covalent bond, the electrons are shared completely equally. The entire molecule is neutral and no part is more negatively or positively charged than any other. Nonpolar covalent bonds form especially readily between the same type of atom, but any two atoms with an electronegativity difference of less than .4 are capable of forming nonpolar covalent bonds.
Clker-Free-Vector-Images (artist). (2012). Water Science Chemistry [digital art]. Retrieved from https://pixabay.com/vectors/water-science-chemistry-molecule-40708/