A covalent bond is a type of chemical bond that holds different atoms together by sharing electrons. Depending on how equally the electrons are shared between the atom, these bonds may be either polar (having an unequal distribution of charge) or nonpolar (having an equal distribution of charge) in nature. In their most stable form, the outer electron shells of an atom are evenly filled. If there are too many or too few electrons to evenly fill these shells, the atom will try to combine with other atoms to reach a lower energy state that is more stable. By sharing electrons, atoms can achieve greater stability.
Atoms can have varying degrees of attraction for its outer shell, or valence, electrons. This measure of attraction is called electronegativity. An atom with high electronegativity is strongly attracted to its electrons and will try to combine with another atom that has a lower electronegativity. The magnitude of the difference between the electronegativity values of the atom will determine how the atoms share electrons. If the difference is large, the atoms tend to form ionic bonds when they combine. If the difference is small, they tend to form covalent bonds. The degree of covalency varies depending on the exact difference.
If the electronegativity difference is less than 0.5 units, the atoms tend to share outer shell electrons equally. When this occurs, the bond is called a nonpolar covalent bond. Chemical compounds with this type of bond are electrostatically neutral and do not tend to form charged ions. Unlike ionic compounds, they do not conduct electricity. Nonpolar covalently bonded molecules tend to be gases, low boiling liquids, or low melting solids. Examples include the noble gases.
When the difference in electronegativity is between 0.5 and 1.7 units, the electrons are not shared equally between the atoms. The electron balance is shifted somewhat toward the more electronegative atom because its attraction for the electrons is stronger. When this situation occurs, the bond is called a polar covalent bond. Because the electron sharing is unequal, the molecules have a polar orientation; in other words, the electrons are not evenly distributed across the bond. Therefore, polar covalent bonds form what is known as a dipole, and the molecule itself is said to be polar. An example of a polar molecule is water (H2O). In molecules made of three or more atoms, the polar bonds may be arranged symmetrically around the central atom. If this happens, the dipoles negate each other and make the entire molecule nonpolar. Examples of molecules with this type of bond symmetry include carbon dioxide (featuring two oxygen atoms balanced on either side of the central carbon atom) and carbon tetrachloride (where four chlorine atoms are uniformly spaced around the carbon atom in the middle. Methane has a similar configuration with its four hydrogens around its central carbon.
In the covalent bonds described above, each atom in the molecule contributes one or more electrons. Another type of covalent bond occurs when both electrons from the shared pair come from the only one of the atoms. These bonds are called coordinate covalent bonds and examples include the ammonium ion NH 4 and the sulfuric acid molecule H2SO 4.
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