One of the major questions college-level chemistry students have pertains to the difference between polar and nonpolar bonds. Many students might have a difficult time understanding the exact definition of both, but there are some general rules that can help to explain the difference. Understanding these bonds represents a critical starting point for chemistry students in their studies.
Covalent bonds form the base of the chemical reactions between atoms of different elements. The only way these bonds can form is when an electron becomes shared by two elements, creating the connection that then results in a new substance. Covalent bonds can exist as polar or nonpolar compounds, but it's important to note that all bonds that are polar or nonpolar in nature must also be covalent.
Polar molecules are the result of polar bonding between atoms in which the electrons are not shared equally. This happens when two different atoms bind together from two different elements, as opposed to two atoms from the same element, which do not form polar bonds. The reason for polar bonds is that every atom pulls electrons at its own level, which means that unless the elements are the same, one atom of an element will be better at pulling electrons than the other in the bond. The molecule will be more negative wherever electrons are "bunched up" unevenly, and more positive on the other side.
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In a nonpolar bond, two atoms share electrons equally with one another. These bonds only occur when two atoms are from the same element, since only matching elements will have the exact same ability to pull electrons. An example of this would be H2 or O2, since the bond is still of only one element. In larger nonpolar molecules with symmetric shapes, the charges are spread out evenly.
A general rule in chemistry states, "like dissolves like," meaning polar substances tend to dissolve each other, as do nonpolar substances. For example, water, a polar liquid, mixes freely with isopropyl alcohol, another polar liquid. However, oils, which are typically nonpolar, do not mix with water; they remain separate.