Ionic and covalent bonds are the two main types of chemical bonding. A chemical bond is a link formed between two or more atoms or ions. The main difference between ionic and covalent bonds is how equally the electrons are shared between atoms in the bond. Here is an explanation of the difference between ionic and covalent bonds, examples of each bond type, and a look at how to tell which type of bond will form.
Key Points
- The two main types of chemical bonds are ionic and covalent bonds. Metals bond via a third type of chemical bond called metallic bonding.
- The key difference between an ionic and covalent bond is that one atom essentially donates an electron to another atom in an ionic bond while electrons are shared between atoms in a covalent bond.
- Ionic bonds form between a metal and a nonmetal. Covalent bonds form between two nonmetals. Metallic bonds form between two metals.
- Covalent bonds are categorized as pure or true covalent bonds and polar covalent bonds. Electrons are shared equally between atoms in pure covalent bonds, while they are shared unequally in polar covalent bonds (spend more time with one atom than the other).
Ionic Bonds
In an ionic bond, one atom donates an electron to another atom. This stabilizes both atoms. Because one atom essentially gains an electron and the other loses it, an ionic bond is polar. In other words, one atom in the bond has a positive charge, while the other has a negative charge. Often, these atoms dissociate into their ions in water. Atoms that participate in ionic bonding have different electronegativity values from each other. If you look at a table of electronegativity values, it is apparent ionic bonding occurs between metals and nonmetals. Examples of compounds with ionic bonds include salt, such as table salt (NaCl). In salt, the sodium atom donates its electron, so it yields the Na+ ion in water, while the chlorine atom gains an electron and becomes the Cl– ion in water.
Covalent Bonds
Atoms are bound by shared electrons in a covalent bond. In a true covalent bond, atoms have the same electronegativity values as one another. This type of covalent bond forms between identical atoms, such as hydrogen (H2) and ozone (O3). In a true covalent bond, electrical charge is evenly distributed between the atoms so the bond is nonpolar. Covalent bonds between atoms with slightly different electronegativity values results in a polar covalent bond. However, the polarity in a polar covalent bond is less than in an ionic bond. In a polar covalent bond, the bonding electron is more attracted to one atom than to the other. The bond between hydrogen and oxygen atoms in water (H2O) is a good example of a polar covalent bond. Covalent bonds form between nonmetals. Covalent compounds may dissolve in water, but they don’t dissociate into their ions. For example, if you dissolve sugar in water, it’s still sugar.
Ionic vs Covalent Bond Summary
Here’s a quick summary of the differences between ionic and covalent bonds, their properties, and how to recognize them:
| Ionic Bonds | Covalent Bonds | |
| Description | Bond between metal and nonmetal. The nonmetal attracts the electron, so it’s like the metal donates its electron to it. | Bond between two nonmetals with similar electronegativities. Atoms share electrons in their outer orbitals. |
| Electronegativity | Large electronegativity difference between participants. | Zero or small electronegativity difference between participants. |
| Polarity | High | Low |
| Shape | No definite shape | Definite shape |
| Melting Point | High | Low |
| Boiling Point | High | Low |
| State at Room Temperature | Solid | Liquid or Gas |
| Examples | Sodium chloride (NaCl), Sulfuric Acid (H2SO4 ) | Methane (CH4), Hydrochloric acid (HCl) |
| Chemical Species | Metal and nometal (remember hydrogen can act either way) | Two nonmetals |
Metallic Bond
Metallic bonding is another type of chemical bonding. In a metallic bond, bonding electrons are delocalized across a lattice of atoms. A metallic bond is similar to an ionic bond. But, in an ionic bond the location of a bonding electron is static and there may be little to no electronegativity difference between bond participants. In a metallic bond, electrons can flow freely from one atom to another. This ability leads to many of the classic metallic properties, such as electrical and thermal conductivity, luster, tensile strength, and ductility. The atoms in metals and alloys are an example of metallic bonding.
References
- Laidler, K. J. (1993). The World of Physical Chemistry. Oxford University Press. ISBN 978-0-19-855919-1.
- Langmuir, Irving (1919). “The Arrangement of Electrons in Atoms and Molecules”. Journal of the American Chemical Society. 41 (6): 868–934. doi:10.1021/ja02227a002
- Lewis, Gilbert N. (1916). “The Atom and the Molecule”. Journal of the American Chemical Society. 38 (4): 772. doi:10.1021/ja02261a002
- Pauling, Linus (1960). The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry. Cornell University Press. ISBN 0-801-40333-2 doi:10.1021/ja01355a027