Water is super cool for lots of reasons! You've probably heard about how it's polar, sticks to things, and dissolves stuff. But have you ever heard of water being a "dipole"? If you're curious about what that means, keep reading!

Basically, a "dipole" is just a fancy word for something that has two different charges. In water's case, it has a positive charge on one side and a negative charge on the other. This happens because of the way the atoms in water are arranged. There are different types of dipoles in chemistry, but we'll focus on the ones that involve water. For example, there's something called a "hydrogen bond" that happens between water molecules because of their dipoles. This is what makes water stick together and form droplets. So, next time you're drinking a glass of water, remember that it's not just a boring liquid - it's got some seriously cool chemistry going on! And now you know a little more about what makes it so special.

What is the Definition of a Dipole in Chemistry?

Dipoles occur when electrons are shared unequally between atoms in the same molecule due to a high difference in the electronegativity of the atoms involved. A dipole is a molecule or covalent bond that has a separation of charges.

Determination and Formation of a Dipole

Dipoles are formed depending on the polarity of a bond, which is determined by how much an atom can attract electrons to itself - this is called electronegativity.

There are three types of bonds: non-polar covalent, polar covalent, and ionic. In non-polar covalent bonds, electrons are shared equally. In polar covalent bonds, electrons are shared unequally, and in ionic bonds, electrons are transferred. Ionic bonds don't have dipoles, polar covalent bonds always have dipoles, and non-polar covalent bonds have dipoles that cancel out due to symmetry.

To figure out what type of bond is present, we need to look at the difference in electronegativity between the atoms involved. If the difference is less than 0.4, it's a non-polar covalent bond. If the difference is between 0.4 and 1.7, it's a polar covalent bond. If the difference is greater than 1.7, it's an ionic bond. The electronegativity values are listed on Pauling's electronegativity scale. You can see a trend on the periodic table - electronegativity increases from left to right and decreases down a group.

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Dipole moments are a measure of the magnitude of a dipole, and we use arrows pointing towards the more electronegative atom to show it. The dipole moment of a bond can be calculated using the partial charges and the distance vector between the two charges. Dipole moments are measured in Debye units, and the bigger the dipole moment of the bond, the more polar the bond is.

The dipole moment of a molecule is the sum of the dipole moments of the bonds. If the molecule is perfectly symmetric, all vectors will add up to zero, and the dipole moment of the whole molecule will be zero.

For example, in PCl3, the bond is polar because of the difference in electronegativity between P and Cl atoms, and the presence of a lone pair of electrons gives it a tetrahedral structure. On the other hand, PCl5 is considered non-polar because its symmetrical shape, which is trigonal bipyramidal, cancels the dipoles out.

To learn more about molecular shapes, you can read about "Valence Shell Electron Pair Repulsion (VSEPR) Theory." If you need to learn how to draw Lewis structures, check out "Lewis Diagrams."

Types of Dipole in Chemistry

The three types of dipole interactions you might encounter are called ion-dipole, dipole-dipole, and induced-dipole induced-dipole (London dispersion forces).

Ion-Dipole

Ion-dipole interactions occur between an ion and a polar molecule, with the strength of the attractive force increasing with the ion's charge. For example, sodium ion in water is held together by ion-dipole forces.

Ion-induced dipole forces occur when a charged ion induces a temporary dipole in a non-polar molecule. This happens when the ion starts to affect the molecule's electrons, causing them to be attracted to the side where the ion is located. This creates a larger concentration of ions on that side, leading to the formation of a dipole on the originally non-polar molecule. An example of this is Fe3+ inducing a temporary dipole in O2.

Dipole-Dipole

When two polar molecules possessing permanent dipoles are near each other, attractive forces called dipole-dipole interactions hold the molecules together. Dipole-dipole interactions are attractive forces that occur between the positive end of a polar molecular and the negative end of another polar molecule. A common example of dipole-dipole forces is seen between HCl molecules. In HCl, the partial positive H atoms get attracted to the partial negative Cl atoms of another molecule.

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A special type of dipole-dipole interaction is hydrogen bonding. Hydrogen bonding is an intermolecular force that occurs between the hydrogen atom covalently bonded to an N, O, or F and another molecule containing N, O, or F.  For example, in water (H2O), the H atom covalently bonded to oxygen gets attracted to the oxygen of another water molecule, creating hydrogen bonding.

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Dipole-induced dipole forces arise when a polar molecule with a permanent dipole induces a temporary dipole in a non-polar molecule.  For example, dipole-induced dipole forces can hold molecules of HCl and He atoms together.

London dispersion forces

Induced-dipole induced-dipole interactions are also known as London dispersion forces. These forces are present in all molecules, but they are most important when dealing with non-polar molecules. London dispersion forces occur due to the random movement of electrons in the cloud of electrons, which produces a weak, temporary dipole moment. An example of this is F2, where London dispersion forces are the only type of attractive force holding the molecules together.

Other examples of molecules containing dipoles include acetone (C3H6O), which is a polar molecule with a bond dipole, and carbon tetrachloride (CCl4), which is a non-polar molecule that contains polar bonds and has dipoles present. However, the net dipole is zero due to its tetrahedral structure, where the bond dipoles directly oppose each other.

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Let's look at one last example! What is the net dipole moment in CO2?CO2 is a linear molecule that has two C=O bond dipoles equal in magnitude but pointing in opposite directions. Therefore, the net dipole moment is zero.

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Dipoles can seem intimidating at first, but they are actually quite simple once you understand the concept. Dipoles occur when electrons are shared unequally between atoms due to a high difference in electronegativity. A dipole moment is a measurement of the magnitude of a dipole, and it is present in polar molecules that have asymmetric shapes.

There are three types of dipoles: ion-dipole, dipole-dipole, and induced-dipole induced-dipole (London dispersion forces), each with their own unique characteristics.

References:

• Saunders, N. (2020). Supersimple Chemistry: The Ultimate Bitesize Study Guide. London: Dorling Kindersley.
• Timberlake, K. C. (2019). Chemistry: An introduction to general, organic, and Biological Chemistry. New York, NY: Pearson.
• Malone, L. J., Dolter, T. O., & Gentemann, S. (2013). Basic concepts of Chemistry (8th ed.). Hoboken, NJ: John Wiley & Sons.
• Brown, T. L., LeMay, H. E., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M., & Lufaso, M. W. (2018). Chemistry: The central science (13th ed.). Harlow, United Kingdom: Pearson.

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