Dipole Moment: Videos & Practice Problems

Electronegativity (EN) is a measure of an atom's ability to attract electrons towards itself, increasing from left to right across the periodic table and up a group. Understanding electronegativity is crucial when examining dipole moments, which occur when there is a significant difference in electronegativity between atoms in a bond, leading to unequal electron sharing. A difference in electronegativity greater than $0.4$ is significant, resulting in a polar bond with a dipole moment. This is depicted by a dipole arrow pointing towards the more electronegative atom, indicating a partial negative charge (δ-) at that end and a partial positive charge (δ+) at the other. The type of bond—nonpolar covalent, polar covalent, or ionic—depends on the difference in electronegativity values. Nonpolar covalent bonds have a difference of $0$ to $0.4$, polar covalent bonds range from $0.5$ to $1.7$, and a difference larger than $1.7$ indicates an ionic bond, where electrons are transferred, and full charges develop, rather than partial charges seen in polar covalent bonds.

Dipole Moment

To grasp the concept of dipole moments in chemical bonds and compounds, it is essential to first understand electronegativity, denoted as EN. Electronegativity measures an element's ability to attract electrons, and it exhibits a periodic trend: it increases from left to right across a period and from bottom to top within a group on the periodic table. This trend indicates that nonmetals, particularly those highlighted in blue, possess higher electronegativity values, with fluorine being the most electronegative element.

Noble gases are typically excluded from discussions of electronegativity due to their stable electron configurations. However, elements like krypton (EN = 3.0) and xenon (EN = 2.6) can exhibit electronegative values, albeit lower than those of nonmetals. The seventh row of the periodic table mainly consists of synthetically produced metals, which are unstable and do not have assigned electronegativity values. In contrast, francium, located in the bottom left corner, has the lowest electronegativity.

Understanding these electronegativity values is crucial when discussing dipole moments, as the difference in electronegativity between bonded atoms leads to the formation of polar bonds. A dipole moment occurs when there is an uneven distribution of electron density, resulting in a molecule having a positive end and a negative end. This concept is foundational for predicting molecular behavior and interactions.

Dipole Moment

A dipole moment arises from the polarity in a bond when there is a significant difference in electronegativities between the bonded atoms. Polarity refers to the unequal sharing of electrons, and a difference in electronegativity greater than 0.4 is considered significant. The difference in electronegativity, denoted as ΔEN, is calculated using the formula:

ΔEN = EN_higher - EN_lower

When a bond exhibits polarity, it also possesses a dipole moment, which is represented by a dipole arrow. This arrow points towards the more electronegative atom, indicating the direction of electron density. For example, when comparing fluorine and carbon, the difference in electronegativity is greater than 0.4, confirming the presence of a dipole moment. In this case, the fluorine end of the bond receives a partial negative charge (δ^-), while the carbon end receives a partial positive charge (δ⁺).

The dipole arrow visually represents this polarity, with the tail of the arrow indicating the less electronegative atom and the head pointing towards the more electronegative atom. Thus, when a significant difference in electronegativity is present, it is essential to illustrate the dipole moment with an arrow, highlighting the partial charges on each end of the bond.

Dipole Moment Example 1

The dipole moment is a measure of the polarity that occurs when there is a significant difference in electronegativity between two elements in a bond. Polarity itself refers to the unequal sharing of electrons between bonding atoms. A difference in electronegativity greater than 0.4 is considered significant, indicating that the bond is polar.

The difference in electronegativity, denoted as \(\Delta \text{EN}\), is calculated using the formula:

\[\Delta \text{EN} = \text{Higher Electronegativity} - \text{Lower Electronegativity}\]

For example, when comparing carbon and fluorine, carbon has an electronegativity value of 2.5, while fluorine has a value of 4.0. Thus, the calculation for the difference in electronegativity is:

\[\Delta \text{EN} = 4.0 - 2.5 = 1.5\]

Since this difference (1.5) is greater than 0.4, the bond between carbon and fluorine is classified as polar, which means it possesses a dipole moment. The dipole moment is represented by a dipole arrow that points towards the more electronegative element, in this case, fluorine. This indicates that fluorine is more electronegative, while carbon, being less electronegative, can be considered more electropositive.

When analyzing the polarity of a bond, remember that a difference in electronegativity greater than 0.4 signifies a polar bond that requires a dipole moment arrow. The arrow will always point towards the more electronegative atom, visually representing the distribution of electron density in the bond.

Arrange the following molecules in order of decreasing dipole moment.
H–IH–FH–BrH–Cl

• A. H–Cl > H–F > H–Br > H–I
• B. H–F > H–Cl > H–Br > H–I
• C. H–F > H–Cl > H–I > H–Br
• D. H–I > H–Br > H–Cl > H–F

Answer: (B)

Dipole Moment

The concept of electronegativity plays a crucial role in determining the type of chemical bond between two elements. Electronegativity is a measure of an atom's ability to attract and hold onto electrons. The difference in electronegativity between two atoms can indicate the bond's polarity: the greater the difference, the more polar the bond becomes.

When the difference in electronegativity is zero, the two atoms have identical electronegativity values, resulting in a pure covalent bond. An example of this is two bromine atoms (Br₂), which share electrons equally, represented by equal arrows in their electron cloud.

As the difference in electronegativity increases, we encounter a nonpolar covalent bond when the difference is between 0.1 and 0.4. For instance, in a bond between carbon (C) and hydrogen (H), carbon has a higher electronegativity (2.5) compared to hydrogen (2.1). Although the electrons are shared, they are slightly more attracted to carbon, indicated by a larger dipole arrow pointing towards carbon.

When the difference in electronegativity falls between 0.5 and 1.7, the bond is classified as a polar covalent bond. A classic example is the bond between chlorine (Cl) and hydrogen (H). Chlorine has an electronegativity of 3.0, while hydrogen is at 2.1. The significant difference results in a dipole arrow pointing towards chlorine, which carries a partial negative charge, while hydrogen carries a partial positive charge.

Finally, when the difference exceeds 1.7, the bond is classified as an ionic bond. In this case, the difference is so substantial that one atom completely transfers its electron to another, forming a cation and an anion. For example, sodium (Na) donates its electron to chlorine, resulting in Na⁺ and Cl^-. Here, the dipole arrow indicates the direction of the charge, with full positive and negative charges present.

In summary, understanding the relationship between electronegativity differences and bond types is essential. The greater the difference in electronegativity, the greater the polarity of the bond, which helps classify the bond as pure covalent, nonpolar covalent, polar covalent, or ionic.

Dipole Moment Example 2

To determine which bond is the most polar, we need to analyze the differences in electronegativity between the atoms involved. Electronegativity is a measure of an atom's ability to attract and hold onto electrons, and the greater the difference in electronegativity between two bonded atoms, the more polar the bond will be.

In this case, we have the following electronegativity values:

• Sulfur (S): 2.5
• Selenium (Se): 2.4
• Hydrogen (H): 2.1
• Chlorine (Cl): 3.0
• Fluorine (F): 4.0

To find the most polar bond, we calculate the differences in electronegativity for the relevant pairs:

• Fluorine and Sulfur: \(4.0 - 2.5 = 1.5\)
• Chlorine and Sulfur: \(3.0 - 2.5 = 0.5\)
• Hydrogen and Chlorine: \(3.0 - 2.1 = 0.9\)
• Hydrogen and Fluorine: \(4.0 - 2.1 = 1.9\)

Among these calculations, the bond between Hydrogen and Fluorine shows the largest difference in electronegativity, which is \(1.9\). Therefore, the bond with the most polar character is the one between Hydrogen and Fluorine, as it has the greatest difference in electronegativity.

Which of the following correctly identifies the chemical bond between a carbon and oxygen atom?

• A. Polar Covalent
• B. Pure Covalent
• C. Nonpolar
• D. Ionic

Answer: (A)

Here’s what students ask on this topic:

What is a dipole moment?

A dipole moment is a measure of the separation of positive and negative electrical charges within a molecule. It's a vector quantity, which means it has both magnitude and direction. The dipole moment arises due to the difference in electronegativity between two atoms bonded together; the more electronegative atom pulls the shared electrons closer to itself, creating a partial negative charge, while the less electronegative atom acquires a partial positive charge.

The magnitude of the dipole moment is measured in Debye units and is calculated by multiplying the amount of charge separation by the distance between the charges. A molecule with a higher dipole moment is generally more polar, meaning it has a stronger charge imbalance, which affects its physical properties and the way it interacts with other molecules. For example, molecules with significant dipole moments tend to have higher boiling points and are more likely to be soluble in polar solvents like water.

How do you calculate dipole moment?

To calculate the dipole moment, you need to consider both the magnitude of the charges involved and the distance between them. The dipole moment (μ) is the product of the charge (Q) on either end of the dipole and the distance (r) between the charges:

μ = Q × r

The charge is usually measured in coulombs (C), and the distance in meters (m), which gives the dipole moment in coulomb-meters (C·m). However, in chemistry, the charges are often in terms of the elementary charge (e), and the distance in angstroms (Å) or picometers (pm), so the dipole moment may be given in Debye units (D).

1 Debye is approximately 3.33564 × 10 - 30 C·m . To convert from the product of elementary charge and distance in Å to Debye, you can use the conversion factor:

1 D = 1 3.33564 × 10 - 30 C·m e · Å

Keep in mind that the direction of the dipole moment vector is from the negative charge to the positive charge.

What is bond polarity?

Bond polarity refers to the distribution of electrical charge over the atoms joined by the bond. In a covalent bond, where two atoms share a pair of electrons, the electrons are not always shared equally. This can happen when the two atoms have different electronegativities, which is a measure of how strongly an atom can attract electrons to itself.

When one atom has a higher electronegativity than the other, it will attract the shared electrons more strongly, causing a partial negative charge (δ^-) to develop on the more electronegative atom, and a partial positive charge (δ⁺) on the less electronegative atom. This creates a dipole, which is a pair of equal and oppositely charged or magnetized poles separated by a distance.

A bond between two atoms with identical or similar electronegativity (like H-H or Cl-Cl) is nonpolar, meaning the electrons are shared equally, and there is no permanent charge difference between the atoms. In contrast, a bond between atoms with different electronegativities (like H-Cl) is polar, with a positive end and a negative end, much like a magnet.

How do you determine bond polarity?

To determine bond polarity, you need to consider the electronegativity of the atoms involved in the bond. Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond. Here's a step-by-step process:

1. Identify the Atoms: Look at the two atoms that form the bond.
2. Electronegativity Values: Find the electronegativity values for each atom. These values are typically found on a chart or in a chemistry textbook.
3. Calculate the Difference: Subtract the smaller electronegativity value from the larger one to determine the difference in electronegativity between the two atoms.
4. Determine Polarity: If the difference is:

• 0, the bond is nonpolar covalent, meaning the electrons are shared equally.
• Less than 0.5, the bond is generally considered to be mostly nonpolar covalent, but with slight polarity.
• Between 0.5 and 1.9, the bond is polar covalent, meaning the electrons are shared unequally, and the atom with the higher electronegativity will have a partial negative charge, while the other atom will have a partial positive charge.
• 2.0 or greater, the bond is usually considered

What determines the polarity of a bond?

The polarity of a bond is determined by the difference in electronegativity between the two atoms forming the bond. Electronegativity is a measure of how strongly an atom attracts electrons in a chemical bond. When two atoms with different electronegativities form a bond, the electrons are not shared equally. The atom with the higher electronegativity will attract the shared electrons more strongly, acquiring a partial negative charge, while the other atom will have a partial positive charge. This creates a dipole, with one end being slightly negative and the other slightly positive, resulting in a polar bond.

If the difference in electronegativity between the two atoms is significant (generally a difference greater than 0.5 on the Pauling scale), the bond is considered polar. If the electronegativity difference is very small or zero, as in the case of two identical atoms, the bond is nonpolar because the electrons are equally shared. The greater the difference in electronegativity, the more polar the bond will be.