# Dipole Moment, what is Q? and other questions

• doxos
In summary, the conversation discusses the concept of dipole moment in the context of a General Chemistry textbook. The example given involves the calculation of the dipole moment of HCl using the equation μ=Q*r, where Q is the charge of an electron and r is the distance between atoms. The resulting dipole moment of 1.11 D indicates that the H-Cl bond is approximately 18% ionic. The conversation also includes questions about the use of Q in the equation, specifically in the context of ionic compounds like Calcium Chloride and covalent molecules like Carbon Dioxide. These questions highlight the importance of understanding the number of electrons involved when calculating dipole moment.
doxos
Hello, I have some questions about Dipole Moment, taken from a General Chemistry textbook, in which they treat Dipole Moment as a scalar. Here is a worked example from this textbook:

## Homework Statement

The dipole moment of HCl is 1.11 D, and the distance between atoms is 127 pm. What is the percent ionic character of the Hcl bond?

## Homework Equations

μ=Q*r

3. The given solution
μ=(1.60*10^-19 C)*(127*10^-12 m)*(1D/3.336*10^-30 C*m) = 6.09D

The observed dipole moment of HCl (1.11D) implies that the H-Cl bond is only about 18% ionic:
(1.11 D)/(6.09 D) = 18.2%

.

Now, here are my questions, along with my train of thought. Please correct me if my understanding is incorrect.

1. We used Q = 1.60*10^-19 C, which is the charge of an electron, and we used it to calculate μ in the scenario when a proton and an electron is at a distance of r apart. But I'm thinking, since the proton has a charge of +1.60*10^-19 C also, shouldn't we use for the value of Q the product of the charges of a proton and an electron, (1.60*10^-19 C)^2 ? I don't understand why we used the value Q = 1.60*10^-19 C in this example problem.

2. I understand the use of μ=Q*r equation in this problem is treating as if we have an ionic compound, where an electron is completely stripped from one atom and transferred to another item. Thus, the actual Dipole Moment of the covalent-bonded molecule in this problem is much lower, because the electron is shared and only partially spends more time around the Cl atom.

Now, suppose a problem asks to find the Dipole Moment in an ionic compound, Calcium Chloride. Between a Ca and a Cl atom, there's a difference of 1 electron lost and 1 electron gained. But wait, the Ca item actually loses 2 electrons, so its charge is even less. Now, taking into account this difference in number of electrons between Ca and Cl, to calculate the μ between Ca and just one Cl atom, what would we use for the Q value?

3. Similarly, suppose a problem asks to find the Dipole Moment in a covalent molecule, Carbon Dioxide. (The molecule has net Dipole Moment of 0, because it's linear and the 2 vectors cancel each other out. But let's look at the Dipole Moment between C and just one O molecule anyway.) The C and O atoms are double-bonded, sharing 2 pairs of electrons. So sometimes, the O atom can have not one more, but up to 2 more electrons spending time around it than around the C atom. Again, taking this into account, what would we use for the Q value?

## 1. What is a dipole moment?

A dipole moment is a measure of the separation of positive and negative electrical charges within a molecule. It is a vector quantity, meaning it has both magnitude and direction, and is typically measured in units of Debye (D).

## 2. How is dipole moment calculated?

Dipole moment is calculated by multiplying the magnitude of the charge (Q) of each atom by the distance (r) between them, and then taking the product of these values and the cosine of the angle (θ) between the bond and the dipole vector. The resulting equation is: μ = Q x r x cos θ.

## 3. What is Q in dipole moment?

Q is the magnitude of the charge of an atom within a molecule. It can be a partial charge, as molecules can have both positive and negative charges. Q is an important component in calculating the dipole moment of a molecule.

## 4. How does dipole moment affect a molecule's polarity?

Dipole moment is directly related to a molecule's polarity. A larger dipole moment indicates a greater separation of charges, resulting in a more polar molecule. This can affect the molecule's physical and chemical properties, such as solubility and reactivity.

## 5. Can a molecule have a zero dipole moment?

Yes, a molecule can have a zero dipole moment if the individual bond dipole moments cancel each other out due to symmetry. This means that the molecule is nonpolar, with no separation of charges. An example of such a molecule is carbon dioxide (CO2).

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