Formal Charges - Online Tutor, Practice Problems & Exam Prep

Formal charges are calculated by subtracting the number of valence electrons (sticks and dots) from the group number of an atom. The equation is: FCi=G-(S+D), where FC is formal charge, G is group number, S is sticks, and D is dots. The net charge is the sum of all formal charges in a molecule. Understanding these concepts helps in determining the stability and reactivity of molecules, ensuring they meet bonding preferences.

Calculating formal and net charge.

So now that we understand bonding preferences so well, I want to move to a really related topic called formal charges. And formal charges are just based on the entire idea of bonding preferences. So let's go ahead and just jump right into it. Basically, a formal charge is assigned whenever there's a difference between the number of valence electrons an atom wants to have and the number of valence electrons it actually has. So remember that the group number is how many it wants. The valence electrons, the sticks, and the dots are the amount it actually has. So all you do to calculate formal charge is you take the group number, whatever that is. That could be group 4, group 5, whatever. And then you subtract the valence electrons, which are just the sticks and the dots. It's really simple math. A lot of times you'll just be able to do this on your fingers. Actually, all the time. So you take your group number then you just subtract the sticks and the dots, and you're good. That's called the formal charge. The net charge, let's write that down, the net charge is the term that we give for the sum of all the formal charges. Now this is an important point because I remember when I was in undergrad, I used to get a little confused over like oh, does the formal charge go on the whole thing or is it just one atom? No. The formal charge is 1 atom at a time. You're just looking at each atom and saying, Does this have a formal charge? Does this have a formal charge? The net charge is the collection of all of those summed together. Hopefully, it's not too complicated. There are usually not that many formal charges on a molecule.

Calculate the formal charges of ALL atoms.

Let's go ahead and do this example where I want to look at each atom and I want to count the formal charges for all of them. What that means is I'm going to be looking at group numbers and I'm going to be looking at sticks and dots. So let's start off with the hydrogens. Even though we already know that this fits the bonding preference. Remember that bonding preferences say that hydrogen wants to have what? One stick. One bond. But let's just do it anyway. Hydrogen is in what group? 1. How many sticks does it have? 1. How many dots does it have? 0. So it's going to be 1 according to my equation. My equation says that formal charge equals group number, which is 1, minus sticks and dots, which is 1. So it's 0. So this has a 0 formal charge. Do you see that? But it also fits with my bonding preference. So as long as your bonding preference agrees with what you see, that's going to be 0.

Let's do this with this oxygen right here. So first of all, does this oxygen fit my bonding preference according to bonding preferences? Yes, it does. It already fits it, so I could already put a 0 here. But now I want to show you mathematically how it works out too. So we said that the formal charge equals the group number. What's the group number of oxygen? It's 6 minus the number of sticks and dots. So how many is that altogether? That's 6. So then I would get a formal charge of 0. See how easy that is?

So now we would do that for carbon, but is carbon fulfilling its bonding preference? It already is. So I can just put a 0 there. Then finally, there was a typo, just so you know. So notice that in your page, the oxygen did not have those lone pairs. Go ahead and just write those in. That just didn't copy. So basically, for this one, the oxygen has 6 electrons total. It wants 6 electrons. I'm sorry. And it has 6 electrons total. In terms of the valence, six sticks and dots, so this would also be 0. This is another way that you can fulfill your bonding preference by having a double bond and having 2 lone pairs. It still counts as 2 bonds and 2 lone pairs. So I hope that you guys see how if it fits its bonding preference, you don't need to calculate. But calculating is always just safe to do.

Calculate the net charge of the molecule

• A. -1
• B. 0
• C. +1
• D. +2

Calculate the total charge of the molecule

• A. -2
• B. -1
• C. 0
• D. +1

Here’s what students ask on this topic:

What is the formula for calculating formal charge?

The formula for calculating formal charge (FC) is:

$\mathrm{FC}=G-(S+D)$

Where:

• $G$ is the group number of the atom.
• $S$ represents the number of sticks (bonds).
• $D$ represents the number of dots (lone pair electrons).

This formula helps determine the formal charge by comparing the number of valence electrons an atom wants to have (group number) with the number it actually has (sticks and dots).

How do you determine the formal charge of an atom in a molecule?

To determine the formal charge of an atom in a molecule, follow these steps:

1. Identify the group number of the atom (this is the number of valence electrons the atom wants).
2. Count the number of bonds (sticks) the atom has.
3. Count the number of lone pair electrons (dots) the atom has.
4. Use the formula: $\mathrm{FC}=G-(S+D)$

For example, for an oxygen atom in a molecule with 2 bonds and 2 lone pairs, the formal charge would be:

$\mathrm{FC}=6-(2+4)=0$

What is the significance of formal charge in determining molecular stability?

Formal charge is significant in determining molecular stability because it helps identify the most stable structure of a molecule. Atoms in a molecule prefer to have formal charges as close to zero as possible. Structures with formal charges that are minimized and evenly distributed are generally more stable. Additionally, formal charges can indicate the likelihood of a molecule's reactivity and its ability to participate in chemical reactions. By ensuring that the formal charges align with the bonding preferences of the atoms, chemists can predict and explain the behavior of molecules in various chemical contexts.

How do you calculate the net charge of a molecule?

The net charge of a molecule is calculated by summing the formal charges of all the atoms in the molecule. The formula is:

$\mathrm{Net} \mathrm{Charge}=\sum \mathrm{FC}\_i$

Where:

• $\sum \mathrm{FC}\_i$ is the sum of the formal charges of all atoms in the molecule.

For example, if a molecule has three atoms with formal charges of +1, -1, and 0, the net charge would be:

$\mathrm{Net} \mathrm{Charge}=1+(-1)+0=0$

Why is it important to check bonding preferences when calculating formal charges?

It is important to check bonding preferences when calculating formal charges because bonding preferences provide a quick way to verify if an atom is likely to have a formal charge of zero. Atoms tend to form bonds in a way that satisfies their bonding preferences, which are based on achieving a stable electron configuration. By checking bonding preferences, you can often predict the formal charge without detailed calculations. However, performing the calculation ensures accuracy and helps identify any deviations from expected bonding patterns, which can be crucial for understanding the molecule's stability and reactivity.