Calculating formal charge is very important in organic chemistry, helping you know the number of electrons in an atom inside a molecule.
To make studying the process a fun experience for you, here is the easy explanation: Consider the formal charge as an electron scorecard.
By recording the number of electrons in an atom, you understand the role of an atom in a molecule.
As an organic chemistry student, you might be thinking: Why is formal charge important in organic chemistry? And you might also be eager to know the right formula to calculate formal charge.
To spark the excitement, we’ll deep dive into the entire process to determine formal charge and answer all the questions that loom. So, let’s get started.
Overview of Formal Charge
We’ll start with an overview of formal charge.
As we’ve discussed, formal charge is used to know the number of electrons in an atom to understand how atoms bond to form a molecule.
And guess what? These atoms also share electrons. So, sometimes an atom gets more electrons, while sometimes it’s short on the required number of electrons.
To break it down, let’s understand that if an atom has:
- More electrons than required, you get a negative formal charge
- Less electrons than required, you get a positive formal charge
- Exact number of electrons needed, the formal charge is zero. It indicates that the atom has a perfect balance of electrons, making it a neutral atom.
Now, if you’re wondering how to know if an atom has its fair share of electrons, let’s discuss the formula of formal charge.
Formula to Calculate Formal Charge
This formula explains how to determine formal charge:
Formal charge = Valence electrons − (Bonding electrons + Non-bonding electrons)
To keep it short and easy for you to calculate, we can write this formula as:
FC = VE – (B + NB)
Here, valence electrons are those electrons located in the outermost shell of an atom. These electrons play a key role in bonding the atom with other atoms to form a molecule.
Bonding electrons are the electrons shared between atoms to form a chemical bond.
And non-bonding or unpaired electrons are lone pairs that are not shared between atoms and stay in their location in an atom.
So, in this formula, you add bonding electrons and non-bonding electrons, and subtract their total from the number of valence electrons.
And with that, you get your formal charge.
Does it seem scary to understand?
Worry not. Let’s understand this formula with an example.
Example: In an acetate ion (CH₃COO⁻), the single-bonded oxygen (O⁻) is the key atom because it has the negative charge. This means that this oxygen has more electrons.
This single-bonded oxygen is made up of three lone pairs that are not bonded to any other atom. Each pair has two electrons, making a total of 6 electrons in this oxygen atom.
There are a total of 6 valence electrons in it.
And there is only 1 single bond to carbon.
So, the formula to calculate is formal charge is:
FC = 6 – (1 + 6)
Formal charge = 6 – 7
FC = -1
This means the formal charge of non-bonded oxygen is -1, making it the strongest atom in the equation.
Now, if you’re wondering how we got the number of electrons for each atom, here we unravel its mystery.
Finding Electrons in an Atom for Calculation
It’s extremely easy to find the electrons in an atom. Check the atomic number of each atom on the periodic table.
The atomic number is used to find the number of protons in an atom.
If the atom is neutral, the number of protons is equal to the number of electrons. It’s because both electrons and protons balance each other’s electrical charge.
But if the atom is charged, it has either a negative charge or a positive charge, indicating whether it has more or fewer electrons.
Example: The atomic number of hydrogen in the periodic table is 1. That means neutral hydrogen has a total of 1 electron.
If the hydrogen atom is negatively charged (H⁻), it indicates that it has an additional electron. It means this atom has a total of 2 electrons.
If this atom is positively charged (H⁺), this indicates that it has 0 electrons. It means it’s short of electrons.
There is one more interesting element to the discussion, and that’s the resonance structure. It beautifully explains how electrons are arranged.
Let’s discuss how it comes into play in the formal charge process.
Relation of Formal Charge and Resonance Structure
While the resonance structure itself is a standalone concept in organic chemistry, here we’ll discuss only the role of formal charge in this structure.
And before we proceed further, there is one very important thing to know. Formal charge is very important to determine which resonance structure is better.
Example: Here, we’ll show two resonance structures of the acetate ion to explain how its formal charge of -1 moves between oxygen atoms.
In this first structure, you can see:
H₃C-C(=O)-O⁻
Here, you see that the formal charge indicates that the single-bonded oxygen atom has an additional electron.
In this second structure, the formal charge shifts from the single-bonded oxygen to the oxygen bonded with carbon.
The structure is as follows:
H₃C-C(-O⁻)=O
Here, you can see that the charge has moved from one atom to the other while the skeletal structure of the acetate ion remains the same.
The formal charge in a resonance structure helps us understand how a charge moves in chemical bonding.
As an organic chemistry student, you can learn different chemical reactions by calculating the formal charge and incorporating it into your resonance structure.
Conclusion
Determining a formal charge is crucial to studying the chemical bonding of atoms and their role in forming molecules.
This is helpful in your organic chemistry exams and practice problems.
If you want to master complex organic chemistry concepts like that, connect with seasoned chemistry tutors for online tutorials here at Orango. And embark on a journey to become an expert in the field of chemistry!
