How To Calculate The Formal Charge of an Atom - Chemistry

The Organic Chemistry Tutor
25 Oct 201713:10
EducationalLearning
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TLDRThis educational video offers a simplified method to calculate the formal charge of atoms in chemical structures. The presenter uses the formula: formal charge = valence electrons - (bonds + lone pairs). Examples include calculating charges for nitrogen in various ions and molecules, emphasizing practice for mastery. The video also explains how formal charge helps determine the most stable Lewis structure, using OCN- as a case study to illustrate the concept of resonance and stability, highlighting the importance of electronegativity in assigning charges.

Takeaways
  • 📚 The video provides a basic introduction to the concept of formal charge and its calculation.
  • 🔍 The formula for calculating formal charge is given as the number of valence electrons minus the sum of the number of bonds and lone pairs (dots).
  • 🌰 An example is demonstrated using a nitrogen atom with two lone pairs, resulting in a formal charge of -1.
  • 📝 Practice problems are suggested to master the calculation of formal charges, starting with the hydronium ion.
  • 🧪 The formal charge on the oxygen atom in the hydronium ion is calculated to be +1, indicating its contribution to the ion's overall charge.
  • 🌐 In sulfur dioxide (SO2), the formal charges are calculated for sulfur and both oxygen atoms, resulting in a neutral molecule with regions of positive and negative charge.
  • 🚫 The cyanide ion example clarifies that the carbon atom, not nitrogen, bears the negative charge due to its calculated formal charge.
  • 🔑 Formal charge is a useful tool for determining the most stable structure of a molecule, as illustrated with the OCN- molecule.
  • ⚖️ The most stable structure is identified by the lowest and closest formal charges to zero, with preference given to more electronegative elements bearing the negative charge.
  • 🔬 The concept of resonance structures is introduced, with the most stable structure being the one that the resonance hybrid will most closely resemble.
  • 🧠 The importance of applying the formula and practicing with different molecules is emphasized for understanding and retaining the concept of formal charge.
Q & A

  • What is the basic concept of formal charge?

    -Formal charge is a property of an atom in a molecule that helps determine the most stable structure of the molecule. It is calculated using the equation: formal charge = valence electrons - (bonds + dots).

  • How can you calculate the formal charge of a nitrogen atom in a molecule?

    -To calculate the formal charge of a nitrogen atom, you first determine the number of valence electrons (5 for nitrogen), count the number of bonds attached to the nitrogen, and count the number of lone pairs (dots). Subtract the sum of bonds and lone pairs from the valence electrons.

  • What is the formal charge of the nitrogen atom in the given example?

    -In the example provided, the nitrogen atom has two bonds and four lone pairs (dots). The formal charge is calculated as 5 (valence electrons) - (2 bonds + 4 dots) = -1.

  • How does the hydronium ion's structure relate to its formal charge?

    -In the hydronium ion, the oxygen atom has three bonds and two lone pairs (dots). The formal charge on oxygen is calculated as 6 (valence electrons) - (3 bonds + 2 dots) = +1, which is the positive charge of the ion.

  • What is the significance of calculating the formal charge for each atom in sulfur dioxide (SO2)?

    -Calculating the formal charge for each atom in SO2 helps to understand the distribution of charge within the molecule. Sulfur has a formal charge of +1, one oxygen has a formal charge of 0, and the other oxygen has a formal charge of -1, resulting in a neutral molecule.

  • Why is the cyanide ion's formal charge on carbon instead of nitrogen?

    -In the cyanide ion, carbon has four valence electrons, three bonds, and two lone pairs (dots), resulting in a formal charge of -1 (4 - (3 + 2) = -1). Nitrogen, with five valence electrons, three bonds, and two lone pairs, has a neutral formal charge (5 - (3 + 2) = 0).

  • What does the formal charge of +1 on nitrogen in the NH4+ ion indicate?

    -The formal charge of +1 on nitrogen in the NH4+ ion indicates that there is an imbalance in the distribution of valence electrons, with nitrogen having four bonds and no lone pairs, resulting in a positive charge that balances the four negative charges from the four hydrogen atoms.

  • How can formal charge be used to determine the most stable structure of a molecule?

    -Formal charge can be used to identify the most stable structure by calculating the formal charge of each atom in different possible structures. The structure with the lowest formal charges and those closest to zero is generally more stable.

  • What is the total number of valence electrons in the OCN- molecule?

    -The total number of valence electrons in the OCN- molecule is 16, calculated as 6 (oxygen) + 4 (carbon) + 5 (nitrogen) + 1 (for the negative charge).

  • How does electronegativity influence the placement of formal charge in a molecule?

    -Electronegativity influences the placement of formal charge by determining which atom is more likely to attract electrons. In the OCN- molecule, it is more stable to place the negative charge on oxygen due to its higher electronegativity compared to nitrogen.

  • What is the concept of resonance structures and how do they relate to formal charge?

    -Resonance structures are different ways of representing the same molecule with the same connectivity of atoms but different electron distributions. Formal charge helps to determine the most stable resonance structure, which is the one that appears most frequently in the resonance hybrid due to its lower energy.

Outlines
00:00
🔍 Introduction to Formal Charge Calculation

This paragraph introduces the concept of formal charge, explaining how to calculate it using a simplified equation. The formula provided is formal charge = valence electrons - (bonds + lone pairs). The speaker illustrates the process with an example focusing on a nitrogen atom, which has two lone pairs. By applying the formula, it's determined that the nitrogen atom has a formal charge of -1. The paragraph encourages viewers to practice calculating formal charges with a series of problems to master the skill.

05:01
📚 Practice Problems: Formal Charge in Ions

The speaker presents practice problems to apply the formal charge formula to different molecules and ions, starting with the hydronium ion. The calculation for the oxygen atom in the hydronium ion results in a formal charge of +1. The next example is sulfur dioxide, where the speaker guides the audience to calculate the formal charge for each atom, resulting in +1 for sulfur, 0 for one oxygen, and -1 for the other oxygen, making the molecule neutral overall. The cyanide ion is also discussed, revealing that carbon, not nitrogen, bears the negative charge in this case. Lastly, the NH4+ ion is briefly mentioned, with nitrogen having a formal charge of +1.

10:03
🧩 Determining the Most Stable Molecular Structure

Formal charge is highlighted as a tool for identifying the most stable molecular structure. The speaker uses the example of OCN- to demonstrate this, explaining the process of determining the most stable Lewis structure by calculating the formal charges of the atoms involved, excluding carbon as it always forms four bonds. The stability of different Lewis structures is evaluated by comparing their formal charges, with the structure having the lowest and closest to zero charges being the most stable. The speaker concludes by emphasizing the importance of electronegativity in determining where to place the negative charge, using the OCN- example to illustrate that oxygen is more suitable for bearing the negative charge due to its higher electronegativity.

Mindmap
Keywords
💡Formal Charge
Formal charge is a theoretical quantity used in chemistry to help predict the most stable structure of a molecule. It is calculated by taking the number of valence electrons of an atom and subtracting the sum of the bonds and lone pairs surrounding that atom. In the video, formal charge is the central concept used to determine the charge distribution in various molecules and ions, such as in the examples of the hydronium ion and sulfur dioxide.
💡Valence Electrons
Valence electrons are the outermost electrons of an atom that are involved in chemical bonding. They play a crucial role in determining the chemical properties of an element. In the context of the video, the number of valence electrons is essential for calculating the formal charge of an atom, as seen in the examples with nitrogen and oxygen atoms.
💡Lone Pairs
Lone pairs are a pair of non-bonding electrons that occupy space around an atom. They contribute to the steric hindrance and influence the geometry of molecules. In the video, lone pairs are considered when calculating the formal charge, as they are part of the 'dots' in the formula, and they affect the stability of the molecule, as illustrated with the nitrogen atom in ammonia.
💡Bonds
Bonds in chemistry refer to the force that holds atoms together in a molecule or compound. The type and number of bonds affect the molecule's properties. In the script, the number of bonds an atom has is subtracted from the valence electrons to calculate the formal charge, which helps in understanding the molecule's stability and charge distribution.
💡Hydonium Ion
The hydronium ion is a specific example used in the video to demonstrate the calculation of formal charge. It consists of an oxygen atom with three hydrogen atoms attached and one lone pair on the oxygen. The formal charge on the oxygen atom in the hydronium ion is +1, which is calculated using the formula provided in the video.
💡Sulfur Dioxide
Sulfur dioxide (SO2) is used in the video as an example to show how to calculate the formal charge of each atom in a molecule. The molecule is neutral overall, but it contains regions of positive and negative charge due to the different formal charges on the sulfur and oxygen atoms.
💡Cyanide Ion
The cyanide ion (CN-) is an example in the video that illustrates the use of formal charge to determine which atom in a molecule bears the negative charge. In this case, the carbon atom has a formal charge of -1, indicating it is the site of the negative charge in the ion.
💡Electronegativity
Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond. It is a concept used in the video to explain why it is more stable to place a negative formal charge on oxygen rather than nitrogen due to oxygen's higher electronegativity.
💡Lewis Structure
A Lewis structure is a graphical representation of the valence electrons in a molecule or ion. It is used in the video to visualize the distribution of electrons and to calculate the formal charge of each atom, which helps in determining the most stable structure of a molecule.
💡Resonance Structures
Resonance structures are different ways of drawing the same molecule, indicating that the actual structure is a hybrid of these forms. In the video, the concept is used to explain that the most stable resonance structure of a molecule is the one with the lowest formal charges, closest to zero.
💡Energy Stability
Energy stability in the context of the video refers to the preference of a molecule to adopt a particular structure based on its lowest energy state. The formal charge is used as an indicator of this stability, with structures having lower formal charges being more stable, as explained in the video with the OCN- molecule.
Highlights

Introduction to the concept of formal charge and its calculation method.

Simplified equation for calculating formal charge: Formal charge = Valence electrons - (Bonds + Dots).

Example calculation of formal charge on a nitrogen atom with two lone pairs.

Formal charge results in nitrogen being -1, indicating a simple method for charge calculation.

Practice problems to master the calculation of formal charges.

Calculation of formal charge on the oxygen atom in the hydronium ion, resulting in a +1 charge.

Explanation of how the positive charge in the hydronium ion is due to the formal charge on oxygen.

Instructions to calculate the formal charge of every atom in sulfur dioxide (SO2).

Formal charge calculation for sulfur in SO2 results in a +1 charge.

Formal charge calculation for oxygen in SO2, with one oxygen being neutral and the other -1.

SO2 is a neutral molecule with regions of positive and negative charge due to formal charges.

Determination of which element in the cyanide ion bears the negative charge using formal charge.

Carbon in the cyanide ion has a formal charge of -1, while nitrogen is neutral.

NH4+ ion example where nitrogen has a formal charge of +1.

Use of formal charge to determine the most stable structure among different Lewis structures.

Calculation of formal charges for OCN- to identify the most stable Lewis structure.

Elimination of the least stable structure based on the highest absolute formal charge.

Preference for placing a negative charge on oxygen due to its higher electronegativity.

Explanation of the resonance hybrid and its relation to the most stable Lewis structure.

Transcripts

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Formal ChargeChemistry TutorialMolecular StabilityLewis StructuresElectronegativityChemical BondsValence ElectronsIon CalculationResonance StructuresChemical Education