How To Identify The Number of Lone Pairs on an Atom Using Formal Charge

The Organic Chemistry Tutor
30 Jan 202315:14
EducationalLearning
32 Likes 10 Comments

TLDRThis video script delves into the complexities of determining the number of lone pairs on atoms in various chemical structures. It explains the general rule of octet fulfillment, exceptions like expanded or incomplete octets, and the use of a formula to calculate lone pairs for atoms with unusual electron configurations. The script provides step-by-step examples for elements like carbon, oxygen, sulfur, and nitrogen, illustrating how to apply the formula to atoms with different charges and bonds, including free radicals. It concludes with an unusual case of NO2, highlighting the identification of radicals through an odd number of electrons.

Takeaways
  • 🧲 The general rule is to add lone pairs to atoms until they have a full octet of eight electrons, except in cases of expanded or incomplete octets.
  • 🚫 Second-row elements like carbon, nitrogen, oxygen, and fluorine cannot have an expanded octet, while third-row elements like phosphorus, sulfur, and chlorine can.
  • 🔢 To determine the number of lone pairs, use the formula: Number of dots = Valence electrons of the free atom - Formal charge - Number of bonds.
  • ⚠️ Carbon with a positive charge (carbocation) will have an incomplete octet, as it has no lone pairs according to the formula.
  • 🌐 Boron, with no formal charge and three bonds, also has an incomplete octet and no lone pairs.
  • 🔄 For atoms with a negative charge, calculate the number of lone pairs by using the formula and considering the extra electron(s).
  • 💧 In water, oxygen has two lone pairs to complete its octet, while in hydroxide (OH-), it has three lone pairs due to the negative charge.
  • 📉 Sulfur can have more than eight electrons in some cases, but typically aims for eight, as shown when calculating lone pairs.
  • 🔄 The azide ion (N3-) has different resonance forms, and the formula helps determine the number of lone pairs on nitrogen in each form.
  • 🌀 NO2 is an example of a molecule where the nitrogen atom does not have a full octet but is part of a free radical due to an odd number of unpaired electrons.
  • 📚 The formula provided is essential for calculating the number of lone pairs on atoms with incomplete or expanded octets, and for identifying free radicals.
Q & A

  • What is the general rule for determining the number of lone pairs on an atom?

    -The general rule is to add lone pairs until the atom has eight electrons, which is known as achieving an octet.

  • What is an expanded octet and when does it occur?

    -An expanded octet refers to an atom having more than eight electrons. It occurs in atoms of elements in the third period or below, such as phosphorus, sulfur, and chlorine.

  • Why can't elements in the second row of the periodic table have an expanded octet?

    -Elements in the second row, like carbon, nitrogen, oxygen, and fluorine, can't have an expanded octet because they are limited to having no more than eight electrons due to their electronic configuration.

  • What is an incomplete octet and when does it occur?

    -An incomplete octet is when an atom has less than eight electrons. It can occur in certain molecules or ions where the atom does not form enough bonds to reach an octet.

  • How can you determine the number of lone pairs on an atom with a formal charge?

    -You can use the formula: Number of dots (lone pairs) = Valence electrons of the free atom - Formal charge - Number of bonds. This formula helps in calculating the number of lone pairs for atoms with incomplete or expanded octets.

  • Why does the carbon atom with a positive charge in the script not have any lone pairs?

    -The carbon atom with a positive charge (carbocation) does not have any lone pairs because the formula calculation results in zero, indicating it has an incomplete octet with only four electrons.

  • What does it mean when the formula results in an odd number of dots for an atom?

    -An odd number of dots indicates the presence of a free radical on the atom, as it cannot form a complete lone pair.

  • How many lone pairs should be on a carbon atom with three bonds and a negative formal charge?

    -The carbon atom should have one lone pair, as the formula calculation results in two dots, which equates to one lone pair.

  • What is the significance of the number of dots on an oxygen atom in a molecule?

    -The number of dots on an oxygen atom helps to determine if it has achieved its preferred electron configuration of eight electrons, which is essential for understanding the molecule's stability and reactivity.

  • How does the formal charge of an atom affect the number of lone pairs it has?

    -The formal charge of an atom directly influences the calculation of the number of lone pairs. A positive formal charge decreases the number of lone pairs, while a negative formal charge increases it.

  • Can you explain the concept of radicals in the context of the formula for calculating lone pairs?

    -Radicals are formed when an atom has an odd number of electrons, which means it cannot form complete lone pairs. This is indicated by an odd result from the lone pair calculation formula.

Outlines
00:00
🔍 Understanding Lone Pairs and Octet Expansion

This paragraph introduces the concept of determining the number of lone pairs on atoms in various chemical structures. It explains the general rule of adding lone pairs until an atom achieves an octet, with exceptions for expanded or incomplete octets. Second-row elements like carbon, nitrogen, oxygen, and fluorine typically cannot have more than eight electrons, whereas elements like phosphorus, sulfur, and chlorine can have expanded octets. The paragraph also discusses how to calculate the number of lone pairs using a formula that considers valence electrons, formal charge, and the number of bonds.

05:01
📚 Calculating Lone Pairs with Formal Charge

The second paragraph delves into the specifics of calculating the number of lone pairs on atoms with different formal charges. It uses examples of carbon, boron, and sulfur atoms to illustrate the formula: number of dots equals valence electrons minus formal charge plus bonds. This formula helps determine whether an atom has an incomplete, normal, or expanded octet. The paragraph clarifies that second-row elements cannot have an expanded octet, while others might.

10:03
🧪 Applying the Formula to Various Atoms and Ions

This paragraph continues the discussion on calculating lone pairs by applying the formula to nitrogen atoms in different chemical environments, including the azide ion and NO2. It explains how to determine the number of lone pairs based on the atom's valence electrons, formal charge, and the number of bonds it forms. The paragraph also touches on the concept of free radicals, which occur when an atom has an odd number of unpaired electrons.

15:05
🎓 Summary of Lone Pair Calculation Techniques

The final paragraph summarizes the process of calculating the number of lone pairs on atoms, reinforcing the method introduced earlier in the script. It emphasizes the importance of understanding the atom's valence electrons, formal charge, and bonding to accurately determine the presence of lone pairs, whether complete or as part of a free radical.

Mindmap
Keywords
💡Lone Pairs
Lone pairs refer to a pair of non-bonding electrons that occupy a single orbital on an atom. In the context of the video, the concept is central to understanding molecular geometry and the electron distribution around atoms. For example, the script discusses how to determine the number of lone pairs on atoms like carbon and oxygen based on their valence electrons and bonding.
💡Octet Rule
The octet rule is a chemical rule of thumb that states atoms tend to form enough bonds to have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. The video explains how this rule is generally followed but also discusses exceptions, such as expanded and incomplete octets.
💡Expanded Octet
An expanded octet refers to a situation where an atom has more than eight electrons in its valence shell. The video mentions that elements in the third period of the periodic table, like phosphorus, sulfur, and chlorine, can have an expanded octet, unlike second-row elements like carbon, nitrogen, and oxygen.
💡Incomplete Octet
An incomplete octet is when an atom has fewer than eight electrons in its valence shell. The script uses the example of boron, which has three valence electrons and forms three bonds, resulting in no lone pairs and an incomplete octet.
💡Valence Electrons
Valence electrons are the electrons located in the outermost shell of an atom and are involved in chemical bonding. The video script explains how the number of valence electrons, combined with the atom's formal charge and the number of bonds, determines the number of lone pairs.
💡Formal Charge
Formal charge is the charge assigned to an atom in a molecule based on the number of electrons assigned to that atom in the Lewis structure. The video uses formal charge in its formula to calculate the number of lone pairs, as seen with the carbon atom with a positive charge and the oxygen atom with a negative charge.
💡Calcogen
Calcogens are elements in group 6A of the periodic table, including oxygen and sulfur. The video script discusses how these elements typically have six valence electrons and may have different numbers of lone pairs depending on their bonding and formal charge.
💡Carbocation
A carbocation is a carbon atom with a positive charge, typically due to the loss of a hydride ion or a pair of electrons. The video explains how to calculate the number of lone pairs on a carbocation using the formula provided, as demonstrated with the carbon atom having three bonds and a positive charge.
💡Resonance
Resonance in chemistry refers to the phenomenon where a molecule can be represented by two or more Lewis structures, known as resonance structures. The video script uses the azide ion as an example, showing different resonance forms and how to calculate lone pairs for each nitrogen atom in these structures.
💡Free Radical
A free radical is a molecule with an unpaired electron, often indicated by an odd number of electrons around an atom. The video script explains that if the formula results in an odd number of dots, the atom is a free radical, as seen with the nitrogen atom in NO2.
Highlights

Determining the number of lone pairs on an atom requires adding lone pairs until the atom has eight electrons in most cases.

Atoms can have an expanded octet with more than eight electrons or an incomplete octet with less than eight.

Second-row elements like carbon, nitrogen, oxygen, and fluorine cannot have an expanded octet.

Third-row elements like phosphorus, sulfur, and chlorine can have an expanded octet.

Oxygen typically has three bonds and one lone pair to achieve eight electrons.

Sulfur can have more than eight electrons and may have three lone pairs with one bond.

A carbon with three bonds and one hydrogen atom intuitively has one lone pair for eight electrons.

A carbon with three bonds and no hydrogen atoms might not follow the eight-electron rule due to charge.

The formula for calculating lone pairs is valence electrons minus formal charge plus bonds equals lone pairs.

A carbocation with three bonds and a positive charge has no lone pairs, resulting in an incomplete octet.

Boron with three bonds and no formal charge has no lone pairs, indicating an incomplete octet.

A carbon with three bonds and a negative charge has one lone pair, aiming for eight electrons.

Oxygen with a positive formal charge and three bonds needs one lone pair to complete its octet.

Sulfur with a negative formal charge and one bond has three lone pairs, indicating an expanded octet.

The formula helps determine the number of lone pairs or dots on an atom, crucial for understanding molecular structures.

In hydroxide, oxygen with a negative charge has three lone pairs, totaling six electrons around oxygen.

The azide ion demonstrates different resonance forms affecting the number of lone pairs on nitrogen atoms.

NO2 provides an example of a molecule where nitrogen does not have a complete octet due to its radical nature.

The odd number of electrons on nitrogen in NO2 indicates a free radical, not a complete octet.

Transcripts

Browse More Related Video

Lewis Structures and Formal Charges Practice Problems | Study Chemistry With Us

How To Calculate The Formal Charge of an Atom - Chemistry

Resonance Structures, Basic Introduction - How To Draw The Resonance Hybrid, Chemistry

Aromaticity and Huckel's Rule

2.2 Drawing Line Angle Structures (aka Bond Line Structures) | Organic Chemistry

1.2 Formal Charges | Organic Chemistry

Rate This

5.0 / 5 (0 votes)

Thanks for rating:

Related Tags
ChemistryLone PairsOctet RuleCarbonOxygenElectronsMolecular StructureCalcogenCarbocationFree RadicalEducational