Lewis Dot Structures

Summary

Professor Dave's tutorial on Lewis dot structures enlightens viewers on the representation of atoms and their valence electrons, crucial for understanding covalent bonding. The video explains how to depict atoms, their coordination sites, and the formation of covalent bonds, including single, double, and triple bonds. It also touches on formal charges and the tendency of atoms like carbon, nitrogen, oxygen, and fluorine to form specific numbers of bonds. The tutorial is designed to enhance comprehension of molecular structures and chemical bonding.

Takeaways

• 🔬 Lewis dot structures represent atoms and their valence electrons in a chemical context.
• 🌐 Atoms are depicted by their chemical symbols, with valence electrons shown as dots around them.
• 📊 Carbon, for example, has four valence electrons and is represented with four dots in its outermost shell.
• 🔄 The drawing of valence electrons follows a specific order: place one electron per coordination site before pairing them up.
• 👫 Elements in the same group have similar Lewis dot symbols due to the same number of valence electrons.
• 🤝 In Lewis dot structures for molecules, unpaired electrons from different atoms can form covalent bonds, represented as lines.
• 🔗 The number of valence electrons an atom has influences how many bonds it forms, such as carbon forming four bonds.
• 💠 Atoms can form single, double, or triple bonds, with the latter being the shortest in length.
• 🎯 Formal charges may appear in Lewis dot structures when an atom contributes a different number of electrons than its typical valence.
• 🌀 Larger atoms like phosphorus and sulfur can form more than four covalent bonds, aiming to fill their outermost shell with eight electrons.

Q & A

What is the purpose of learning how to draw Lewis dot structures?

The purpose of learning how to draw Lewis dot structures is to visually represent the valence electrons of atoms and how they form covalent bonds with other atoms, which is crucial for understanding the structure of molecules.

How are valence electrons represented in a Lewis dot structure?

Valence electrons are represented as dots around the chemical symbol of an atom. For example, carbon, which has four valence electrons, is represented with four dots around the 'C' symbol.

Why do we assume there are four coordination sites for small atoms like carbon?

We assume there are four coordination sites for small atoms like carbon because that is the number needed for such atoms to fill their outermost shell, or octet, by forming covalent bonds.

How do you draw the valence electrons for an atom like carbon?

For carbon, you first place one dot in each coordination site before pairing them up, resulting in a structure that looks like this: C:..., not like this: C::.

What is a covalent bond in the context of Lewis dot structures?

A covalent bond in Lewis dot structures is represented by a line between two atoms, indicating that unpaired electrons from each atom have combined to form a bond, which always contains two electrons.

What is a sigma bond and how does it differ from a pi bond?

A sigma bond is the first covalent bond formed between two atoms. A pi bond is the second bond in a double bond or the second and third bonds in a triple bond. Pi bonds are represented in Lewis structures as lines that form a 'sideways' bond.

Why do single, double, and triple bonds differ in length?

Single covalent bonds are the longest because they consist of one pair of electrons between the atoms. Double bonds are shorter because they have two pairs of electrons closer together, and triple bonds are the shortest due to three pairs of electrons being even closer.

What is a formal charge and how does it relate to Lewis dot structures?

A formal charge is the charge an atom appears to have in a Lewis dot structure when the number of electrons it is contributing differs from its typical valence. It is calculated by the formula: Formal charge = (Valence electrons of the atom) - (Non-bonding electrons) - (1/2 × Bonding electrons).

Why does nitrogen in ammonia have a neutral charge while in the ammonium ion it has a formal positive charge?

In ammonia (NH3), nitrogen contributes all five of its valence electrons, one per covalent bond, and has no formal charge. In the ammonium ion (NH4+), nitrogen contributes four electrons to the structure, one less than its typical valence, resulting in a formal positive charge.

How do larger atoms like phosphorus and sulfur form more than four covalent bonds?

Larger atoms like phosphorus and sulfur can have more than four valence electrons and thus can form more than four covalent bonds. They can expand their octet by utilizing d-orbitals to accommodate more than eight electrons, a phenomenon known as 'expanded octet.'

Outlines

00:00

🔬 Introduction to Lewis Dot Structures

Professor Dave introduces the concept of Lewis dot structures, explaining how they represent atoms and their valence electrons. The paragraph details the process of drawing these structures, starting with the chemical symbol for an atom and then surrounding it with dots representing valence electrons. It emphasizes the importance of placing electrons in coordination sites before pairing them up, and how elements in the same group will have similar Lewis dot symbols due to the same number of valence electrons. The paragraph also explains the formation of covalent bonds, where unpaired electrons from different atoms combine, represented by a line instead of dots, and the preference of atoms to form bonds based on their valence electron count.

Keywords

💡Lewis dot structures

Lewis dot structures are a way to visually represent the valence electrons of atoms, which are the electrons in the outermost shell that participate in chemical reactions. In the context of the video, these structures are essential for understanding how atoms form covalent bonds. The script explains that each atom is represented by its chemical symbol, and its valence electrons are depicted as dots. For example, carbon, which has four valence electrons, is represented with four dots around it.

💡Valence electrons

Valence electrons are the electrons in the outermost shell of an atom, which are involved in chemical bonding. The video script emphasizes that these electrons are crucial for forming covalent bonds, as they can be shared between atoms. The number of valence electrons determines the bonding behavior of an atom, as illustrated by the examples of carbon, nitrogen, oxygen, and fluorine, each with a different number of valence electrons.

💡Covalent bonds

Covalent bonds are a type of chemical bond formed when two atoms share one or more pairs of electrons. The video script describes how unpaired electrons from different atoms can come together to form covalent bonds, which are represented by lines in Lewis dot structures. The script further explains that single, double, and triple bonds are possible, with the latter two involving additional pi bonds.

💡Sigma bond

A sigma bond is the first covalent bond formed between two atoms, where the electron pair is shared along the axis connecting the nuclei of the two atoms. The video script mentions that when two atoms form a covalent bond, they create a sigma bond, which is the strongest type of covalent bond and is always present in double and triple bonds as well.

💡Pi bond

A pi bond is a type of covalent bond that forms when two atoms share electrons in a side-to-side overlap, resulting in a bond that lies above and below the plane of the sigma bond. The video script explains that after forming a sigma bond, if there are still unpaired electrons, they can form a pi bond, which is part of double and triple bonds.

💡Formal charge

Formal charge refers to the charge an atom appears to have in a Lewis structure when it is calculated by comparing the number of valence electrons assigned to the atom in the structure to the number it would have if it were neutral. The video script uses the example of nitrogen in ammonia and the ammonium ion to illustrate how formal charges are determined and how they affect the representation of atoms in Lewis dot structures.

💡Octet rule

The octet rule states that atoms tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The video script explains that atoms like carbon, nitrogen, and oxygen aim to fill their outermost shell by sharing, gaining, or losing electrons to achieve a stable octet.

💡Hydrogen

Hydrogen is a unique element in that it only needs two electrons to fill its outermost shell, known as the duet rule, rather than the octet. The video script points out that hydrogen's small size allows it to achieve stability with just two electrons, which is different from other elements that typically aim for an octet.

💡Phosphorus and Sulfur

Phosphorus and sulfur are mentioned in the script as examples of larger atoms that can form more than four covalent bonds. This is due to their ability to expand their valence shell to accommodate more than eight electrons, a phenomenon known as 'expanded octet.' The script suggests that these elements can form five or six covalent bonds, which is important for understanding their bonding behavior in complex molecules.

💡Polyatomic ion

A polyatomic ion is a group of two or more atoms that are bonded together and have an overall charge. The video script uses the ammonium ion as an example of a polyatomic ion, where nitrogen has a formal positive charge due to contributing fewer electrons to the Lewis structure than its typical valence would suggest.

Highlights

Introduction to drawing Lewis dot structures for covalent bonding.

Representing an atom by its chemical symbol in Lewis dot structures.

Drawing valence electrons as dots around the atom for Lewis dot structures.

Carbon's electron configuration and its representation in Lewis dot structures.

Assumption of four coordination sites for small atoms like carbon.

Procedure for drawing valence electrons, starting with unpaired electrons.

Similarity in Lewis dot symbols for elements within the same group.

Drawing Lewis dot structures for molecules by pairing unpaired electrons.

Covalent bonds represented as lines between atoms in Lewis dot structures.

Electrons in Lewis dot structures are either in covalent bonds or lone pairs.

The number of valence electrons dictates the number of bonds an atom tends to form.

Tendency of carbon to form four bonds, nitrogen three, oxygen two, and fluorine one.

Explanation of sigma and pi bonds in covalent bonding.

Length differences between single, double, and triple covalent bonds.

Formal charge in Lewis dot structures due to deviation from typical valence.

Example of nitrogen's formal charge in ammonia and ammonium ion.

Atoms' desire to fill their octet with eight electrons in Lewis dot structures.

Hydrogen's unique requirement of only two electrons to fill its shell.

Larger atoms like phosphorus and sulfur can make more than four covalent bonds.

Encouragement to practice drawing valence electrons and combining unpaired ones.

Comprehension check and invitation to subscribe for more tutorials.

Transcripts

professor Dave here, let's learn how to draw Lewis dot structures

we've learned about how ions come together to form ionic compounds but much more

interesting is the way atoms form networks of covalent bonds. in order to

talk about this we're going to have to learn how to draw Lewis dot structures

when we draw these we will represent an atom by its chemical symbol. here's

carbon. we then draw that atom's valence electrons as dots around the atom. carbon is

one 1s2 2s2 2p2 so that's four electrons in the outermost shell, which

you can also tell by counting over from the left

for carbon and other small atoms we assume that there are four coordination

sites that can accommodate electrons or bonds because this is how carbon can

fill its octet, or its

outermost shell. when we draw the valence electrons we put one in each

coordination site first before pairing them up so carbon will look like this

not like this

elements in the same group will have similar looking lewis dot symbols

because they have the same number of valence electrons. now to draw a lewis dot

structure for a molecule just draw each atom with all of its valence electrons

unpaired electrons from two different atoms can come together to make covalent

bonds. when that happens instead of two dots we make a line. this is a covalent

bond which always contains two electrons. for our purposes here we can assume that

electrons need to be paired so a good lewis dot structure will have all the

electrons either inside covalent bonds or in lone pairs. the number of valence

electrons an atom has dictates how many bonds it tends to form. carbon likes to

make four bonds. nitrogen, with five valence electrons likes to have three

bonds and one lone pair. oxygen likes to have two bonds and two lone pairs. and

fluorine likes to have one bond and three lone pairs. when two atoms have one

covalent bond between them we call that a sigma bond, but atoms can have double

or even triple bonds. look at carbon dioxide. after we make a sigma bond

between the atoms there are still unpaired electrons. if adjacent atoms

each have unpaired electrons they can become another covalent bond. the second

covalent bond in a double bond is called a pi bond. the second and third bonds in

a triple bond are also pi bonds. single covalent bonds are the longest, double

bonds are a little shorter, and triple bonds are shorter still. more on these

later. sometimes an atom in a lewis dot structure will have a formal charge. this

happens if the number of electrons an atom is contributing to a lewis dot

structure is different from its typical valence. nitrogen has five valence

electrons so in ammonia

where it contributes those 5, 1 per covalent bond plus the lone pair, it will

be a neutral nitrogen atom. for the ammonium ion, nitrogen is now

contributing four electrons to the lewis dot structure, one per bond. that's one

less than its typical valence and one fewer negatively charged particles means

the nitrogen will have a formal positive charge. that makes this a polyatomic ion

these elements want to fill their octet by having eight electrons around them

filling n=2 shell. hydrogen is very small so that only needs two to

fill the n=1 shell, and larger atoms like phosphorus and sulfur can

make five or six covalent bonds. just draw valence electrons and start

combining unpaired ones. let's check comprehension

thanks for watching, subscribe to my channel for more tutorials and as always feel free to email me