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Lewis Dot Structures

4 min readโ€ขdecember 1, 2021

S

Saarah Hasan

S

Saarah Hasan

Visual Representation of Molecules with Lewis Dot Structures.

We draw Lewis Structures to get a visual representation of the molecules we examine, helping us determine ๐ŸŽจ

  1. Whether they are polar or nonpolar

  2. The number and type of bonds they form, andย 

  3. The number of lone pairs of electrons in the molecule and where those lone pairs lie!

For example, here's the Lewis structure for water, aka H2O! ๐Ÿ’ง

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ztzAlL4lXpJv.png?alt=media&token=f8de5b09-03fa-47c1-acd3-7b6e60d99b09

Image courtesy of Wikipedia

Writing a Lewis structure for molecules of representative elements is based on the octet rule. Drawing them may get a little tricky. With some practice and understanding the directions to formatting Lewis Structures, weโ€™ll get the hang of it! ๐Ÿ˜„

Rules for Writing Lewis Structures for Covalent Compounds

Thereโ€™s one main formula we need to keep in mind: S = N - A, where

  • S = the total number of electrons shared

    • S/2 = number of bonds

  • N = the sum of the number of electrons needed for all atoms involved to get a noble gas configuration. (N will be 2 for Hydrogen, 4 for Beryllium, 6 for Boron and Aluminum, and 8 for any other atom!)

  • A = the sum of the number of valence electrons available in each of the representative elements. Donโ€™t forget to adjust this for ions!

  • U = the number of unshared electrons

Some Guidelines for a Skeleton of a Molecules

  • Choose a structure that is as symmetric as possible.

  • The least electronegative element is usually the central atom.

  • Use multiple bonds when needed as you place the number of shared electrons in the structure.

  • Place the unshared electrons as lone pairs to satisfy the octet rule for bonded atoms first! If you have any leftover, then place them on the central atom.

  • If there are more than one non-equivalent structure possible, use formal charges to choose between them.

  • Oxygens DO NOT bond to each other except in O2 and peroxides!

Formal Charges

Formal charges are hypothetical charges on an atom in a molecule. We use them when we need to determine which Lewis structure is more likely. Here are our steps to do it:

  • Assign a formal charge for each atom.

    • Formal Charge = Group Number - (Number of Bonds + Number of Unshared Electrons)

  • Choose the better structure.

    • The formal charge on each atom is at or closer to 0.

    • The negative formal charges are on the more electronegative elements!

Resonance

Resonance occurs when we have two or more Lewis structures that both accurately describe the bonding in a molecule. The true actual structure is a hybrid of the drawn structures.ย  In organic chemistry, resonance comes up a lot more than in general chemistry. ๐Ÿ’ก

Limitations of the Octet Rule

  • Radicals: Species that have an odd number of available electrons ๐Ÿ˜ฒ

  • The weird structures of covalent compounds: Elements with an incomplete octet include Beryllium, Boron, and Aluminium ๐Ÿงฉ

    • Beryllium has 2 valence electrons, so it usually only forms 2 covalent bonds. We use 4 electrons as the number of needed electrons.

    • Boron and Aluminium have 3 valence electrons, so they usually form 3 covalent bonds. We use 6 electrons as the number of needed electrons.

  • Bigger valence shells: There are a lot of compounds where the central atom has to have more than 8 electrons to accommodate all bonded atoms. ๐Ÿš

Practice

Letโ€™s do some practice with Lewis Structures! ๐Ÿ˜Œ

1 Whatโ€™s the Lewis Structure of ammonia, NH3?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-iYdzE6gCcPGH.png?alt=media&token=fc2981f3-6454-4db5-ad1d-8c9124171164

Image courtesy of Wikimedia Commons

Remember: S = N - A and U = A - Sย 

  • N: (8) + (2 x 3) = 14

  • A: (1 x 3) + 5 = 8

  • S: 14 - 8 = 6

    • Number of bonds = S/2 = 6/2 = 3

  • U: 8 - 6 = 2

    • Number of lone pairs = U/2 = 2/2 = 1

Nitrogen is our central atom, and we draw 3 bonds connecting each hydrogen to it. Since our hydrogens are fulfilled with these bonds, we place one lone pair of electrons on the nitrogen. The one lone pair on our central atom lets us know that the molecule is polar. โœ…

2 Whatโ€™s the Lewis Structure of CCl4?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-e9Zbl3ZnNzqf.png?alt=media&token=a03cde8d-6b4b-4479-87c6-f74b01d62ae4
  • N: (8) + (8 x 4) = 40

  • A: (4) + (7 x 4) = 32

  • S: 40 - 32 = 8

    • Number of bonds = S/2 = 8/2 = 4

  • U: 32 - 8 = 24

    • Number of lone pairs = U/2 = 24/2 = 12

Carbon is our central atom; we draw 4 bonds connecting each chloride to it. We add 3 lone pairs to each chloride, leaving us with no lone pairs to place on the carbon. โœ…

3 Whatโ€™s the Lewis Structure of H2O?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-jow5lh21bx9Y.png?alt=media&token=7d2696bf-55d6-4406-ad05-d635e270a190

Image courtesy of Wikipedia

  • N: (2 x 2) + 8 = 12

  • A: (1 x 2) + (6) = 8

  • S: 12 - 8 = 4

    • Number of bonds = S/2 = 4/2 = 2

  • U: 8 - 4 = 4

    • Number of lone pairs = U/2 = 4/2 = 2

In this case, oxygen is our central atom; we draw one bond for each hydrogen atom to connect them. Since hydrogen is already fulfilled with these bonds, we add our two lone pairs to oxygen. โœ…

Lewis Dot Structures

4 min readโ€ขdecember 1, 2021

S

Saarah Hasan

S

Saarah Hasan

Visual Representation of Molecules with Lewis Dot Structures.

We draw Lewis Structures to get a visual representation of the molecules we examine, helping us determine ๐ŸŽจ

  1. Whether they are polar or nonpolar

  2. The number and type of bonds they form, andย 

  3. The number of lone pairs of electrons in the molecule and where those lone pairs lie!

For example, here's the Lewis structure for water, aka H2O! ๐Ÿ’ง

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ztzAlL4lXpJv.png?alt=media&token=f8de5b09-03fa-47c1-acd3-7b6e60d99b09

Image courtesy of Wikipedia

Writing a Lewis structure for molecules of representative elements is based on the octet rule. Drawing them may get a little tricky. With some practice and understanding the directions to formatting Lewis Structures, weโ€™ll get the hang of it! ๐Ÿ˜„

Rules for Writing Lewis Structures for Covalent Compounds

Thereโ€™s one main formula we need to keep in mind: S = N - A, where

  • S = the total number of electrons shared

    • S/2 = number of bonds

  • N = the sum of the number of electrons needed for all atoms involved to get a noble gas configuration. (N will be 2 for Hydrogen, 4 for Beryllium, 6 for Boron and Aluminum, and 8 for any other atom!)

  • A = the sum of the number of valence electrons available in each of the representative elements. Donโ€™t forget to adjust this for ions!

  • U = the number of unshared electrons

Some Guidelines for a Skeleton of a Molecules

  • Choose a structure that is as symmetric as possible.

  • The least electronegative element is usually the central atom.

  • Use multiple bonds when needed as you place the number of shared electrons in the structure.

  • Place the unshared electrons as lone pairs to satisfy the octet rule for bonded atoms first! If you have any leftover, then place them on the central atom.

  • If there are more than one non-equivalent structure possible, use formal charges to choose between them.

  • Oxygens DO NOT bond to each other except in O2 and peroxides!

Formal Charges

Formal charges are hypothetical charges on an atom in a molecule. We use them when we need to determine which Lewis structure is more likely. Here are our steps to do it:

  • Assign a formal charge for each atom.

    • Formal Charge = Group Number - (Number of Bonds + Number of Unshared Electrons)

  • Choose the better structure.

    • The formal charge on each atom is at or closer to 0.

    • The negative formal charges are on the more electronegative elements!

Resonance

Resonance occurs when we have two or more Lewis structures that both accurately describe the bonding in a molecule. The true actual structure is a hybrid of the drawn structures.ย  In organic chemistry, resonance comes up a lot more than in general chemistry. ๐Ÿ’ก

Limitations of the Octet Rule

  • Radicals: Species that have an odd number of available electrons ๐Ÿ˜ฒ

  • The weird structures of covalent compounds: Elements with an incomplete octet include Beryllium, Boron, and Aluminium ๐Ÿงฉ

    • Beryllium has 2 valence electrons, so it usually only forms 2 covalent bonds. We use 4 electrons as the number of needed electrons.

    • Boron and Aluminium have 3 valence electrons, so they usually form 3 covalent bonds. We use 6 electrons as the number of needed electrons.

  • Bigger valence shells: There are a lot of compounds where the central atom has to have more than 8 electrons to accommodate all bonded atoms. ๐Ÿš

Practice

Letโ€™s do some practice with Lewis Structures! ๐Ÿ˜Œ

1 Whatโ€™s the Lewis Structure of ammonia, NH3?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-iYdzE6gCcPGH.png?alt=media&token=fc2981f3-6454-4db5-ad1d-8c9124171164

Image courtesy of Wikimedia Commons

Remember: S = N - A and U = A - Sย 

  • N: (8) + (2 x 3) = 14

  • A: (1 x 3) + 5 = 8

  • S: 14 - 8 = 6

    • Number of bonds = S/2 = 6/2 = 3

  • U: 8 - 6 = 2

    • Number of lone pairs = U/2 = 2/2 = 1

Nitrogen is our central atom, and we draw 3 bonds connecting each hydrogen to it. Since our hydrogens are fulfilled with these bonds, we place one lone pair of electrons on the nitrogen. The one lone pair on our central atom lets us know that the molecule is polar. โœ…

2 Whatโ€™s the Lewis Structure of CCl4?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-e9Zbl3ZnNzqf.png?alt=media&token=a03cde8d-6b4b-4479-87c6-f74b01d62ae4
  • N: (8) + (8 x 4) = 40

  • A: (4) + (7 x 4) = 32

  • S: 40 - 32 = 8

    • Number of bonds = S/2 = 8/2 = 4

  • U: 32 - 8 = 24

    • Number of lone pairs = U/2 = 24/2 = 12

Carbon is our central atom; we draw 4 bonds connecting each chloride to it. We add 3 lone pairs to each chloride, leaving us with no lone pairs to place on the carbon. โœ…

3 Whatโ€™s the Lewis Structure of H2O?

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-jow5lh21bx9Y.png?alt=media&token=7d2696bf-55d6-4406-ad05-d635e270a190

Image courtesy of Wikipedia

  • N: (2 x 2) + 8 = 12

  • A: (1 x 2) + (6) = 8

  • S: 12 - 8 = 4

    • Number of bonds = S/2 = 4/2 = 2

  • U: 8 - 4 = 4

    • Number of lone pairs = U/2 = 4/2 = 2

In this case, oxygen is our central atom; we draw one bond for each hydrogen atom to connect them. Since hydrogen is already fulfilled with these bonds, we add our two lone pairs to oxygen. โœ…



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APยฎ and SATยฎ are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


ยฉ 2024 Fiveable Inc. All rights reserved.

APยฎ and SATยฎ are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.