The Octet Rule & Lewis Dot Structures

Lewis Dot Structures or Electron Dot Structures are diagrams that show how elements in a molecule use their valence electrons to form bonds. 

Lewis Dot Symbols

Concept: Understanding Lewis Dot Symbols

1m
Video Transcript

We already discussed that the two major forms of chemical bonding that we're going to cover are ironic bonding and covalent bonding. Before we can move on to those bonding models, it's important for us to be able to draw the electron dot symbol of an element when it's based on only its number of valence electrons.
First we're going to say another name for the electron dot symbol is the Lewis dot symbol. We're going to say in the Lewis dot symbol, the electron symbol represents the nucleus and the inner electrons. Remember, we’ve talked about inner or core electrons. The surrounding dots represent the valence electrons. For this first example, which is pretty basic, we just have to draw the electron dot symbol for each of the following elements.
Remember, we’ve said this before as well. Major group elements, groups 1A to 8A. The number of valence electrons that they possess is equal to their group number.

For Main Group Elements, the number of valence electrons is based on the group number of that element. 

Example: Draw the electron-dot symbol for each of the following elements. 

1A            2A             3A            4A              5A            6A          7A            8A

Li          Be          B           C           N          O          F         Ne

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Problem: Draw the electron-dot symbol for the following ion.

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Problem: Draw the electron-dot symbol for the following ion.

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Problem: Draw the electron-dot symbol for the following ion.

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Drawing Lewis Dot Structures

Concept: Rules for Drawing Molecular Compounds 

7m
Video Transcript

Welcome back, guys! In this new video, we’re going to look at some basic rules that we can follow in order to draw a Lewis dot structure. Just realize that I know it might seem complicated right now like your professor is just throwing elements on the board, writing bonds and throwing on some dots. But if we follow these rules that I’ve built up and made from looking at various examples, you'll be able to be better prepared when it comes to drawing a Lewis dot structure.
Here if we take a look at the first rule, it says rule number one: Least electronegative element goes in the center. We're going to say remember, electronegativity, when it comes to our periodic table, it increases going from left to right. and it increases going from the bottom up to the top of any group. Remember with chemistry, there's one thing you can always depend on in chemistry. There's always an exception to something you come up with. We say that the least electronegative element goes in the center but that's not always the case. When it comes to hydrogen and fluorine, we're going to say they never go in the center, never ever go in the center. They only make one bond. That’s a huge rule to remember.
Sometimes, hydrogen may be the least electronegative element but we can never put it in the center. Hydrogen only wants to make one bond because every time you make a bond, you gain one electron. Hydrogen on our periodic table has an atomic number of 1. If it gains one more electron, it’s going to have an atomic number of 2 and become just like helium. It doesn't need to gain more than one more electron, so that's why it only makes one bond. Fluorine, fluorine doesn't want to make more than one bond. It only needs one electron to become just like Neon, so that's why it only makes one bond. Just remember this big rule here.
Next we’re going to say rule two: The number of valence electrons that an element has is equal to its group number. You're not going to see bonds between transition metals so don't worry about that. Next, we’re going to say carbon must make four bonds. If we take a look at the periodic table, and this one’s kind of incomplete but neon is right here. Just realize, how far is carbon away from being a noble gas? It’s four spaces away – 1, 2, 3, 4. Every time we make a bond in Lewis dot structure, we pick up an electron. Carbon would need to make four bonds in order to gain four more electrons to become just like neon. That's why we say carbon must make four bonds. But there are rare occasions where it doesn't make four bonds. In those rare occasions, it's going to make three bonds.
We’re going to say if carbon were positive or negative, then it would make three bonds. Next, we're going to say nitrogen likes to make how many bonds? Nitrogen is three spaces away from being a noble gas. Therefore, nitrogen wants to make three bonds. Every bond it makes, it picks up an electron. Nitrogen likes to make three. Based on that trend, we can say that oxygen likes to make two bonds. It’s two spaces away from being a noble gas so it wants to make two bonds so that it picks up two electrons, so it has the same number of electrons just like neon.
Then we’re going to say halogens. They’re group 7A elements. When they are not in the center of our Lewis dot structure, they only make one bond. Remember, fluorine can never go in the center. But the other halogens – bromine, chlorine, and iodine – we could put them in the center.
The last and final rule. This is called the expanded valence shell theory. We said earlier that nonmetals when they form covalent compounds want to follow the octet rule. The octet rule says that that central element wants eight electrons around it, eight valence electrons around it. But here's the thing. That is not always the case. We get into expanded valence shell theory territory. In this case, we’re going to say that non-metal starting from period 3 down two period 7 can have more than eight valence electrons around them. They actually use their D shells, their D orbitals to compensate the extra electrons they’re picking up. Remember, hydrogen is here. This is period 1, 2, 3, 4, 5, 6. There’s also francium down here which will be 7. We’re saying period 3 nonmetals, so phosphorus, sulfur, chlorine and lower all the way down to 7, so selenium, bromine, iodine and of course, the noble gases as well.
We're going to say here when it comes to the noble gases making bonds, we’re usually only going to see Krypton and lower making bonds. Because they're larger, they’re able to form bonds with other elements. Neon, helium, and argon are higher up. They’re too stable to want to form any type of bonds with anyone. We usually don't see any bonds forming between helium, neon, and argon. But the bottom three, you can see bonds where they're forming connections to other elements.
Again, remember these rules. They could save you on the exam. This is just a composite of different rules and patterns that I've noticed from reading various books. I just condensed it as far as I could to seven basic rules. Learn these basic rules. It will help you a lot when it comes to drawing different types of Lewis dot structure.

Incomplete Octets

Concept: Understanding Incomplete Octets

2m
Video Transcript

We say that nonmetals form covalent bonds. Covalent bonds have the sharing of electrons between different nonmetals or the same nonmetals. We're going to say, generally, they follow the octet rule. The octet rule is when the element is surrounded by eight valence electrons, the central element in our compound. This isn't always the case. Sometimes we can have the central element having less than eight valence electrons around it or more than eight valence electrons around it.
We're going to say sometimes elements form compounds in which they have less than eight valence electrons. When they make less than eight valence electrons around them, they're called incomplete octets or they are electron deficient. Just remember, electron deficient is the same thing as having an incomplete. Meaning you have less than eight valence electrons around you.

An incomplete octet occurs when the central element in a Lewis Dot Structure is surrounded by less than 8 valence electrons. 

Example: Draw the following molecular compound. 

BH3

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Problem: Draw the following molecular compound. BeCl2

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Expanded Octets

Concept: Understanding the Expanded Octet

1m
Video Transcript

Remember, we say that covalent bonds are the connections between non-metals, they tend to share electrons with one another in order to achieve the octet rule where the central element has 8 valence electrons around it. But remember, certain elements can have less than 8 or more than 8 electrons. When we talk about the expanded octet, we mean that the central element will have more than 8 electrons around it. Now we're going to say nonmetals starting from period 3 to period 7 can have more than 8 valence electrons around them when in the center. 

An expanded octet occurs when the central element in a Lewis Dot Structure is surrounded by more than 8 valence electrons. 

Example: Draw each of the following molecular compounds. 

IF3                                                

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Example: Draw each of the following molecular compounds. 

KrF5+                                                

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Problem: Draw the following molecular compound. SBr4

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Problem: Draw the following molecular compound, the triiodide ion, I3-.

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Drawing Polyatomic Ions

Concept: Shortcut to Drawing Polyatomic Ions 

2m
Video Transcript

When it comes to drawing Lewis dot structures, polyatomic ions are considered some of the hardest types to draw. The good thing is from all the years of looking at different structures, I've noticed a pattern. From this pattern, I've come up with a shortcut. We’re going to use this shortcut anytime we have to draw complex polyatomic ion. The good thing is it covers a majority of the polyatomic ions that you’re used to seeing.
We're going to say if you have carbon, sulfur, selenium, phosphorus, halogen, or a noble gas and they're connected to oxygen. One of these guys will be connected to oxygen. Then the negative charge tells you how many oxygens are single bonded. The remaining oxygens will be double bonded to the central element.
What the heck does that mean? Let's take a look at the first example and you'll see what I mean by this shortcut.

Example: Draw the following molecular compound.

SO42­-     

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Example: Draw the following molecular compound.

PO4­3–     

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Example: Draw the following molecular compound.

H2SO4  

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Problem: Draw the following molecular compound, selenate ion.

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Problem: Draw the following molecular compound.

XeO64-

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The Octet Rule & Lewis Dot Structures Additional Practice Problems

How many lone pairs are on the central atom in BCl 3? Express your answer numerically as an integer.

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Give the proper Lewis structure of the following molecule/ion. Show the formal charge of any atom with a non-zero formal charge.

Iodine pentachloride

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Which of the following is the correct Lewis structure for CS 2?

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Which of the following are possible Lewis structures for C 2H6O?

A. 1 only

B. 2 only

C. 3 only

D. 1 and 2

E. 2 and 3

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How many dots should be included in the Lewis structure of SeCl  42+?

(A) 32

(B) 34

(C) 36

(D) 100

(E) 102

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Chemists use Lewis dot structures primarily to help visualize:

(A) protons in ionically bonded compounds

(B) protons in covalently bonded compounds

(C) electrons in ionically bonded compounds

(D) electrons in covalently bonded compounds

(E) pretty pictures

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How many "dots" would be drawn in the Lewis structure of KrF 22+?

(A) 14

(B) 12

(C) 20

(D) 22

(E) 24

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How many valence electrons would be drawn in the Lewis structure of SiCl 4?

(A) 32

(B) 82

(C) 5

(D) 160

(E) 28

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Consider the following molecules

I. BCl3        II. CHBr3 (C is the central atom)      III. Cl2        IV. XeBr2          V. CO

These molecules do not follow the octet rule.

  1. I, IV
  2. I, III, IV, VI
  3. III, V, VI
  4. I, IV, VI
  5. I, II, IV, VI
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Which of the following species is a radical?

  1. CH2O
  2. HCN
  3. HClO
  4. ClONO2
  5. ClO
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Which of the following is the correct Lewis formula for formaldehyde (CH  2O)?

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In the Lewis structure for ICI2-,how many lone pairs of electrons are on the central iodine atom.

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Consider the following set of molecules:

I). BF3      II). CHBr3 (central atom is C)     III). Br2      IV). XeCl2       V). CO      VI). SF4

Select the group that best fits the following statement: These molecules violate the octet rule.

a) I, III, IV, VI

b) I, IV, VI

c) I, II, IV

d) III, V, VI

e) I, II, IV, VI

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In the nitrite ion, how many total valence electrons are used for all three atoms? HINT: nitrite ion, not nitrate ion.

a. 16 valence electrons are used

b. 26 valence electrons are used

c. 18 valence electrons are used

d. 25 valence electrons are used

e. 17 valence electrons are used

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In the spaces below draw a Lewis dot diagram showing the presence of all electrons in the molecule or ion. All of these obey the rule that there are eight electrons around each atom. There may be more than one possible structure but you should draw only one configuration but draw it clearly placing your final answer in the box.

Also indicate the formal charge on each atom. You may use a line ( —) to represent a two electron chemical bond, you may use( • or o or x) to represent an electron but your drawings must be clear and definite.

 

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Which of the following molecules does NOT contain an exception to the octet rule?

A. SF4

B. NO

C. BH3

D. XeF2

E. N2O

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Draw the Lewis electron dot diagrams for the following chemical species and include all reasonable resonance diagrams, if any. Hint: Minimize formal charge without breaking the Octet rule unless the atom is in period 3 or higher.

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Which of the following have their valence electrons in the same shell?

a) Mg, Al, Cl

b) B, Si, As

c) N, As, Bi

d) He, Ne, F

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Which of the following statements is/are true?

i. B in BF3 violates the octet rule

ii. Xe in XeF2 violates the octet rule

iii. S in SF2 violates the octet rule.

a. i only

b. iii only

c. i and ii

d. i and iii

e. ii and iii

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Choose the best Lewis structure for ICI5.

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How many valence electrons does an atom of arsenic have?

a) 3

b) 33

c) 15

d) 5

e) 8

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Draw the best dot structure you can for HNNH, where the atoms are connected as written. 

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Which one does not have a completely full quantum level?

(A) Na+

(B) O2

(C) O2-

(D) He

(E) The quantum levels are completely full in all of the above.

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Write Lewis dot structures for the following molecule (note: you must indicate the total number of valence electrons in the molecule and show all bonding pairs and lone pairs in the structure).

 AsCl3

 

 

 

 

 

 

 

 

 

 

 

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Assuming the octet rule is obeyed, how many covalent bonds will a nitrogen atom form to give a formal charge of zero?

1) 1

2) 2

3) 3

4) 4

5) 5

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Write the Lewis Dot structures for the following molecule (note: the first atom is the central atom).  Be sure to give the total number of valence electrons and show all bonding and lone pair electrons.

 CH2Cl2

 

 

 

 

 

 

 

 

 

 

 

 

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White phosphorous is composed of molecules of P4 in which each P atom is connected to three other P atoms in the shape of a tetrahedron. Does it obey the octet rule?

1. No

2. Yes

3. Unable to determine

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