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Week 2: Lewis Structure

Simple bonding theory • Lewis structure • Shapes of structure • Resonance • Formal charge

2

Dot Structures: valence electrons and the periodic table

Dot Structures: closed shell elements He

2

Ne

2 8

Kr 2, 8, 8, 8, 18

Xe 2, 8, 8, 18, 18

2

Ar

8 8

Rn 2, 8, 8, 18, 18, 32

Step 1: Lewis Structure by Bond Determination Determination of valence electrons for all elements. (Connectivity) From the chemical formula, determine the atom connectivity for the structure. • Given a chemical formula, ABn, A is the central atom and B flanks the A atom. i.e., NH3, NCl3, NO2. In these examples, N is central in the structure. In general, the central atom is the first atom in the chemical formula. • Except when the first atom in the chemical formula is Hydrogen (H) or fluorine (F).

How to choose a central atom for Lewis diagram? • Almost always the least electronegative atom is the central atom. • For example, in ClO2, the Cl is the central atom; in SF5 the S is the central atom. • Occasionally, you will need to choose the unique atom, even when it is the most electronegative: e.g., the O in Cl2O. • A wrong choice usually will be signaled by your being unable to write a valid structure. http://chemistry.umeche.maine.edu/CHY251/therules.html

6

Find the central atom 1. 2. 3. 4. 5. 6.

CH2ClF SeCl2 O3 CO2 NH3 PO43-

Find the central atom 1. H2O 2. PCl3 3. SO3 4. CO325. BeH2 6. IO3-

Atomic connectivity The atomic arrangement for a molecule is usually given. H2Se H2SO4 CH2ClF HNO3 CH3COOH Cl O N O O H C F H H

H O H CC H O H

O H Se H H O S O H O

O3 OOO

Step 2: Lewis Structure by Bond Determination (# of electrons in Bonding) Determine the number of bonds in the compound, by calculating the theoretical Octet electrons (Oe) minus the total valence electrons (TVe) and dividing by 2 (2 electrons = 1 bond). Oe is the theoretical number of electrons necessary for each atom in the structure to obtain a Noble Gas electron configuration, while TVe is the actual number of total valence electron for each atom in the structure.

Octet rule When compounds are formed, they tend to follow the Octet Rule. Octet Rule: Atoms will share e- until it is eight valence electrons. • Octet rule works mostly for second period elements. • H prefers 2 e- (electron deficient)

Do you know the valence electron for these elements? • Sulfur • Nitrogen • Chlorine • Beryllium • Calcium • Potassium • Carbon

• Oxygen • Aluminium • Boron • Neon • Fluorine • Magnesium • Phosphorus

Valence electrons • Recall that the valence electrons for the elements can be determined based on the elements position on the periodic table.

Lewis Dot Symbol

How many bonds will they form? • Sulfur • Nitrogen • Chlorine • Beryllium • Calcium • Potassium • Carbon

• Oxygen • Aluminium • Boron • Neon • Fluorine • Magnesium • Phosphorus

The number of bonds • Number of bonds depends on the number of valence electrons.



F am ily H a lo g e n s F , B r, C l , I C a lc o g e n s O, S N itr o g e n N, P C a rb o n C, Si

X O N C

   

# C o v a l e n t B o n d s*

1 bond

often

2 bond often 3 bond often 4 bond

always

Step 3: Lewis Structure by Bond Determination (Remaining e-) Calculate the number of remaining electrons in the compound by taking the total valence electron (TVe) minus the number of electrons that was used to form bonds. Complete the Lewis structure by drawing atomic connectivity. Write bonds in the structure and the place remaining electrons to selected atoms in the structure to give each atom an octet. Keep in mind that the H-atom is satisfied with 2 electrons.

Bond table Setting up the bond table requires the chemical formula, and determining the number of electrons around each atom. Chemical Formula I.e., HNO3 Oe - Octet Electrons (either 8 or 2) and Tve - Total Valence Electron. HNO3

Octet e-

Total valence e-

H N 3O

1x2=2 1x8=8 3 x 8 = 24 34

1x1=1 1x5=5 3 x 6 = 18 24

Calculating the Number of Bonds and the Remaining electrons After setting up the bond table, calculate the number of bonds in the chemical specie and the number of electrons. The remaining electrons are place around the atoms in the chemical specie such that each atom obeys the octet rule HNO3

# Bonding e= 10

Bond Table

Oe Tve 34 – 24

# of Bonds

# bonding e-/2 = 10/2 = 5

Remaining e-

Tve – # bonding e- = 24 – 10 = 14

Combination of all 1) Chemical formula: HNO3

2) Atomic sequence:

O N O O H

3) # bonds = 5 and Remaining e- = 14 4) Lewis Structure with 5 bonds: O

N

O

O H

O

N

5) Complete Lewis Structure with 14 remaining

O

electrons

H

O

Using bond table, draw the lewis structure of CO2 and ClO4CO2

Octet e-

Valence e-

ClO4-

1C

1Cl

2O

4O

Total

Total

Octet e-

Valence e-

• Calculate Octet electrons (Oe-) and Total Valence electrons to determine number of bonds CO2 1C 2O Chg

Oe 1•(8)= 8 2•(8)=16 24

• Calculate the number of bonds in compound structure. • # bonds = (Oe - TVe) = (24- 16) = 8 = 4 bonds • 2 2 2 • • Calculate the remaining electrons to add to structure to complete Lewis dot structure. • Remaining e- = TVe - e- used in bonding. • = 16 - 8 = 8 e-Remaining

O

C

O

1,2. Write atom connectivity for CO2.

O

C

O

3,4,5. Draw the four bonds in the structure.

6. Place the remaining 8 electrons in the structure to complete the Lewis Structure

TVe 1•(4) = 4 2•(6)=12 16

ClO41 Cl 4O Chg

Oe 1•(8)= 8 4•(8)= 32 40

TVe 1•(7) = 7 4•(6)= 24 1 32

• Number of Bonds. # bonds =

(40- 32) = 8 = 4 bonds 2 2

• Remaining electrons. Remaining e- = 32 - 8 = 24 e-Remaining

Alternative ways

Step 1

Molecular formula

Draw the individual atoms/ions - with their valence electrons. H has 1, Carbon has 4, N has 5, O has 6, etc. Determine the total number of valence electrons. Account for the net charge. Make the molecule. Place the atom with Step 2 lowest EN in center (CO2, SO4 ).

Atom placement

Step 3

Make single bonds

Make the bonds. Add 2 e- to each bond Follow the octet rule. Add Step 4 remaining e- (double, triple bonds, account for net charge)

Impose octet rule

Step 5

to C, N, O, F, Lewis structure

Check the formal and total charges, and octet rule.

Example: Lewis Dot Structure NF3

NF3

Example: Lewis Dot Structure NF3 Molecular formula count valence eAtom placement Add in valence edone

NF3

Example: Lewis Dot Structure NF3 Molecular formula count valence eAtom placement Add in valence edone

NF3

Lewis Diagram from Gilbert.N. Lewis, 1916 • Creating Lewis diagrams is simple that only account for the valence electrons on each atom. • Valence electrons are represented as dots. • When two electrons are paired (lone pairs), they are represented by two adjacent dots located on an atom, and when two paired electrons are shared between atoms (bonds), they are shown as lines. • The drawing of Lewis electron-dot structures is guided largely by the octet rule: that atoms form bonds to achieve eight electrons in their valence shell.

• Exception to this rule is the first row elements, H and He. These two elements have n=1 as their valence shell, and so they have only two electrons in a full valence shell. • Although H and He are exceptions to the "octet rule", they still form bonds to achieve a full valence shell.

Exception: 1. H and He can have only 1 and 2 e respectively. 2. Lithium (atomic number 3) • He is the closest nobel gas. • Meaning Li will tend to lose 1 electron to have configuration like He thus becoming ion (Li+). 3. Stable with less than octet 4. More than octet (atom in period 3 and above) 30

Can you draw the lewis structure of BeCl2 and BeCl3?

Answers:

• Molecules that do not have complete octets are called electron deficient. These molecule usually react with species containing lone pairs, acquiring octets by formation of coordinate covalent bonds.

Can you draw the Lewis structure of

ClF3?

The heavier p-block elements : Expanded shell

Can you draw a Lewis structure yet maintain octet rule for ClF3 ?

Can? or cannot?

What do you think?

Inserting the next fluoride ion

Exercise • Draw the Lewis structures for H2O, CO2, and N2. Both the atoms and molecules.

If I say that CO2 and N2 have resonance structure, can you draw them out? 41

Resonance • Describes possible ways of arranging valence electron in Lewis structure. • Contributing structures are called resonance contributors. • Actual structures are called resonance hybrid. • The overall electronic energy of a molecule is lowered and stabilized if the molecule has several resonance structure.

Look back at the resonance structure of CO2 and N2, which one is best to represent a structure?

A

B

C

X

Y

Z 44

Can you draw all the resonance structure of carbonate ion?

45

So, which one is best to represent carbonate ion?

Resonance contributors

Resonance hybrid 47...