Lewis Structures

Valence electrons for Elements Recall that the valence electrons for the elements can be determined based on the elements position on the periodic table. Lewis Dot Symbol

Valence electrons and number of bonds Number of bonds elements prefers depending on the

number of valence electrons. In general -

X

F a m i l y → # C o v a l e n t B o n d s*

H a l o g e n s

F , B r , C l , I

C a l c o g e n s

O , S

N i t r o g e n

N , P

C a r b o n

C , S i →

O

N

C

→

→

→ 1 bond often

2 bond often

3 bond often

4 bond always

The above chart is a guide on the number of bonds formed by these atoms.

Lewis Structure, Octet Rule Guidelines

When compounds are formed they tend to follow the Octet Rule. Octet Rule: Atoms will share electrons (e-) until it is surrounded by eight valence electrons. 4 unpaired 3unpaired 2unpaired 1unpaired up = unpaired e- 4 bonds 3 bonds 2 bonds 1 bond

O=C=O N≡ N O = O F - F

Atomic Connectivity The atomic arrangement for a molecule is usually given. CH2ClF HNO3 CH3COOH H2SO4

H C F

Cl

H H

N O O

O H O S O H

O

O

H C C

O H H

H O

In general when there is a single central atom in the molecule, CH2ClF, SeCl2, O3 (CO2, NH3, PO4

3-), 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). In which case the central atom is the second atom in the chemical formula.

Find the central atom for the following:

1) H2O 2) PCl3

3) SO3 4) CO32-

5) BeH2 6) IO3-

Bond table requires the chemical formula, and determining the number of electrons around each atom. A) Chemical Formula I.e., HNO3 B) Oe - Octet Electrons. This is always either 8 or 2 (for H) C) Tve - Total Valence Electron.

Setting up Bond Table

ChemFormula Octet e- Tot Val e- HNO3 Oe Tve

H N O

1 x 2 = 2 1 x 8 = 8 3 x 8 = 24

1 x 1 = 1 1 x 5 = 5

3 x 6 = 18 34 24

Note this recipe works only if the chemical specie obeys the octet rule. For chemical specie which violates the octet rule, this method must be modified.

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

Calculating the Number of Bonds and the Remaining electrons

HNO3 Oe Tve # Bonding e-

Bond Table # of Bonds

Remaining e-

34 - 24 = 10 10/2 = 5

Tve(24) - electrons in Bond (10) = 14

1) Chemical formula: 2) Atomic sequence: HNO3

3) Number of bonds = 5

4) Remaining electrons = 14

5) Lewis Structure with 5 bonds:

6) Complete Lewis Structure with 14 remaining electrons

Putting it Together

H

N O O O

H

N O O O

H

N O O O

Lewis Dot Structure of CO2 by Bonds Table

B. Calculate the number of bonds in compound structure.

# bonds = (Oe - TVe) 2

= (24- 16) = 8 = 4 bonds 2 2

C. 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

Writing Lewis Structure: Use information from the above calculations. Total of 16e- in CO2, of which 8 electrons are used to form 4 bonds and 8 remaining electrons are used to complete Lewis structure.

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

A. Calculate Octet electrons (Oe-) and Total Valence electrons to determine number of bonds

CO2 Oe TVe 1 C 1•(8)= 8 1•(4) = 4 2 O 2•(8)=16 2•(6)=12 Chg

24 16

1,2. Write atom connectivity for CO2.

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

O C O O C O

Lewis Dot Structure of ClO4-

B. Number of Bonds.

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

C. Remaining electrons.

Remaining e- = 32 - 8 = 24 e-Remaining

Writing Lewis Structure:

6. Place the remaining 24 electrons in the structure such that each atom has an octet to complete the Lewis Structure

A. Calculate (Oe-) and (TVe)

ClO4- Oe TVe

1 Cl 1•(8)= 8 1•(7) = 7 4 O 4•(8)= 32 4•(6)= 24 Chg 1

40 32

1,2. Write atom connectivity for ClO4

-. 3.4.5. Draw the four bonds in the structure.

O

Cl OO

O

OClO OO

O

Cl OO

O

Summary Lewis Structure Determination: • Molecular Formula • Atomic Sequence (H and F are terminal) • Determine the # of bonds

Oe- and TVe- # of Bonds = (Oe - TVe-) / 2

• Determine remaining electrons Re = (TVe-) - (# e- in bonding)

• Make sure all atoms satisfy octet rule (Except H which is satisfied with 2 electrons)

Rules for molecules with an overall charge:

1.  When figuring out the number of electrons available (ie. counting the valence electrons), add or subtract electrons as indicated by the charge.

2.  Create the Lewis structure the same as always

3. Put [square] brackets around the structure

4. Write the charge as a superscript

Try it: CO3-2

C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-

O O

C

O

Try it: CO3-2

C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-

O O

C

O

Try it: CO3-2

C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-

O O

C

O

Try it: CO3-2

O O

C

O

All 24 e- have been used, buy C does NOT have a complete octet….. Need to make a double bond….Does it matter which O it makes the double bond with?

Try it: CO3-2

O O

C

O

The bond can go here but it could also go between the C and either of the other Os

Finished Product J

C C C

-2

Resonance Structures •  The Lewis structure for CO3 shows two different types of

bonds, single and double. •  Double bonds also have a shorter bond length, the distance

between the nuclei of the two atoms in the bond, than single bonds do. Thus, if the above Lewis structure for carbonate were correct, the carbonate ion would have one bond that is shorter and stronger than the other two.

•  This is not the case. All three of the bonds in the carbonate ion are the same strength and the same length.

•  Interestingly, the behaviour of the bonds suggests they are longer than double bonds and shorter than single bonds.

•  They are also stronger than single bonds but not as strong as double bonds.

Resonance Structures (2) •  So the carbonate ion would actually 3 bonds, with the bond

lengths between the characteristic single and double bond lengths

•  The important points to remember about resonance forms are: àThe molecule is not rapidly oscillating between different discrete forms àThere is only one form of the each molecule, we need to draw two (or more) Lewis structures because a single structure is insufficient to describe the real structure

Drawing them: •  Expressing resonance when drawing Lewis

structures is done by drawing each of the possible resonance forms and placing double-headed arrows between them

Do you remember polyatomic ions?

Formula Name Formula Name NH4

+/NH3 Ammonium/Ammonia CrO4-2 Chromate

C2H3O2- Acetate Cr2O7

-2 Dichromate CH3COO- Acetate MnO4

- Permanganate CN- Cyanide MnO4

-2 Manganate CO3

-2 Carbonate NO2- Nitrite

HCO3- Bicarbonate NO3

- Nitrate C2O4

-2 Oxalate OH- Hydroxide ClO- Hypochlorite PO4

-3 Phosphate ClO2

- Chlorite SCN- Thiocyanate ClO3

- Chlorate SO3-2 Sulfite

ClO4- Perchlorate SO4

-2 Sulfate S2O3

-2 Thiosulfate HSO4- Hydrogen sulfate

BrO- Hypobromite IO3- Iodate

AsO2-3 Arsenite SeO4

-2 Selenate BrO3

- Bromate HSO3- Hydrogen sulfite

Lewis Structures: Examples Example

a) CH2ClF

H C F

Cl

H

b) SO2

O S O

c) SO42-

O S OO

O

d) H3PO4

H

P O O O

O

H H

Co-ordinate Covalent Bond

•  A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei.

•  In a simple covalent bond, each atom supplies one electron to the bond - but that doesn't have to be the case.

•  A co-ordinate bond is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

Example: NH4+

Carbonate ion:

C C C

-2

Try these:

1.  O3 2.  NO3

-1 3.  HCO2

-1 4.  CN-1

5.  SO32-

6. BrO-1

7. PO43-

8. IO3-1

9. CHO2 -1 10. NO2

+1