Drawing molecules

High School Chemistry Rapid Learning Seriesl - 16

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Drawing Molecules

HS Ch i t R id L i S i

Rapid Learning Centerwww.RapidLearningCenter.com/© Rapid Learning Inc. All rights reserved.

HS Chemistry Rapid Learning Series

Wayne Huang, PhDKelly Deters, PhDRussell Dahl, PhD

Elizabeth James, PhD



Learning Objectives

Valence Bond Theory

Th O t t R l

By completing this tutorial, you will learn…

The Octet Rule

Lewis Structures for:Elements

Covalent Compounds

Polyatomic Ions

Ionic Compounds

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p

Concept MapPrevious content

New contentChemistry

Studies

Matter

Compounds

One type is

ValenceBond

Theory

1 bonding theory is

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StructuresLewis

Structures

Shown with



Valence Bond Theory and the Octet Rule

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Definition: Valence Shell

Valence Shell – Outermost shell of electrons; the electrons with the highest principal energy level number; the electrons that form chemical bonds.

Cl: 1s2 2s2 2p6 3s2 3p5 7 l l t

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Cl: 1s2 2s2 2p6 3s2 3p5 7 valence electrons

4s2 3d10 4p5Br: [Ar] 7 valence electrons



Definition: Valence Bond Theory

Valence Bond Theory –Bonds are formed by overlap of valence orbitalsoverlap of valence orbitals from bonding atoms.

H HH H

Valence Bond

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H HH Hs-orbital

Both atoms get to “count” the electrons that are being shared between the two.

Definition: Octet Rule

Octet Rule – Most atoms are more stable with a full valence

h ll ( hi h i blshell (which is a noble gas configuration). A full shell has 8electrons (“oct-” = 8).

More exceptions will be discussed soon, but for now, Hydrogen is an exception.

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Hydrogen’s valence shell only contains a 1s orbital, which can only hold 2 electrons.

Therefore, hydrogen is most stable with 2 electrons (one single bond).

HONC Valence Rule Mnemonic: The electrons needed for full valence shell and covalent bonds formed are H(1), O(2), N(3) and C(4) = HONC



Determining # of Valence Electrons

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Valence Electrons and Periodic Table

The main groups of the periodic table have # of valence electrons = main group #.

1 2 3 4 5 6 7 8

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The transition metals don’t have easy patterns. Here are some of the common elements:

Valence Electrons of Transition Metals

Element Valence Electrons

Cu

Zn

Cd

2

2

2

Configuration

[Ar] 3d104s1

[Ar] 3d104s2

[Kr]4d105s2

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Ag

Au

1

3

Valence electrons in transition metals are not as reliable and predictable as the ones in main group elements.

[Kr]4d105s1

[Xe]4f145d106s1

Lewis Structures

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Definition: Lewis Structure

Lewis Structure – 2D visualization of how electronsvisualization of how electrons are shared to form bonds between atoms.

Also called:

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Also called:Electron Dot StructuresDot StructuresLewis Dot StructuresLewis Dot Diagrams

Lewis Structures of Elements

Use the element’s symbol to represent the nucleus and core (non-valence) electrons.1

How to draw an element’s Lewis structure:

( )

Determine the number of valence electrons from the position on the Periodic Table.

Draw the electrons around the “nucleus” - one on each side before doubling up (Hund’s Rule - place one in each orbital before doubling).

2

3

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Example: Draw the Lewis Structure for an oxygen atom.

OOxygen is in the 6th main group.

There are 6 valence electrons.



Lewis Structures - Covalent Compounds with 2 Elements

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Drawing Binary Covalent Structures

Arrange the atoms symmetrically.

Determine the # of valence electrons

1

For compounds with only 2 different non-metals:

Determine the # of valence electrons for each atom.Draw the valence electrons - do not double up where 2 atoms are bonding.When atoms have 8 (2 for H) electrons, the structure is done.

2

3

4

E l D th L i St t f CH

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Example: Draw the Lewis Structure for

CCarbon is in the 4th main group.There are 4 valence electrons.

HHH

HHydrogen is in the 1st main group.There is 1 valence electron.

Carbon now has 8 electrons it’s sharing.

Each hydrogen has 2 electrons it’s sharing.

CH4



Binary Covalent Structure #2


Determine the # of valence electrons

1

Another example:

Determine the # of valence electrons for each element.Draw the valence electrons - do not double up where 2 atoms are bonding.When atoms have 8 (2 for H) electrons, the structure is done.

2

3

4

E l D th L i St t f NH

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Hydrogen is in the 1st main group.There is 1 valence electron.

Nitrogen is in the 5th main group.There are 5 valence electrons.

H

Nitrogen now has 8electrons it’s sharing.

Each hydrogen has 2 electrons it’s sharing.


N HH

NH3

Lone Pairs and Bonding PairsLone Pair

Electrons not shared in a

bond.

Bonding PairElectrons shared between two atoms.

N HHH

Lone Pairs are “counted” only by the one atom.

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y yLone Pairs are important and must be drawn even though they aren’t bonding.

Bonding Pairs are “counted” by both atoms that are sharing them.



Lewis Structures - Multiple Bonds

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Multiple Bond Example #1 - 1


Determine the # of valence electrons for each

1

Begin with the same steps:

Determine the # of valence electrons for each element.Draw the valence electrons - do not double up where 2 atoms are bonding.When atoms have 8 (2 for H), the structure is done.

2

3

4

E l D th L i St t f CO

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Carbon is in the 4th main group.There are 4 valence electrons.

Oxygen is in the 6th main group.There are 6 valence electron.

O C OCurrently, carbon only has 6.

And each oxygen only has 7.

Example: Draw the Lewis Structure forCO2



When the previous steps do not result in full valences:

Move two unpaired electrons on adjacent atoms to bond


5 jtogether.


Repeat until all atoms have full valence shells.

CO2

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O C ONow the carbon has 8.

And each oxygen also has 8.

Example: Draw the Lewis Structure for CO2




1

Begin with the same steps:


2

3

4

Example: Draw the Lewis Structure for HCN

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Carbon has 4 valence electrons.Hydrogen has 1 valence electron.Nitrogen has 5 valence electrons.

But the carbon and nitrogen each only have 6 electrons, not 8.

C NH

Example: Draw the Lewis Structure for HCN.



Move two unpaired electrons on adjacent atoms to bond

When the previous steps do not result in full valences:


5 jtogether.

Example: Draw the Lewis Structure for HCN


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C NH

Example: Draw the Lewis Structure for HCN.

Now all valence shells are full.

Double and Triple Bonds

C OO

Triple Bond

Double Bond

C OO

C NH

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A double bond is 2 pairs of electrons being shared.Double bonds are shorter and stronger than single bonds.

A triple bond is 3 pairs of electrons being shared.Triple bonds are shorter and stronger than double bonds.



Lewis Structures - CovalentLewis Structures Covalent Compounds with More Than 2 Elements

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What Order do the Elements Go In?When there are more than two elements, how do you arrange them?

“COOH” is a carboxylic acid.1Hydrogen and halogens (F, Cl, Br, I) can’t go in the middle.Of the elements that can go in the middle, write in the order they’re given.Write the hydrogen and halogen atoms around what they’re next to in the formula.

2

3

4

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Example: H5C2OH (i.e. CH3CH2OH)

Write in this order C C OHH

H H

HH



Finishing the Lewis StructuresOnce you’ve arranged the atoms, finish the process:

Arrange the atoms according to the formula.


1


2

3

4

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Example: H5C2OH (i.e. CH3CH2OH)

C C OHH

H H

HH

Each carbon has 4 electrons.Each hydrogen has 1 electron.The oxygen has 6 electrons.

Example #2 - 1

“COOH” is a carboxylic acid.

Hydrogen and halogens (F Cl Br I) can’t go in the1

Hydrogen and halogens (F, Cl, Br, I) can t go in the middle.Of the elements that can go in the middle, write in the order they’re given.Write the hydrogen and halogen atoms around what they’re next to in the formula.

2

3

4

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Example: BrH2CCH2COOH

C C CBrH

H H

HOCarboxylic acid

OH



Example #2 - 2



1


2

3

4

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C C CBrH

H H

HO

O

HCurrently, some of the atoms are full…

But one carbon and one oxygen each only have 7.

Move two unpaired electrons on adjacent atoms to bond together.

Example #2 - 3

5 j g


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All valence shells are currently full.


C C CBrH

H H

HO

O

H



Moving Hydrogen Atoms AroundSometimes, in order to have all atoms with full valence shells, a hydrogen must be bonded in a different location.

Example:

Each carbon has 4 electrons.Each hydrogen has 1 electron.

C3H6

C C CH

H H

H

H

HTwo carbons do not have full valence shells.Th dj h d bl b d

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They are not adjacent - they cannot double bond.If one hydrogen is moved to another carbon…

Now two carbons right next to each other have un-full shells and can double bond.

This move of the hydrogen is not prohibited by the given information (the formula C3H6).

Lewis Structures -Polyatomic Ions

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Definition: Polyatomic Ion

Polyatomic Ion – A group of atoms covalently bonded that

h h htogether have a charge.

Examples: NH4+ and SO4

2-.

2

Common Polyatomic Ions:

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+1 Cations

Ammonium (NH4+), Hydronium (H3O+), Mercury (I) (Hg2

2+).

-1 Anions Acetate (C2H3O2), Bicarbonate (HCO3-), Chlorate (ClO3

-), Cyanide (CN-), bisulfate (HSO4-),

Hydroxide (OH-), Nitrate (NO3-), Nitrite (NO2

-), Perchorate (ClO4-), Permanganate (MnO4

-).

-2 Anions Carbonate (CO32-), Chromate (CrO4

2-), Dichromate (Cr2O72-), Hydrogen Phosphate (HPO4

2-),Peroxide (O2

2-),Sulfate (SO42-), Sulfite (SO3

2-), Thiosulfate (S2O32-).

-3 Anions Phosphate (PO43-)

Polyatomic CationA cation is a positively charged ion (loss of electrons).


D t i th # f l l t f h l t

1

2

In this case, there is no choice but to double up on a side with a bond.


2

3

4

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Example: NH4+

N HHH

HNitrogen has 5 electrons.Each hydrogen has 1 electron.The +1 charge means we can remove 1 electron!

H’s electron is removed to result in +1 charge.

+1



Polyatomic AnionAn anion is a negatively charged ion (gain of electrons).


D t i th # f l l t f h l t

1

2 Determine the # of valence electrons for each element.Draw the valence electrons - do not double up where 2 atoms are bonding.When atoms have 8 (2 for H), the structure is done.

2

3

4When placing electrons around the oxygen, do not place them where there is already a pair of electrons from sulfur.

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Example: SO42-

S OOO

OSulfur has 6 electrons.Each oxygen has 6 electrons.The -2 charge means we can add 2 electrons!

-2

2 electrons are added to result in the -2 charge.Warning: This structure satisfies the Octet rule.

However, the more realistic structure is where two S=O bonds are formed with resonance structures.

Lewis Structures -Another Approach

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Another Approach - 1

Arrange the atoms symmetrically or according to the chemical formula.1

O C O

Example: Draw the Lewis Structure for CO2.

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Determine the total # of valence electrons for each element.2


O C OCounting electrons:1 Carbon = 1 × 4 = 4 electrons2 Oxygens = 2 × 6 = 12 electrons


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2 Oxygens 2 × 6 12 electronsTotal = 16 electrons



Put one pair of electrons in between each set of atoms.

3




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2 Oxygens 2 × 6 12 electronsTotal = 16 electrons 161412Electrons to Fill =

Place lone pairs around the most electronegative atom first (closest to F on the periodic table). Stop when you

t f l t

4


run out of electrons.


Example: Draw the Lewis Structure for Oxygen is more electronegative than carbon.

CO2.

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2 Oxygens 2 × 6 12 electronsTotal = 16 electrons 121086420Electrons to Fill =



If any atoms do not have full valences, move a lone pair from an adjacent atom in to form a

lti l b d

5


multiple bond.

O C O


Each oxygen has 8 electrons.But carbon only has 4 electrons.

CO2.

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Note: There is a simpler notation where a single bonding pair is replaced with a single line, double bond with a double line and triple bond with a triple line. CO2 becomes …

O = C = C....

..

..

A Larger Compound - 1

Arrange the atoms symmetrically or according to the chemical formula.1


O

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C CBrH

H H

HO

O

HC



Determine the total # of valence electrons for each element.2


Counting electrons:


O

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1 Bromine = 1 × 7 = 7 electrons5 Hydrogens = 5 × 1 = 5 electrons3 Carbons = 3 × 4 = 12 electrons2 Oxygens = 2 × 6 = 12 electronsTotal = 36 electrons

C C CBrH

H H

HO

O

H

Put one pair of electrons in between each set of atoms.

3


Counting electrons:


O

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3616

C C CBrH

H H

HO

O

H

Electrons to Fill =



Place lone pairs around the most electronegative atom first (closest to F on the periodic table). Stop when you

t f l t

4


run out of electrons.

Counting electrons:

Example:

Oxygen is most electronegative, followed by bromine.

BrH2CCH2COOH

O

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C C CBrH

H H

HO

O

H

Electrons to Fill =

If any atoms do not have full valences, move a lone pair from an adjacent atom in to form a multiple bond.

5


Example:

All have full valences except one carbon and one oxygen.

BrH2CCH2COOH

O

46/56Now all have full valences!

C C CBrH

H H

HO

O

H



Lewis StructuresLewis Structures - Ionic Compounds

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Definition: Ionic Compound

Ionic Compound – Metals transfer electrons to non-metals. The resulting ions form an electrostatic attraction.

e.g. KCl, Na2SO4

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e.g. KCl, Na2SO4



Ionic Compound ExampleAn ionic compound is between metals and non-metals.

Determine the # of valence electrons for each atom.1

Draw the valence electrons.

Transfer electrons from the metals to the non-metals to fill valence shells.

2

3

The metal will be left with no electrons in the electrons shell “drawn”.However, the next inner shell is full and it now a full “valence” shell.

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Example: KCl

ClKPotassium has 1 electron.Chlorine has 7 electrons.

+1 -1

Polyatomic Ionic Compound Example

An ionic compound is between metals and non-metals.

Determine the # of valence electrons for each atom.1

Draw the valence electrons.

Transfer electrons from the metals to the non-metals to fill valence shells.

2

3

The total charge should = 0

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Example: Na2SO4 NaEach sodium has 1 electron.SO4

2- is a polyatomic ion - it must be covalently bonded first.

+1

S OOO

O -2

Na+1

-1



Exceptions to the Octet Rule

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Common Exceptions to the Octet Rule# of Valence

Electrons when “Full”Element(s)

2 H He (i e H He)2

4

H, He (i.e. H2, He)

Be (i.e. BeCl2)

A l t i th 3rd i d

6 B (i.e. BCl3)

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>8 Any element in the 3rd period and below. (i.e. PF5, SF6)

Elements in period 3 and below have empty “d” orbitals that can be used to hold more than 8 valence electrons (18 Electrons Rule). Free radicals with one unpaired electron clearly do not follow the Octet rule.

Octet Exception Mnemonic: H&He: 2; Be: 4; B = 6; S&P > 8 = “2 Hawks and Hens (2-legs); 4 Bears (4-legs); 6 Bugs (6-legs); many SPiders (<8 legs).”



Exception Examples

B is “full” with 6 electrons.

B FF B FFF

S

BF3 = 24 Valence Electrons(Trigonal Planar Geometry)

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S has 10 electrons.

F

SF5+ = 40 Valence Electrons

(Trigonal Bipyramidal Geometry)

Elements bond to obtain a Elements bond to obtain a

Learning Summary

Covalent compoundsCovalent compounds

full valence shell - for most elements, that

means 8 (the Octet Rule).

full valence shell - for most elements, that

means 8 (the Octet Rule).

Lewis Structures areLewis Structures are

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Covalent compounds share electrons, while

ionic compounds transfer electrons from one atom to another.

Covalent compounds share electrons, while

ionic compounds transfer electrons from one atom to another.

Lewis Structures are used to show the

valence electrons and their arrangement in

compounds.

Lewis Structures are used to show the

valence electrons and their arrangement in

compounds.



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