Drawing molecules

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Drawing Molecules

HS Ch i t R id Learning Series
Chemistry Rapid L
i
S i
Wayne Huang, PhD
Kelly Deters, PhD
Russell Dahl, PhD
Elizabeth James, PhD

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© Rapid Learning Inc. All rights reserved.

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1


Learning Objectives
By completing this tutorial, you will learn…
Valence Bond Theory
The O t t R l
Th Octet Rule
Lewis Structures for:
Elements
Covalent Compounds
Polyatomic Ions
Ionic Compounds
p

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Concept Map
Previous content
Chemistry

New content
Studies

Matter
One type is

Valence
Bond
Theory

1 bonding
theory is

Compounds

Shown with

Lewis
Structures

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2


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 valence electrons
l
l t

Br: [Ar] 4s 2 3d10 4p5

7 valence electrons

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3


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

H

H

H

H
s-orbital

Both atoms get to “count” the electrons that are being
shared between the two.
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Definition: Octet Rule
Octet Rule – Most atoms are
more stable with a full valence
shell (which is a noble gas
h ll ( hi h i
bl
configuration). A full shell has 8
electrons (“oct-” = 8).
More exceptions will be discussed soon, but for now, Hydrogen
is an exception.
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).

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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


4


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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5


Valence Electrons of Transition Metals
The transition metals don’t have easy patterns.
Here are some of the common elements:
Element

Valence Electrons

Configuration

Cu

2

[Ar] 3d104s1

Zn

2

[Ar] 3d104s2

Cd

2

[Kr]4d105s2

Ag

1

[Kr]4d105s1

Au

3

[Xe]4f145d106s1

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

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6


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

Also called:
Electron Dot Structures
Dot Structures
Lewis Dot Structures
Lewis Dot Diagrams
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Lewis Structures of Elements
How to draw an element’s Lewis structure:

1

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

2

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

3

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

Example:

Draw the Lewis Structure for an oxygen atom.

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

O

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7


Lewis Structures - Covalent
Compounds with 2 Elements

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Drawing Binary Covalent Structures
For compounds with only 2 different non-metals:

1

Arrange the atoms symmetrically.

2

Determine the # of valence electrons
for each atom.

3

Draw the valence electrons - do not
double up where 2 atoms are bonding.

4

When atoms have 8 (2 for H) electrons,
the structure is done.

Example:
E
l

Draw th L i Structure for CH4
D
the Lewis St
t
f

H

Carbon is in the 4th main group.
Hydrogen is in the 1st main group.
There is 1 valence electron.

H C H
H

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Carbon now has
8 electrons it’s
sharing.
Each hydrogen
has 2 electrons
it’s sharing.

8


Binary Covalent Structure #2
Another example:

1


2

Determine the # of valence electrons
for each element.

3

Draw the valence electrons - do not
double up where 2 atoms are bonding.

4

When atoms have 8 (2 for H) electrons,
the structure is done.

Example:
E
l

Draw th L i Structure for NH3
D
the Lewis St
t
f

Nitrogen is in the

5th

main group.

Hydrogen is in the

1st

main group.

There is 1 valence electron.

H N H
H

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Nitrogen now
has 8
electrons it’s
sharing.
Each hydrogen
has 2 electrons
it’s sharing.

Lone Pairs and Bonding Pairs
Lone Pair
Electrons not
shared in a
bond.

H N H
H

Bonding Pair
Electrons shared
between two atoms.

Lone Pairs are “counted” only by the one atom.
y y
Lone Pairs are important and must be drawn
even though they aren’t bonding.
Bonding Pairs are “counted” by both atoms
that are sharing them.
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9


Lewis Structures
- Multiple Bonds

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Multiple Bond Example #1 - 1
Begin with the same steps:

1


2

Determine the # of valence electrons for each
element.

3

Draw the valence electrons - do not double up
where 2 atoms are bonding.

4

When atoms have 8 (2 for H), the structure is done.

the Lewis St
t
f CO
Example: D
E
l Draw th L i Structure forCO2
Carbon is in the 4th main group.
Oxygen is in the

6th

O C O

main group.

There are 6 valence electron.

Currently, carbon
only has 6.
And each oxygen
only has 7.

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10


When the previous steps do not result in full valences:

5

Move two unpaired electrons on
j
adjacent atoms to bond
together.
Repeat until all atoms have full valence
shells.

Example:

Draw the Lewis Structure for CO2

Now the carbon has 8.

O C O
And each oxygen also has 8.
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Begin with the same steps:

1


2

element.

3


4


Example:

Draw the Lewis Structure for HCN
HCN.

Carbon has 4 valence electrons.

H C

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

N

But the carbon and nitrogen each
only have 6 electrons, not 8.
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11


When the previous steps do not result in full valences:

5

Move two unpaired electrons
j
on adjacent atoms to bond
together.
Repeat until all atoms have full valence
shells.

Example:

Draw the Lewis Structure for HCN
HCN.

Now all valence shells are full.

H C

N

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Double and Triple Bonds

O C

O

Double Bond

H C

N
Triple Bond

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

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


12


Lewis 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?

1

“COOH” is a carboxylic acid.

2

Hydrogen and halogens (F, Cl, Br, I) can’t go in the
middle.

3

Of the elements that can go in the middle, write in
the order they’re given.

4

Write the hydrogen and halogen atoms around
what they’re next to in the formula.

Example:

H5C2OH (i.e. CH3CH2OH)

Write in this order

HH
H C C O H
HH

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13


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

1

Arrange the atoms according to the formula.

2

element.

3


4


Example:

H5C2OH (i.e. CH3CH2OH)

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

H H
H C C O H
H H

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Example #2 - 1
1

“COOH” is a carboxylic acid.

2

Hydrogen and halogens (F Cl Br I) can’t go in the
(F, Cl, Br, can t
middle.

3

Of the elements that can go in the middle, write in
the order they’re given.

4

Write the hydrogen and halogen atoms around
what they’re next to in the formula.

Example:

BrH2CCH2COOH
Carboxylic acid

HHO
Br C C C O H
HH

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14


Example #2 - 2
1


2

element.

3


4


Example:
BrH2CCH2COOH
Currently, some of the atoms
are full…

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But one carbon and one
oxygen each only have 7.

H H O
Br C C C O H
HH

Example #2 - 3
5

Move two unpaired electrons on
j
g
adjacent atoms to bond together.
Repeat until all atoms have full valence shells.

Example:
BrH2CCH2COOH
All valence shells are
currently full.

H H O
Br C C C O H
HH

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15


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

C3H6

Each carbon has 4 electrons.

H H H
C C C
H H H

Two carbons do not have full valence shells.
They
Th are not adjacent - they cannot double bond.
dj
h
d bl b d
If one hydrogen is moved to another carbon…
Now two carbons right next to each other have un-full shells and
can double bond.

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

Lewis Structures Polyatomic Ions

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16


Definition: Polyatomic Ion
Polyatomic Ion – A group of
atoms covalently bonded that
together have a charge.
h h
h
Examples: NH4+ and SO42-.
Common Polyatomic Ions:
+1
Cations

2+
Ammonium (NH4+), Hydronium (H3O+), Mercury (I) (Hg22 ).

-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 (CrO42-), Dichromate (Cr2O72-), Hydrogen Phosphate (HPO42-),
Peroxide (O22-),Sulfate (SO42-), Sulfite (SO32-), Thiosulfate (S2O32-).

-3 Anions

Phosphate (PO43-)

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

1


2

Determine th # of valence electrons for each element.
D t
i the
f l
l t
f
h l
t

3

Draw the valence electrons - do not double up where
2 atoms are bonding.

4

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

Example:

NH4+

Nitrogen has 5 electrons.
The +1 charge means we can
remove 1 electron!

H
H N H
H

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

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17


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

1


2

Determine th # of valence electrons for each element.
D t
i the
f l
l t
f
h l
t

3

Draw the valence electrons - do not double up where
2 atoms are bonding.

4

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

SO42-

O
Each oxygen has 6 electrons.
O S O
The -2 charge means we can add 2 electrons!
Warning: This structure satisfies the Octet rule.
O
However, the more realistic structure is where two

-2

Sulfur has 6 electrons.

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2 electrons are
added to result
in the -2 charge.

S=O bonds are formed with resonance structures.

Lewis Structures Another Approach

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18


Another Approach - 1
1

Arrange the atoms symmetrically or
according to the chemical formula.

Example:

Draw the Lewis Structure for CO2.

O C O

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2

Determine the total # of valence
electrons for each element.

Example:


Counting electrons:
1 Carbon = 1 × 4 = 4 electrons
2 Oxygens = 2 × 6 = 12 electrons

O C O

Total = 16 electrons

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19


3

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

Example:


Counting electrons:

O C O

Electrons to Fill = 16
12
14

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4

Place lone pairs around the most
electronegative atom first (closest to F
on the periodic table). Stop when you
run out of electrons.
t f l t

Example:


Counting electrons:

Oxygen is more electronegative than carbon.


O C O

10
0
2
6
4
8

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20


5

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

Example:


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

O C O

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
..

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A Larger Compound - 1
1

Arrange the atoms symmetrically or
according to the chemical formula.

Example:

BrH2CCH2COOH

H H O
Br C C C O H
HH
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21


2

Determine the total # of valence
electrons for each element.

Example:

BrH2CCH2COOH

Counting electrons:
1 Bromine = 1 × 7 = 7 electrons
5 Hydrogens = 5 × 1 = 5 electrons
3 Carbons = 3 × 4 = 12 electrons

H H O
Br C C C O H
HH

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3

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

Example:

BrH2CCH2COOH

Counting electrons:
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H H O
Br C C C O H
HH

16


22


4

Place lone pairs around the most
electronegative atom first (closest to F
on the periodic table). Stop when you
run out of electrons.
t f l t

Example:

BrH2CCH2COOH

Oxygen is most electronegative, followed by bromine.
Counting electrons:

H H O
Br C C C O H
HH

0
6

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5

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

Example:

BrH2CCH2COOH

All have full valences except one carbon and one oxygen.

H H O
Br C C C O H
HH
Now all have full valences!
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23


Lewis 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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24


Ionic Compound Example
An ionic compound is between metals and non-metals.

1

atom.

2

Draw the valence electrons.

3

Transfer electrons from the metals to the nonmetals to fill valence shells.

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.
Example:

KCl

Potassium has 1 electron.
Chlorine has 7 electrons.

K

+1

Cl

-1

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Polyatomic Ionic Compound Example
An ionic compound is between metals and non-metals.

1

atom.

2

Draw the valence electrons.

3

Transfer electrons from the metals to the nonmetals to fill valence shells.
The total charge should = 0

Example:

Na2SO4

Each sodium has 1 electron.
SO42- is a polyatomic ion - it must
be covalently bonded first.

Na
Na

+1

+1

-2
O -1
O S O
O

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25


Exceptions to
the Octet Rule

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Common Exceptions to the Octet Rule
# of Valence
Electrons when “Full”

Element(s)

2

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

4

Be (i.e. BeCl2)

6

B

>8

Any l
A element i the 3rd period
t in th
i d
and below. (i.e. PF5, SF6)

(i.e. BCl3)

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.

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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).”


26


Exception Examples
B is “full” with 6
electrons.

F B F
F
BF3 = 24 Valence Electrons
(Trigonal Planar Geometry)

S
F
S has 10
electrons.
SF5+ = 40 Valence Electrons
(Trigonal Bipyramidal Geometry)
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Learning Summary

Elements bond to obtain a
full valence shell - for
most elements, that
means 8 (the Octet Rule).

Lewis Structures are
used to show the
valence electrons and
their arrangement in
compounds.

Covalent compounds
share electrons, while
ionic compounds
transfer electrons from
one atom to another.

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27


Congratulations
You have successfully completed
the core tutorial

Drawing Molecules

Chemistry :: Biology :: Physics :: Math

What’s N t
Wh t’ Next …

Step 1: Concepts – Core Tutorial (Just Completed)
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Step 3: Recap – Super Review Cheat Sheet

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