ATOMIC STRUCTURE

Chapter 1 Classically, atoms are shown as the nucleus surrounded

by small “balls” representing orbiting electrons.
“Structure and Bonding”
Modern quantum theory, however, suggests that they are
better described as a very tiny nucleus surrounded by
clouds of electrons, where each cloud has a distinct
(quantized) energy level.

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ATOMIC STRUCTURE ATOMIC STRUCTURE

Atoms are incredibly small… a single copper penny
Classically, atoms are shown as the nucleus surrounded
contains approximately,
by small “balls” representing orbiting electrons.
Modern quantum theory, however, suggests that they are 28,000,000,000,000,000,000,000 atoms!
better described as a very tiny nucleus surrounded by
clouds of electrons, where each cloud has a distinct Obviously, atoms and subatomic particles
(quantized) energy level. are very small. The mass of a proton is
1.672623 × 10-24 grams.
“Core electrons” are close to
the nucleus and are not
generally involved in the The mass of an neutron is similar to that of a proton
formation of chemical bonds. and is 1.674929 × 10-24 grams.

The mass of an electron is significantly less than that

“Valence electrons” occupy the
outer shell (the highest energy of a proton and is 9.109390 × 10-28 grams.
level) and participate in the
formation of chemical bonds.
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ELEMENTS THE PERIODIC TABLE

An Element is the simplest form of matter, and in its
pure state, contains only one type of atom. There are
over 115 elements known.

Every element is identified by a chemical symbol, that

is generally an abbreviation of its name. Many elements The average atomic
are readily identified by their symbols (for example, mass of carbon.
Carbon is C), but some symbols are based on ancient
Latin or Greek names (ferrum for iron; Fe).

Chemists have classified the elements according to their

chemical properties and represent them in a table format
know as the Periodic Table of the Elements.

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ELECTRON CONFIGURATION ELECTRON CONFIGURATION
“Core electrons” are close to The energy levels that electrons occupy are called
“Valencehas
Carbon-12 electrons”
six occupy the the nucleus and are not
outer shell (the highest energy generally involved in the Quantum Levels, and the quantum level (n) coincides
neutrons and six protons
level) and participate in the formation of chemical bonds. with the period of the element in the periodic table.
in theformation
nucleus, and isbonds.
of chemical
surrounded by a cloud The maximum number of electrons that can reside
within a quantum level is 2n2… or, 2, 8, 18, 32, 50, etc.
containing six electrons.
I VIII
The electrons on carbon,
1 II III IV V VI VII
however, do not occupy a
2
single “cloud”, but are
distributed among two 3
major energy levels. 4

7
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ELECTRON CONFIGURATION ELECTRON CONFIGURATION

Within each quantum level, electrons reside in orbitals. Within each quantum level, electrons reside in orbitals.
Orbitals are “regions of space” where electrons can reside The first level holds two electrons in the 1s orbital.
and each orbital can hold two electrons. They are given For Level 1, 2n2 = 2; two
electrons fill the 1s
the names of s, p, d & f. The lowest energy are orbital.
1s
spherical s orbitals,
There are three p
holding up toeach
orbitals, 2 electrons.
holding
I VIII
up to 2 electrons.
There are five d 1 II III IV V VI VII
orbitals, each holding
up to 2 electrons. (and 2
seven f orbitals) 3

7
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ELECTRON CONFIGURATION ELECTRON CONFIGURATION

In the second level, the first two electrons go into the 2s In the second level, the first two electrons go into the 2s
orbital and the next six electrons go into the three 2p orbital and the next six electrons go into the three 2p
orbitals. orbitals.
2s For Level 2, 2n2 = 8. The For Level 2, 2n2 = 8. The
first two electrons go next six electrons go into
into the 2s orbital. the three 2p orbitals.
I VIII I VIII
1 II III IV V VI VII
1 II III IV V VI VII

2 2

3 3

4 4

5 5

6 6

7 7
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ELECTRON CONFIGURATION ELECTRON CONFIGURATION
In the third level, the first two electrons go into the 3s In the third level, the first two electrons go into the 3s
orbital and the next six electrons go into the three 3p orbital and the next six electrons go into the three 3p
orbitals. orbitals.
3s For Level 3, 2n2 = 18.
The next six electrons go
The first two electrons
into the three 3p orbitals.
go into the 3s orbital.
I VIII I VIII
1 II III IV V VI VII
1 II III IV V VI VII

2 2

3 3

4 4

5 5

6 6

7 7
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ELECTRON CONFIGURATION ELECTRON CONFIGURATION

In the third level, the first two electrons go into the 3s In the transition metals in the fourth level, the 4s and 4p
orbital and the next six electrons go into the three 3p levels hold a total of eight electrons, and ten electrons
orbitals. now fill the five d orbitals.
The 3d orbitals are higher Six electrons go into
energy than the 4s so they 4s
the three 4p orbitals.
do not fill with electrons (yet).
I VIII I VIII
1 II III IV V VI VII
1 II III IV V VI VII

2 2

3 3

4 4

5 5
Ten electrons go into the
6 6 five 3d orbitals.

7 7
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ELECTRON CONFIGURATION HUND’S RULE

Quantum mechanics assigns electrons a property called Every orbital in a subshell is singly occupied with one
“spin”. This spin results in a local magnetic field that electron before any one orbital is doubly occupied, and
can be oriented “up” or “down”. Electrons within orbitals all electrons in singly occupied orbitals have the
are often identified using arrows (↑and ↓) to represent same spin. What this simply means is that electrons fill
this spin. orbitals with single electrons (of the same spin) before
any electrons are paired in the orbital.

Thus, hydrogen (one electron in the 1s-orbital) is

shown as:

1s ↑ 1s1

and, helium (two electrons in the 1s-orbital) is shown

as:
1s ↑ ↓ 1s2

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IN-CLASS PROBLEM LEWIS DOT STRUCTURES

Carbon is an element in the second period of the In order to help us understand chemical bonding
periodic table; the atomic number of carbon is 6. Draw between atoms, it is often useful to show these
a representation of the electron configuration for valence electrons using the scheme suggested
carbon. many years ago by G.N. Lewis. In this method,
valence electrons are shown as “dots” placed around
the atom.

Using carbon as an
example, you first write
the chemical symbol C
and then show all the
valence electrons as
individual dots.

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LEWIS DOT STRUCTURES LEWIS DOT STRUCTURES

In order to help us understand chemical bonding In order to form bonds in molecular compounds,
between atoms, it is often useful to show these atoms share valence electrons so that each atom has
valence electrons using the scheme suggested eight valence electrons. This type of bonding is
many years ago by G.N. Lewis. In this method, termed covalent.
valence electrons are shown as “dots” placed around
the atom. In molecular chlorine,
two atoms with seven
Draw the Lewis structure valence electrons each
for fluorine: (seven bond to form Cl2.
valence electrons) Cl Cl

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IN-CLASS PROBLEM IN-CLASS PROBLEM

Draw the Lewis structure for ammonia (NH3). Draw the Lewis structure for molecular nitrogen (N2).

I VIII
1 II III IV V VI VII
I VIII
1 II III IV V VI VII 2

2 3

3 4

4 5

5 6

6 7
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 IN-CLASS PROBLEM VALENCE OF COMMON ATOMS
Draw the Lewis structure for carbon monoxide (CO). Valence: the number of bonds an atom forms in
its neutral ground-state.

H
•• ••
H O H H N H H B H H C H
I VIII ••
1 II III IV V VI VII
H H H
2
Oxygen: two
3
Nitrogen:three
4

5
Boron: three
6 Carbon: four
7
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2s 2px 2py 2pz

THE GEOMETRY OF CARBON COMPOUNDS THE GEOMETRY OF CARBON COMPOUNDS

Carbon is an element in the second period of the

periodic table; the atomic number of carbon is 6. Draw
y
a representation of the electron configuration for
carbon. z
The 2s orbital has
The 1s is filled: 1s ↑↓ x spherical symmetry...
2s 2px 2py 2pz
The 2s is filled: 2s ↑↓

The 2p orbitals will have two electrons: 2p ↑ ↑

The full configuration is: 1s ↑↓ 2s ↑↓ 2p ↑ ↑

This is commonly written as: 1s2 2s2 2p2

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2s 2px 2py 2pz

THE GEOMETRY OF CARBON COMPOUNDS THE GEOMETRY OF CARBON COMPOUNDS

z The four valences of carbon

...while the 2p orbitals must be constructed from
are aligned along the x these orbitals, or from a low
x, y and z axes. energy combination of these
2s 2px 2py 2pz orbitals.

py py py py py py

pz pz pz pz pz pz

px px px px px px

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THE GEOMETRY OF CARBON COMPOUNDS THE GEOMETRY OF CARBON COMPOUNDS

The geometry of molecular compounds can be explained

simply using the valence shell electron pair repulsion
theory. In VSEPR theory, the most important factor in
determining geometry is the relative repulsion between
Tetrahedral electron pairs.

Square-planar

109.5o
Trigonal pyramidal
109.5o

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THE GEOMETRY OF CARBON COMPOUNDS THE GEOMETRY OF CARBON COMPOUNDS

Computer rendering of the lowest
The tetrahedral geometry arises from The tetrahedral geometry arisesOccupied
energy Highest from
the combination of the four orbitals to the combination of the four
Molecular orbitals
Orbital to of
(HOMO)
form a low energy sp3 hybrid. form a low energy sp3 hybrid.
methane…

109.5o 109.5o

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THE MOLECULAR ORBITALS OF METHANE (CH4) FRONTIER MOLECULAR ORBITAL THEORY

The filled (HOMO) and empty (LUMO) molecular orbitals The Frontier Molecular Orbital Theory suggests that
of methane (CH4). reactions occur between filled (HOMO) and empty
(LUMO) molecular orbitals. Consider cyanide anion:
Computer rendering of the Highest
HOMO LUMO+3 Occupied Molecular Orbital
(HOMO) of cyanide anion. The

HOMO-1 LUMO+2
N C size of a lobe is a function of
electron density.

LUMO+1
HOMO-2

LUMO
HOMO-3

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FRONTIER MOLECULAR ORBITAL THEORY FRONTIER MOLECULAR ORBITAL THEORY
Cyanide anion reacts with bromomethane to give Cyanide anion reacts with bromomethane to give
acetonitrile by the following reaction: acetonitrile by the following reaction:
H H H H H H

N C C Br N C C Br N C C Br N C C Br
H H
H H
H H
The Lowest
Unoccupied Molecular
Orbital (LUMO) of
bromomethane.

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IN-CLASS PROBLEM IN-CLASS PROBLEM

Draw the Lewis structure for ethane (CH3CH3). Draw the Lewis structure for ethene (CH2CH2).

I VIII I VIII
1 II III IV V VI VII
1 II III IV V VI VII

2 2

3 3

4 4

5 5

6 6
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IN-CLASS PROBLEM HYBRIDIZATION TO FORM AN SP2 CARBON

y
Draw the Lewis structure for ethene (CH2CH2).
z

H H Draw the atoms, then bond

them to give each atom eight x
valence electrons.

C C p
y
p
y 120˚
p p
H H p
x
z
p
x
z
One s and two p-orbitals
from each carbon are
used to construct a
sigma bonding network
with trigonal geometry...

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HYBRIDIZATION TO FORM AN SP2 CARBON HYBRIDIZATION TO FORM AN SP2 CARBON
p
y
p
z

p
x

The remaining two p-orbitals are orthogonal (at The overlap of these orbitals forms a
right angles) to the sigma network and are used continuous “π–cloud” above and below
to construct a pi (π) bonding network above and the plane of the sigma bonds.
below the plane of the sigma bonds.

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HYBRIDIZATION TO FORM AN SP2 CARBON HYBRIDIZATION TO FORM AN SP2 CARBON

π-molecular orbital cut open to π-molecular orbital cut open to

show the atoms... show the atoms...

This “π–bond” is represented as

The overlap of these orbitals forms a
the second bond in a “double
continuous “π–cloud” above and below
the plane of the sigma bonds. bond”. The π–bond is weaker than
the sigma-bond in the molecule,
but still provides significant
bonding energy.
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HYBRIDIZATION TO FORM AN SP2 CARBON ROTATION ABOUT AN SP2 CARBON

The fact that there is hindered rotation around double
bonds means that compounds can exist which differ
only in the geometry around the double bond.

One consequence of the π– Rotation around the

trans- on opposite sides cis- on the same side
overlap is the requirement that sigma bond makes
the two π–orbitals remain the orbitals
perpendicular, Isomers which differ only in the way in which they
parallel so that bonding overlap
disallowing overlap are arranged in space are called stereoisomers.
can occur.
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HYBRIDIZATION TO FORM AN SP CENTER HYBRIDIZATION TO FORM AN SP CENTER
One s and one p-orbital from each carbon are used to One s and one p-orbital from each carbon are used to
construct a sigma bonding network with linear construct a sigma bonding network with linear
geometry. geometry. The remaining p-orbitals are used to
construct a pi (π) bonding network with electron
density surrounding the plane of the sigma bonds.
p
y yy
p
z zz

x xp
x
p p
y y
p p
z 180˚ z
p p
x x

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HYBRIDIZATION TO FORM AN SP CENTER HYBRIDIZATION IN CARBON COMPOUNDS

The overlap of these orbitals forms a continuous “π–
cloud” surrounding the plane of the sigma bonds.
These “π–bonds” are represented as the second and
third bonds in a “triple bond”.

a “single bond”; a “double bond”;

sp3 hybridization sp2 hybridization

a “triple bond”;
sp hybridization

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FUNCTIONAL GROUPS IN-CLASS PROBLEM

1. Indicate the carbon atom that displays sp3
hybridization.
2. The C-C-C bond angle that is highlighted will
have what value?
3. Indicate the oxygen atom that will have sp2
hybridization.
Alkanes – CnH2n+2 Alkenes – CnH2n 4. The C-C-O bond angle that is highlighted will
all carbons are sp3 contains one sp2 have what value?
hybridized hybridized carbon 5. What is the total number of sp2 hybridized
atoms in the molecule?

Alkynes – CnHn
contains one sp
hybridized carbon
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IN-CLASS PROBLEM IN-CLASS PROBLEM
6. What is the total number of sp hybridized 8. What is the total number of unshared pairs of
atoms in the molecule shown below? electrons in the molecule shown below?

7. What is the total number of sp3 hybridized

atoms in the molecule shown below?

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THE REPRESENTATION OF CARBON COMPOUNDS THE REPRESENTATION OF CARBON COMPOUNDS

Carbon compounds can Carbon compounds can

exist as chains of carbon exist as chains of carbon
atoms, rings, polycyclic atoms, rings, polycyclic
rings... rings...

Morphine
Glucose

…rings containing heteroatoms (not carbon), or all

of these structural features, including multiple
bonds.
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THE REPRESENTATION OF CARBON COMPOUNDS THE REPRESENTATION OF CARBON COMPOUNDS

C4H10 CH3CH2CH2CH3
Molecular formula Condensed structure

H H
H H
C H H
C H H
H C C H
H C C H
H H C
H H C
H H
H H “line” or structural
drawing

Ball and Dreiding model Space-filling model 1. A CH2 is shown as a simple vertex.
stick model 2. A CH3 is shown as a truncated line.

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THE REPRESENTATION OF CARBON COMPOUNDS THE REPRESENTATION OF CARBON COMPOUNDS

CH3

CH3
H H CH3 H3C H H H H
C H H CH3
H C C H H H
CH3 CH3
H H C H H CH3
H H
H3 C ...Newman
...structural ...sawhorse
or line projection projection
CH3 drawing

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IN-CLASS PROBLEM IN-CLASS PROBLEM

Convert the following structures into “line drawings” Convert the following structures into “line drawings”

CH3 C(CH3 )2 CH2 CH2 CH3 CH3 CH3

CH3 CH2 CH2 CH3 CH2 H
H H H
H H CH2
H C C H H CH3 CH3
C
H C C H CH3
C H CH3
H H
H H
CH3

CH3 CH2 CH2

H
H CH3 Br

CH3 © ChemistryOnline, 2009-2015

C
C H3 © ChemistryOnline, 2009-2015
H3C
C H3

H3C C H2C H2C H3

CH

C H3

C H3

C
C H3
C H3C H2
C H3

IN-CLASS PROBLEM IN-CLASS PROBLEM

CH CH H 3 2
C
H
Convert each of the structures shown below into Convert each C
of
C Hthe structures shown below into
3

a “skeletal structure”. a “skeletalH structure”.

CH 3

Br
Br C H3
C
C H3
H3C H C C
C H3 C C C H3

H3C H H
C H2C H2C H3
CH
H H H H H
C H3
H C C C C C C H2
C H3
H H H H H C H3
C
C H3
C H3C H2
C H3 H H

C
C H3C H2 H2C C H2
H
C
H H2C C H2
C H3 C
C
H C H3 H H
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Br C H3 H H

C C
H C C H H
C C C H3 C C

H H C C
H H
C C
H H H H H
H H
H C C C C C C H2

H H H H H C H3

H H

C
H2C C H2

H2C C H2
C

H H
 C H3

C
C H3
C H3C H2
C H3

C H3C H2 H
C
H
C H3
C
H C H3

Br C H3

H C C
C C C H3

H H

H H H H H

H C C C C C C H2

IN-CLASS P
H
ROBLEM
H H H H C H3

H H
Convert eachCof the structures shown below into
a “skeletalH Cstructure”.
2CH 2

H2C C H2
C

H H

H H

C C
H H
C C

C C
H H
C C

H H

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