The Atom and
Periodic Table
General Chemistry: Unit 3
Fall 2010
Atomic Model Scientists

John Dalton

1803
John Dalton (Wikipedia.org)
Dalton’s Atomic Theory

All matter is made up of tiny, unbreakable
particles called atoms (Democritus proposed this in 460-370
BC)
Atoms of the same element are identical, but
they are different from atoms of other
elements.
 Chemical reactions occur when atoms are
separated, joined, or rearranged

Successes and Problems
Verified theories and experiments by
Democritus and Lavoisier (law of conservation of matter)
and Proust (law of definite proportions).
 We know today that atoms are made of smaller
particles and can be split, and there can be
isotopes of the same element.

Atomic Model Scientists (cont.)

Dmitri Mendeleev


1869
Developed a Periodic
Table based on atomic
mass
Dmitri Mendeleev
(Wikipedia.org)
Atomic Model Scientists (cont.)

Joseph John Thomson



1897
Found rays bent toward a
positively charged plate and
away from a negatively charged
plate.
Determined cathode rays are
made up of negatively charged
particles referred to as electrons.
JJ’s Successes
Scientists were able to use JJ Thompson’s
cathode ray tube to discover protons.
 The amount of charge on an electron and
proton is equal but opposite, but the mass of a
proton is much greater that that of an electron.

Thompson Again!
He also discovered that Neon consisted of
atoms of two different masses.
 Later, these were called isotopes: atoms of an
element that are chemically alike but differ in
mass (# of neutrons).
 Scientists were then able to discover the
neutron (1930s): a neutrally charged particle of
equal mass to a proton.

Atomic Model Scientists (cont.)

Ernest Rutherford


1911
Discovered the positively
charged nucleus through gold foil
experiment.
Ernest Rutherford
(Nobelprize.org)
Rutherford’s Gold Foil Experiment
http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_05.JPG
Atomic Model Scientists (cont.)

Niels Bohr

1913
Niels Bohr (The
University of New
York)
Bohr Continued



Rutherford’s atomic model did not
explain the chemical properties of
elements
A description of the behavior of
electrons was needed
Bohr described fixed energy levels
an electron can possess.


In order to move up an energy
level, energy must be absorbed
and vice versa but energy is
released
The more energy an e- has, the
further from the nucleus it is
Bohr concluded
A quantum of energy is the amount of energy
needed to move an e- from one level to the
next
 The higher the energy level, the less energy it
takes to move from that level to the next
 This idea failed to explain more complex
elements… this is where Shrodinger comes in

Erwin Schroedinger
(quantum mechanical model)
Electrons are not stuck in a “planetary orbit,”
or exact path, around the nucleus.
 Instead, they’re spherical regions, of space
around the nucleus in which electrons are
most likely to be found.

 You

can’t determine the exact location of an e-
Called “electron clouds” or “atomic orbitals”
A
region of space in which there is a high
probability of finding an electron
Subatomic =
smaller than
an atom
Atomic Structure

Atoms are made up of three subatomic
particles:
Live
where?
Have
mass?
What
charge?
In nucleus
Yes,
substantial
+
Protons
Neutrons
Electrons
In nucleus
Outside
nucleus
Yes,
substantial
none
No,
negligible
-
Elements



Substances made of
only one type of atom
Identified by atomic
number (protons!!!)
Can not be broken down
into simpler substances
Element Symbols


Shorthand name of the element
Most are based on the Latin name


Ex: Gold = Au
The symbol is either:
1.
One capital letter
-ex: Carbon = C
2.
Two letters…one capitol, one lower case
-ex: Krypton = Kr
Electron Clouds

The different layers of clouds hold different
numbers of electrons
 1st cloud = 2
 2nd cloud = 8
 3rd
cloud = 8
 And
These cloud layers
conveniently match the rows
on the periodic table:
1st row = 2 elements
2nd row = 8 elements
3rd row =8 elements…
And then it gets complicated
then it gets complicated...we’ll save that
discussion for future chemistry courses
Electron Clouds (cont.)

To draw the electron clouds:
 Figure
out how many total electrons
 Fill in the electrons starting with the first
cloud

Sulfur has:
 16
electrons
Electron Clouds (cont.)

Practice Problem #3
 Draw
the electron clouds for an Al atom
This is WAY
too much
work…there
must be a
simpler way!
Lewis Dot Structures

A smart man named Gilbert Newton Lewis
figured out an easier way!
 For
Lewis Dot Structures draw only the important
electrons…
 The outer, or valence, electrons
Just draw the outer electrons
around the atomic symbol
S
Lewis Dot Structures (cont.)

Practice Problem #4
 Draw
Lewis Dot Structures for:
A
Calcium atom
A
Chlorine atom
 An
Oxygen atom
Ca
Cl
O
Lewis Dot Structures (cont.)

All elements want to be full of electrons:
 So
they gain or lose electrons until they are full
 This gives the atom a charge
 Negative
charge if they gain electrons
 Positive charge if they lose electrons
Charged atoms are called
ions: cations if they are
positive and anions if
they are negative
Called the
“Octet Rule”
-gaining/losing
enough e- to
have a full
valence
Atomic Number
1

H

Top Number indicates Atomic
Number
Atomic Number = Number of
Protons

12
Hydrogen:


Mg
Magnesium:


Pb

Atomic Number 12 = 12 protons
Lead:

82
Atomic Number 1 = 1 proton
Atomic Number 82 = 82 Protons
IF YOU CHANGE THE NUMBER OF
PROTONS OF AN ATOM, YOU
CHANGE ITS IDENTITY!!!!!!!!
Atomic Mass


H
Bottom Number indicates Atomic Mass
Atomic Mass = Total Mass (Number of
Protons + Number of Neutrons)

Hydrogen:

1

Magnesium:


Mg
24

Pb – (Atomic Mass)


207
207 (Protons + Neutrons)
If we take the Atomic Mass and subtract the
Atomic Number, we can figure out the
number of neutrons.

Pb
24 (Protons + Neutrons)
Lead:


1 (Protons + Neutrons)


207
– Atomic Number)
82
=
125
Atomic Structure (cont.)

Atomic mass
Average mass of an element, based
on amount of each isotope found in
nature
 Not a whole number because it is
an average


Atomic number
Number of protons in an element
 Also, number of electrons when it is
neutral (has no charge)


Mass number

Mass of a particular isotope
7
N
14.011
Determining Composition of atoms


Number of neutrons = mass number – atomic number
# of protons = atomic number


Charge is positive?


If neutral atom… # protons = # electrons
That # fewer electrons than protons
Charge is negative?

That # more electrons than protons
Atomic Structure (cont.)

For a lithium atom:
 What
is the atomic
number? 3
many protons? 3
 How many electrons
since it is neutral? 3
 How
 What
is the atomic
weight? 6.939
 How
many neutrons?
Usually 4
Who was the scientist
that came up with the
planetary model of the
atom?
Atomic Structure (cont.)


Practice Problem #1
For a sodium atom:

What is the atomic 11
number?
How many protons? 11
 How many electrons since
11
it is neutral?


What is the atomic
weight? 22.99

How many neutrons?
Usually 12
Atomic Structure (cont.)


Practice Problem #2
For a boron atom:

What is the atomic
number? 5
How many protons? 5
 How many electrons since it
is neutral?
5


What is the atomic
weight?
10.811

How many neutrons?
Often 6
THE PERIODIC TABLE OF ELEMENTS…
Is a tool to organize the elements
PERIODIC TABLE DEVELOPMENT
By 1860, scientists had discovered 60
elements
 They noticed some elements had similar
properties.
 They also noticed differences between the
elements.

J.W. DOBEREINER



1829
Classified elements into
groups of 3
Called them triads.


The elements in a triad
had similar chemical
properties
Physical properties varied
in an orderly way
according to their atomic
mass
DMITRI MENDELEEV
 1869
 Developed
a
Periodic Table
based on atomic
mass
 He left blank
spaces
Dmitri Mendeleev (Wikipedia.org)
DMITRI MENDELEEV

Realized chemical +
physical properties of
elements repeated in
an orderly way.

Periodicity- the
tendency to recur at
regular intervals
WHAT ABOUT THE BOLD LINE THAT LOOKS
LIKE STAIR STEPS?
METAL PROPERTIES




Luster
Conductive
Malleable (can be
bent and formed into
shapes)
Ductile (can be pulled
into wires)
NONMETAL PROPERTIES



Dull
Nonconductive
Brittle
(Shaded regions)
METALLOID PROPERTIES


Share properties of
metals and nonmetals
Some are
semiconductors
I study
metalloids
Columns and Rows
Called “groups” or
“families”
Called “periods”
PERIODS




An atom can have up to 7 energy levels of electrons.
An element’s period (row) tells us the number of …?
For example, a sodium (Na) atom has ____ electron
orbitals?
Fluorine (F) has ____ electron orbitals?
GROUPS/ FAMILIES




An element’s family (aka group) tells us ...?
The outer 2 shells of the Group B elements are considered
valence electron orbits. We will be able to ignore Group B for
now.
For example Sodium (Na) has ____ valence electrons
Fluorine (F) has ____ valence electrons
ALKALI METALS
Group 1A
 Only one valence
electron
 VERY reactive!!!!
 Hydrogen is NOT
included

ALKALINE EARTH METALS
Group 2A
 Two valence electrons
 Not as reactive as the alkali metals
 Named because of where they are found on
Earth

TRANSITION METALS



Found in the middle
of the table
In the “B Groups”
Can change their
number of valence
electrons
Bottom Rows are also known as
the Rare Earth Metals!!!
HALOGENS



Group 7A
Seven valence
electrons
VERY reactive!!!!
NOBLE GASES



Group 8A
8 valence electrons—
outer energy level is
full
Very UNREACTIVE—
what do they need to
be 
NOW FOR A LITTLE
PRACTICE…
For carbon (C):
(a)
How many electron shells does it have?
(b)
How many valence electrons does it have?
(c)
Is it a metal, nonmetal, or metalloid?
Answer for carbon (C):
(a)
2 electron shells
(b)
4 valence electrons
(c)
nonmetal
For potassium (K):
(a)
How many electron shells does it have?
(b)
How many valence electrons does it have?
(c)
Is it a metal, nonmetal, or metalloid?
Answer for potassium (K):
(a)
4 electron shells
(b)
1 valence electron
(c)
metal
For copper (Cu):
(a)
How many electron shells does it have?
(b)
Is it a metal, nonmetal, or metalloid?
(c)
How many protons does it have?
Answer for copper (Cu):
(a)
4 electron shells
(b)
metal
(c)
29 protons
For uranium (U):
(a)
How many electron shells does it have?
(b)
Is it a metal, nonmetal, or metalloid?
(c)
How many protons does it have?
Answer for uranium (U):
(a)
7 electron shells
(b)
metal
(c)
92 protons
RADIOACTIVITY
Radioactivity is the process of nuclear decay, in which an
unstable nucleus gives off matter and energy.
Nuclei with too many or too few neutrons compared to the
number of protons are radioactive.
 The three types of nuclear radiation are alpha, beta, and
gamma radiation
ALPHA PARTICLES
When alpha radiation
occurs, an alpha
particlemade of
two protons and two
neutrons is emitted
from the decaying
nucleus
BETA PARTICLES
•
A second type of
radioactive decay is
called beta decay.

Sometimes in an
unstable nucleus a
neutron decays into a
proton and emits an
electron.
BETA PARTICLES
Because the atom now has one more proton, it
becomes the element with an atomic number one
greater than that of the original element.
 However, because the total number of protons
and neutrons does not change during beta decay,
the mass number of the new element is the same
as that of the original element.

GAMMA RAYS

They have no mass
and no charge and
travel at the speed
of light.
ARCHITECTURE OF ATOMS

Isotopes
 Same
element, different number of neutrons
 Ex. Uranium-235 & Uranium-237
 Also written as
Some isotopes are
radioactive, while
others are not.
235
92
U&
237
92
U
BALANCING NUCLEAR REACTIONS
Mass
# and _____________
atomic # are always
_________
conserved due to the Law of Conservation of
Mass.
Th

90
232
Ra

He
88
228
4
2
224
88
Ra  He  Rn
241
95
Am 
4
2
220
86
14
7
N 
227
88
Ra
BALANCING NUCLEAR REACTIONS (CONT.)

Alpha decay of U-238
U  He 
238
92

4
2
Th
Beta decay of Th-235
Th  e 
235
90

234
90
0
1
235
91
Pa
Gamma decay of Th-235
Th 
235 m
90
Th  
235
90
0
0
Alpha decay emits a
Helium atom
Beta decay
decomposes a neutron
to a proton and emits
an electron
Gamma decay causes
rearrangement of the
nucleus and emits
gamma radiation
NUCLEAR RADIATION

Alpha, beta, and gamma rays:
 particles
Form of
radiation
or waves emitted during radioactive decay
Symbol
Charge
Penetration
Alpha
α
+
Least
Beta
β
γ
-
Middle
Gamma
No charge
Most
RADIATION (CONT.)

Most damaging radiation is…
 Alpha,
if it gets in the body
If alpha
radiation
gets in the
body it
does NOT
leave!
RADIATION (CONT.)

People are exposed
to radiation
 Naturally
 Cosmic
rays,
radioisotopes from
rocks, soil,
groundwater
RADIATION (CONT.)

People are exposed to radiation
 Human
sources
 Nuclear
weapons testing, air travel, nuclear power, Xrays, medical treatments with radiation
SMOKE DETECTORS

Some smoke detectors give off alpha particles
that ionize the surrounding air.
RADIATION (CONT.)


Radon
is…
A noble gas
that decays
into radioactive
materials
RADIATION (CONT.)

Radon in homes is a concern because…
 It
seeps in through cracks in foundations
 It stays trapped in air-tight homes
 It decays into heavy metals that emit alpha
particles
10-14% of U.S.
deaths from lung
cancer attributed
to radon
RADIATION (CONT.)

Radon levels in the U.S.
 Zone
>
4 pCi/L
 Zone
 2-4
 Zone
2
1
2
pCi/L
3
pCi/L
12% of lung
cancer deaths
are caused by
Radon
BENEFITS OF RADIOISOTOPES

2 ways of using
radioisotopes in
medicine
 To
locate problems
 To kill cancer cells
BENEFITS OF RADIOISOTOPES (CONT.)

Tracers are…
 Radioisotopes
injected in
people for diagnostic
purposes.
 They are injected and then
traced as they travel through
the body.
BENEFITS OF RADIOISOTOPES (CONT.)

Irradiation
 Kills
cancer cells
 It is applied directly to a cancerous location
on the body
BENEFITS OF RADIOISOTOPES (CONT.)

Irradiation is…
 Exposure
to
radiation of any
kind
 Usually refers to
food irradiation
The FDA requires that all
irradiated foods be
labeled with the above
symbol, the radura
BENEFITS OF RADIOISOTOPES (CONT.)

Irradiate
strawberries, poultry,
potatoes, etc.
 Reduces
spoiling
 Kills bacteria and
parasites that lead to
foodborne illnesses
HALF-LIFE

Half-life
is…
 The
time it
takes for
half the
atoms to
decay
HALF-LIFE (CONT.)

Hydrogen-3 has a half-life of 12 years…start
with 1,000 atoms
 After
12 years
There will be 500 atoms
 After 24 years
There will be 250 atoms
 After 48 years
There will be 62.5 atoms
HALF-LIFE (CONT.)

7,000 atoms of Radon-222, half life of 3.28
days, in 6.56 days there are how many
atoms?


222
86
Rn
1,750 atoms
100,000 atoms of Uranium-238, half life of
4.5 billion years, in 13.5 billion years there
are how many atoms?

12,500 atoms
238
92
U
HALF-LIFE (CONT.)

60,000 atoms of Potassium-40,
half life of 1,280,000,000 years,
how long until 7,500 atoms left?
 3,840,000,000

40
19
K
years
Radon-222, half life of 3.28 days,
how long until 12.5% of initial
222
sample?
Rn
 9.84
days
86
HALF-LIFE (CONT.)

500 atoms of Radon-222, half life of 3.28
days, it has been 19.68 days since the
experiment began, how much did you begin
with?


222
86
Rn
32,000 atoms
Carbon-14, half life of 5,730 years, what
fraction remains after 11,460 years?

¼
14
6
C
HALF-LIFE (CONT.)
Uranium powers our nuclear
power plants. After the fuel is
spent, we are left with
radioactive waste.
 High-level radioactive waste
Uranium’s half
with U-238 is a problem
life is 4.5 billion
because…
years, it will be

around for a
long time.
NUCLEAR FISSION

Nuclear
fission: The
process of
splitting a
nucleus into
several
smaller
nuclei.
Nuclear fusion
 Combining
of two nuclei to form a new, heavier
nucleus
Fusion is much
more powerful
than Fission