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Unit 02
Atomic Structure
Just How Small is an Atom?
You don’t need to write.
• A speck 0.1 mm in diameter (about half the
size of a period at the end of the sentence)
requires one million atoms.
• It would require a million atoms, edge to
edge, to match the thickness of a page of
paper.
Can you see an atom?
• Technically, you cannot "see" anything smaller than the shortest
wavelength of light that you can see it with.
• But there are ways to "visualize" it, like Atomic Force Microscopy. But
these are all just measurements converted to computer images, and
are not in any real sense "seeing" the atom.
• You can't see atoms in any normal sense of using an optical
microscope.
• You don't get an optical image, but it does allow you to map out an
image of the atoms of a molecule. To do this you use a metallic tip
which interacts with the atoms you want to image. As you move the
tip over the atoms, you pass a current, called a tunneling current,
between the tip and the atom. This current is extremely sensitive to
the distance between the atom and the tip.
- REMEMBER FROM: Elements,
Mixtures, and Compounds -
Element
- a pure substance made up of one
type of atom.
- organized on periodic table
- each element has a unique
number of protons…its atomic number
Atomic Structure:
Atoms contain three subatomic particles…
These are
1. Protons…positive charge
located in
2. Neutrons…neutral charge
NUCLEUS!
3. Electrons…negative charge
Electrons surround
the nucleus in
orbitals
Atomic Structure
An atom is considered electrically neutral.
Electrically neutral means the number of
protons (+) = the number of electrons (-)
4 red protons = 4 blue electrons
Properties of Subatomic Particles
Relative
Particle
Symbol
electrical
Relative mass
Actual mass (g)
charge
Electron
e-
1-
1/1840
9.11 × 10-28
Proton
p+
1+
1
1.67 × 10-24
Neutron
n0
0
1
1.67 × 10-24
Protons and Neutrons have the same mass.
A. Discovery of the Atom
Ernest Rutherford discovered the nucleus by shooting
alpha particles (have a positive charge) at a very
thin piece of gold foil.
He predicted that the particles would go right through
the foil at some small angle.
Rutherford’s Gold Foil Experiment
Rutherford’s Gold Foil Experiment
• some particles (1/8000) bounced back from
the foil
• this meant there must be a “powerful force”
in the foil to hit particle back
Predicted Results
Actual Results
Discovery of the Atom
Purpose:
The students will find the shape of different items and relate this to the
early scientist that made discoveries about the shape and size of the atom.
Procedure:
1. Title the left side of your spiral Discovery of the Atom.
2. For each item you will write the letters then draw your predicted shape
of the item.
3. Then you will write 1 sentence describing why they think your prediction
is the shape of the item.
A: Item in brown bag – Use your hands to feel the shape of the item.
B: Item in clay – Using the toothpicks provided find the shape of the object
enclosed in the modeling clay.
C: Black box – Maneuver the black box with a marble inside to discover the
shape of the object enclosed.
B. Models of the Atom
J.J. Thomson
“Plum pudding” atom negatively
charged e- stuck into a lump
of positively charged material
– similar to chocolate chip
cookies
Ernest Rutherford
• In Rutherford’s gold foil experiment he
discovered electrons surround a dense
positive nucleus
Bohr Model
• electrons are arranged in fixed orbits around
the nucleus.
ex. Orbits gum
Quantum Mechanical Model
• Quantum mechanics was developed by Erwin
Schrodinger
• Estimates the probability of finding an e- in a
certain position
• Electrons are found in an “electron cloud”
I. Nuclear Symbols
11
5
A. Mass Number
• mass # = protons + neutrons
 always a whole
number
 NOT on the
Periodic Table!
© Addison-Wesley Publishing Company, Inc.
B. Isotopes
• Atoms of the same element with
different mass numbers. (different
number of neutrons)
 Nuclear symbol:
Mass #
Atomic #
12
6
 Hyphen notation: carbon-12
C
B. Isotopes
C. Nuclear Symbols
You must know how to find:
A. # of protons = atomic number
B. mass # = # of n0 + # of p+ (atomic #)
–
What’s in the nucleus of the atom
C. # of electrons = # of protons (in a neutral atom)
Boron
5
B
10.811
atomic number
(Not the same as the mass #)
(average) atomic mass
How to write a Nuclear Symbol
Element Symbol
Mass Number
=
p+
+
n0
Atomic Number
= p+
11
5
B
Charge if ion
-3
C. Nuclear Symbols
• Chlorine-37
–atomic #:
17
–mass #:
37
–# of protons:
17
–# of electrons:
17
–# of neutrons:
20
37
17
Cl
Nuclear Symbol Examples
35
17
Cl
Atomic Number
Mass Number
17
35
Number of
Protons
Number of
Electrons
Number of
Neutrons
17
17
18
27
12
Mg
Atomic Number
12
Mass Number
27
Number of
Protons
Number of
Electrons
Number of
Neutrons
12
12
15
D. Relative Atomic Mass
•
12C
atom = 1.992 × 10-23 g
 atomic mass unit (amu)
 1 amu = 1/12 the mass of a
12C
atom
 1 p = 1.007276 amu
1 n = 1.008665 amu
1 e- = 0.0005486 amu
© Addison-Wesley Publishing Company, Inc.
E. Average Atomic Mass
• weighted average of all naturally
occuring isotopes
• on the Periodic Table
• round to 2 decimal places
Avg.
Atomic
Mass
(mass)(% )  (mass )(% )

100
E. Average Atomic Mass
• EX: Calculate the avg. atomic mass of oxygen if its
abundance in nature is 99.76% 16O, 0.04% 17O, and
0.20% 18O.
Avg.
(16)(99.76 )  (17)(0.04)  (18)(0.20)
Atomic 
 16.00
100
amu
Mass
E. Average Atomic Mass
• EX: Find chlorine’s average atomic mass if
approximately 8 of every 10 atoms are chlorine-35
and 2 are chlorine-37.
Avg.
Atomic
Mass
(35)(8)  (37)(2)

 35.40 amu
10
Example:
A sample of cesium is 75% 133Cs, 20%
and 5% 134Cs. What is the average atomic
mass?
132Cs
Answer:
.75 x 133 = 99.75
.20 x 132 = 26.4
.05 x 134 = 6.7
132.85 = average atomic mass
II. The Periodic Table
Periodic Law – properties of elements can
be predicted by their position on the
periodic table
A. History of the Periodic Table
• Dmitri Mendeleev (1871)
– Developed the first periodic table
– It was arranged by atomic mass because atomic
number had not been discovered
– He was able to predict properties of elements
A. History of the Periodic Table
• Henry Moseley (1913)
- developed the modern periodic
table
- arranged in order of increasing
atomic number
B. Organization of the Periodic Table
• Period
– horizontal rows numbered 1 - 7
– Energies of outermost electrons are similar
B. Organization of the Periodic Table
• Groups/ Families
– vertical columns
– have similar chemical & physical properties
Group 1 (IA) – Alkali Metals
Group 2 (IIA) – Alkaline Earth Metals
Group 18 (VIIIA) – Noble Gases
Group 17 (VIIA) - Halogens
B. Organization of the Periodic Table
Metals- Left of stair step
Nonmetals- Right of stair step
Metalloids- On the stair step
NONMETALS
METALLOIDS
METALS
B. Organization of the Periodic Table
Group A- Representative
Group B - Transition
REPRESENTATIVE
“Group A”
TRANSITION “Group B”
INNER TRANSITION
The Extended Periodic Table