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Atomic Structure
Chapter 4
4.1 Defining the Atom
• Democritus (460 BC –
370 BC)
• Had an atomic
philosophy
• Matter was made of
small invisible
particles called atoms
• No scientific
evidence, but it was
logical
Dalton’s Atomic Theory
• John Dalton (17661844) was an English
chemist and school
teacher
• Dalton’s theory was
the first scientific
theory of the atom
John Dalton looks like another
famous John
Dalton’s Atomic Theory
Dalton’s Atomic Theory
1. Matter is composed of tiny indivisible atoms
2. All atoms of a given element are exactly the
same
3. Different elements are made of different atoms
4. Atoms of different elements can combine in
simple whole number ratios to form compounds
5. Chemical reactions involve separating,
combining, or rearranging atoms.
How big are atoms?
• Pretty small
• Just 4 grams of helium (He) contains 6.02
x 1023 atoms!
• Most atoms have a radius in the range of
5 x 10-11 m to 2 x 10-10 m
• Atomic sizes are in the pico- and
nanometer range
Can we see atoms?
• Amazingly, yes!
• STM
• Scanning tunneling
microscope has made
it possible to see
atoms.
• These are gold atoms
• They are only 2.3 Å
apart!
Here is a picture of a mixture of tin,
lead, and silicon atoms!
Custance's team demonstrated
their "fingerprinting" technique by
using an atomic force microscope
(AFM) to distinguish atoms of tin
(blue) and lead (green) deposited
on a silicon substrate (red). (Credit:
Oscar Custance)
physicsworld.com/cws/article/news/
27208
The colors are added to help
distinguish the atoms
4.2 Structure of the Nuclear Atom
When do I know I have learned this
section? When…
• I can describe 3 types of subatomic particles
• I can describe the structure of the nuclear atom
• I can explain Thomson’s discovery
– experiment and model of the atom
• I can explain Millikan’s discovery
• I can explain Rutherford’s discovery
– experiment and model of the atom
4.2
Subatomic Particles
• A cathode ray is deflected by electrically charged
plates.
Subatomic Particles
• In Dalton’s theory, atoms had no charge
• Thomson studied cathode rays (the
cathode ray experiment)
• Resulted in discovery of the electron
• Electrons are negatively charged particles
• Discovered the charge to mass ratio of
electron
JJ Thomson’s Plum Pudding
Model
• Atom was a sphere
• Atoms were
electrically neutral
• (-) charged electrons
embedded in a (+)
charged matrix
• Like a chocolate chip
cookie
Millikan’s Oil Drop Experiment
• Discovered the actual quantity of charge
for the electron
• Using Thompson’s charge/mass ratio,
Millikan was also able to determine the
mass of the electron
• Mass of electron is 1/1840 the mass of a
hydrogen atom.
Protons
• If H atoms are neutral, and they give off
negatively charged electrons, what must
be left?
• Positive charge
• Proton discovered by Eugen Goldstein
noticed a ray traveling in opposite direction
of the electrons in the cathode ray tube
• Toward the negative end (cathode)
Neutrons
• Discovered by James Chadwick
Quarks
• Quarks are subnuclear particles
• Protons and neutrons are composed of
quarks
The Atomic Nucleus
• Ernest Rutherford discovered the nucleus
• Gold foil experiment
Simulation of Rutherford’s Gold
Foil Experiment
Interpretation of Rutherford’s
Findings
Rutherford’s Conclusions
• All positive charge and nearly all the mass
of the atom are concentrated in a very
small nucleus
• Atom contains mostly empty space
• Rutherford’s model of the atom was like a
mini solar system
Summary of Atomic Models
• Democritus
• Dalton
Invisible particles
Tiny homogeneous sphere
5 (4) statements
• Thomson
• Rutherford
Plum pudding
Nuclear atom
Thomson’s Model of the Atom
• Plum Pudding
• Negatively charged
electrons embedded
in a positive matrix
Rutherford’s Model
• Nuclear Atom
• Most of the volume of the
atom is empty space
• All positive charge & most
mass in a small nucleus
• Nucleus contains protons
and neutrons
• Electrons occupy space
outside the nucleus
Distinguishing Among Atoms
• Atomic Number
= number of protons in nucleus
Each element has unique atomic number
• Mass Number
= number of protons and neutrons in nucleus
# neutrons = mass # - atomic #
Practice
Symbols of Elements
Elements are symbolized by one or two letters.
First letter is always capitalized.
Second letter, if there is one, is always lower case.
Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
Mass Number
The mass of an atom in atomic mass units (amu)
is the total number of protons and neutrons in
the atom.
Isotopes
• Isotopes are atoms of the same element with
different masses.
• Isotopes have different numbers of neutrons.
11
C
6
12
C
6
13
C
6
14
C
6
Different Ways of Indicating
Isotopes
• Chemical symbols
16 O
18 O
8
8
• Symbol and mass number
O-16
O-18
• Name and mass number
oxygen-16
oxygen-18
Isotopes of Neon
Atomic #
Mass #
Atomic Mass
• Measured in amu’s (atomic mass units)
because their mass is so small
• Definition: one atom of carbon-12 equals
exactly 12 amu’s
• 1 amu = exactly 1/12 the mass of a C-12
atom
• 1 amu = 1.6605 x 10-24g
Different Isotopes Have Different
Abundance & Atomic Masses
Isotope
Abundance
Atomic
mass
(amu)
Chlorine-35
75.77%
34.969
Chlorine-37
24.23%
36.966
What is the atomic mass of chlorine on the periodic table?
Average Atomic Mass
•Determining abundance of isotopes when
average atomic mass is known
•Multiply the mass of each isotope by its
relative abundance (decimal equivalent)
and add the products
Isotope
Atomic
Abundance
Mass
R-125
125
________ ________
R-120
120
________ ________
124
Periodic Table - Preview
• Allows you to easily compare and predict
properties of elements
• Groups = Columns
– Numbered 1-18 or
– 1A-8A (representative elements)
– 1B-8B (transition metals)
• Periods = Rows
– Numbered 1-7