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THE PERIODIC TABLE
AND THE ATOM
Overview
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The atom, while now a familiar concept, is a
relatively new theory.
It is truly a theory, since nobody has every actually
seen one individual atom.
As we will see, the evidence for this theory is
overwhelming.
Dalton’s Atomic theory

In 1802:
 An element is made up of tiny
particles called atoms
 Atoms of an element are identical.
Atoms of different elements are
different in some fundamental way.
 Compounds are formed by
combinations of different elements,
whose ratio of atoms remains the
same.
 Reactions involve reorganizing atoms;
the atoms themselves are
unchanged.
The First Picture
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J. J. Thomson in 1897
Experimenting with Cathode Ray
Tubes
Found that the beam deflected when
a magnet was brought near
If the beam was made of one charge
of matter, the rest of matter must
have opposite charge
Plum Pudding Model of the Atom
Cathode Ray Tube
Plum Pudding Model
Positive
matter
Negative
Particles
Getting to the Core
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Ernest Rutherford in 1912
“Gold Foil Experiment”
Assumed:
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Like Thompson said, the atom is full of positive
matter with negative chunks distributed
throughout
Assumed the large positive radiation would
blast through the atoms’ matter
All radiation would be detected on other side
of foil
Gold Foil Experiment
Rutherford’s Model
Actual Results
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Majority of
radiation went
through
Few deflections to
sides
No radiation stuck in
foil; all went through
Atomic Model
•
Atom is mostly
empty space
•
Positively charged
nucleus
•
Electrons very
little mass,
randomly in
empty space
Rutherford’s Model
Why is this thing so important?
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Gives information on individual
elements or atoms
Visual representation of
properties of the elements
Predicts the existence and
properties of elements yet to be
discovered
Individual Atoms
Atomic Mass
79
Symbol
Se
Name
Selenium
Atomic Number
34
Atom

smallest particle of an element that retains the
properties of the element.
Atom

Contains…
 Protons
(Nucleus)
 Neutrons (Nucleus)
 Electrons (Shell)
Atom
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Protons (Positive Charge)
Neutrons (Neutral Charge)
Electrons (Negative Charge)
What are those things?
Name:
 Typically
named by discoverer, but must be approved
by the IUPAC (International Union of Pure and Applied
Chemistry)
 Typically named after people or places
 Thorium:
Thor, god of thunder
 Einsteinium: Albert Einstein, scientist
 Berkelium: University of California at Berkely
What are those things?
Symbol:

Once named, the symbol has 1 or 2 letters
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Second letter always lowercase
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N (Nitrogen)
Ti (Titanium)
Co (Cobalt: pretty blue dye used in art)
CO (Carbon Monoxide: colorless gas used in death)
Sometimes symbol is based off of something unusual
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Pb (lead): based on Latin word “plumbus”, which by the way gave us the
word “plumber” as well, since pipes used to be made out of lead!
W (Tungsten): element found in the mineral Wolframite
What are those things?

Atomic Number:
Represents number of protons in one atom
 Under usual conditions protons = electrons
 PT organized by atomic number
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Atomic Mass:
Represents the mass of one atom
 It is calculated by adding the protons + neutrons
 So why isn’t the value shown on the PT a whole number?
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Example
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How many protons, neutrons,
and electrons are in one atom
of Gold?
197
Au
 Protons
= 79
 Electrons =79
 Neutrons =197-79 = 118
Gold
79
Periodic Table Basics
Valence Electron
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Electrons found in the outer shell.
 Rules…
 1st
shell can hold 2 electrons.
 2nd, 3rd, 4th… can hold 8 electrons.
Review
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Atoms are mostly ____.
A. positive
B. negative
C. solid spheres
D. empty space
Review
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Whose work led to the modern atomic theory?
A. Dalton
B. Rutherford
C. Einstein
D. Aristotle
Review
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What are the two fundamental subatomic
particles found in the nucleus?
A. proton and electron
B. proton and neutron
C. neutron and electron
D. neutron and positron
Isotopes
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Atoms with the same number of protons but
different numbers of neutrons are called isotopes.
Uses: dating matter, nuclear power, smoke detectors
Mass of Atoms
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The mass number is the sum of the protons and
neutrons in the nucleus.
Mass of Atoms
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The atomic mass of an element is the weighted
average mass of the isotopes of that element.
Review
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Elements with the same number of protons and
differing numbers of neutrons are known as
what?
A. isotopes
B. radioactive
C. abundant
D. ions
Review
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An unknown element has 19 protons, 19
electrons, and 3 isotopes with 20, 21 and 22
neutrons. What is the element’s atomic number?
A. 38
B. 40
C. 19
D. unable to determine