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Chapter 3
History of the Atom
1
Element Symbols




Rule 1
 First letter is capitalized
Ex:
Rule 2
 Second letter is lower case
Ex:
Rule 3
 Some are Latin names
Ex:
Rule 4
 Some elements are named for places, scientists, greek gods…
Ex:
2
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Foundations of Atomic Theory

__________________________________________
 Mass
is neither created nor destroyed.
HgO  Hg + O
433.2 g
401.2g +
32g
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Dalton’s Atomic Theory (1808)
1. All matter is made of indivisible and indestructible atoms.
2. All atoms of the same element are identical in their physical and
chemical properties.
3. Atoms of different elements have different properties.
4. Atoms of different elements combine in simple whole-number
ratios to form compounds.
5. Atoms cannot be subdivided, created, or destroyed when they
are combined, separated, or rearranged in chemical reactions.
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Changes to Dalton’s Theory

Are atoms actually indivisible?

Are all atoms identical?
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The Atom



the smallest unit of an element that retains that
elements properties
Made of protons, neutrons, and electrons
Can you see inside an atom?
 How
did they know it contains protons, neutrons and
electrons?
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Cathode Ray Tube



When connected to electric current the remaining
the gas forms a _________________________.
The beam always started at the
_____________electrode and flowed to the
_______________ electrode.
The electrode is named by what type of particle it
attracts
 Cathode:
Negative (-)
 Anode: Positive (+)
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Cathode Ray Conclusions
1. The beam was attracted to a
_____________magnet.
2. The beam must be made of
______________particles.
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JJ THOMSON DISCOVERED A
NEGATIVE PARTICLE CALLED THE:
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Plum Pudding Model
Radioactivity

Becquerel
 discovered
 uranium
______________
would expose photographic plates in the dark
 The
properties of an element changed as it gave off
radiation

Curie
 Discovered

radium and polonium
The radioactive emissions of ________, _________
and ____________ rays were identified.
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13
(Uranium compound)
Types of Radioactive Decay
Decay
Type
Reaction
Symbol Charge
Strength
Particle
Can be
stopped
by…
Alpha
Beta
Gamma
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Radioactive Decay
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Ernest Rutherford’s Gold Foil
Experiment
Set up Gold Foil with a detection sheet around it.
 Set up radioactive source emitting alpha particles.


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____________________shot at gold foil.
Rutherford’s Gold Foil Experiment
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Rutherford: It’s like shooting a cannon at a piece of tissue
paper and having it bounce back at you!
_______particles went through the gold foil
SOME particles ____________back
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Gold Foil Conclusions
1.
2.
3.
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Rutherford’s Model of
the Atom
+
NUCLEUS
–
EMPTY
SPACE
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Niels Bohr – 1913



Developed a new diagram of the atom
Electrons can only be at certain energies
Electrons must gain a specific amount of energy to
move to a higher level, called a quantum
**
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Bohr’s Model of the Atom
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Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
AX
Z
Isotopes:
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Do You Understand Isotopes?
How many protons, neutrons, and electrons are in 146
C?
How many protons, neutrons, and electrons are in 116
C?
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Hyphen Notation
The mass number is written with a hyphen after the
name of the element.
Element – Mass#
Uranium – 235
Calcium – 40
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Ion: an atom, or group of atoms, that has a net
positive or negative charge.
cation – _________________________________
If a neutral atom loses one or more electrons
it becomes a cation.
Na
Na+
anion – __________________________________
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
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Cl-
Do You Understand Ions?
+
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How many protons and electrons are in 13 Al ?
2- ?
Se
How many protons and electrons are in 78
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Relative Atomic Mass


Carbon is the standard for all masses on the
periodic table.
Carbon: 6 p and 6 n = 12 amu
Atomic Mass Unit

Periodic table lists weighted average atomic
masses of elements (like a GPA or final grade
calculation)
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Calculation AVERAGE Atomic Mass
75%
20%
5%
133Cs
132Cs
Steps:
1.
134Cs
2.
3.
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