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Atomic Structure
How do you discover and study something you can’t see?
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WHAT IS A THEORY?
A hypothesis is a proposed explanation made as a starting point for further investigation (It’s bright outside because the sun is probably out)
A theory is an explanation confirmed through repeated observation and experimentation (When the sun is out, it tends to be bright outside)
A law is a fact of nature that is observed so often it is accepted as truth.
Theories EXPLAIN laws.Theory vs. Law- Ted Ed
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ATOMIC THEORYWHAT IS IT??
The idea that matter is made up of atoms, the smallest pieces of matter.
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THE ATOM
The atom is the smallest particle of an element that retains the properties of that element.
A scanning tunneling microscope allows individual atoms to be seen.
Imagine you could increase the size of an atom to make it as big as an orange. At this new scale, an orange would be as big as Earth.
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History of the Atom
As a class we will be creating a timeline of the discovery of the atom…
Your group will research the following information and present it to the class:
– Date – What was the time or year of the scientist’s work concerning the atom?
– Observations/Experiments/Evidence –Describe or explain what observation, evidence, or experiment the scientist used to devise his theory of the atom.
– Discoveries/Conclusions – Explain what discovery or conclusion the scientist came to about the atom. What was his atomic theory?
– Contribution – Explain how the scientist helped in the development of atomic theory. How did his work contribute to the theory? What did his experiment/discovery/conclusion do for the theory? Think about the larger picture and how the theory progressed as a result of his work.
– Be able to draw a diagram of the scientist’s model of the atom
Each group will be responsible for one of the following scientists:
Democritus
Rutherford
Bohr
JJ Thomson
Schrodinger/Heisenberg (quantum mechanics)
Chadwick
Dalton
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Greek philosophers believed all matter was made up of four basic elements: fire, earth, water, and air.
History of Atomic Theory
And that matter could be endlessly divided into smaller and smaller pieces
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History of Atomic Theory
Democritus: 460 – 370 B.C.
– Proposed the idea that matter was not infinitely divisible, but made of individual particles called “atomos”.
Aristotle (484-322 B.C.) disagreed because he did not believe empty space could exist. His viewswent unchallenged for 2000 years.
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History of Atomic TheoryJohn Dalton revived the idea of the atom in the early 1800’s with his “Atomic Theory of Matter”
Which parts are still considered true? Which are not considered true?
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LAW OF CONSERVATION OF MASS
John Dalton also easily explained the law of conservation of mass (which was proposed by Lavoisier) in a chemical reaction as the result of the combination, separation or rearrangement of atoms.
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LAW OF CONSTANT COMPOSITION
Dalton’s theory also helps explain a law proposed by
Proust in the early 1800’s. The law of constant
composition states that the elemental composition
of a compound is always the same.
ex: water is always 11% hydrogen and 89%
oxygen (by mass)
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Discovery of Atomic StructureJJ Thomson (1897)
Cathode Ray Tube experiment
_
+
Thomson released cathode rays
from one side of the tube and they
were attracted to the positive plate. Discovered first subatomic particle: the Electron.
What attracts to a positive charge?
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JJ Thomson Video
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Discovery of Atomic Structure
Rutherford (1895)Gold foil experiment
Most particles passthrough, but some arebounced back towards thesource.
Discovered a positively charged nucleus and that atoms are mostly empty space.
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Alpha particles
Plum pudding atom Nuclear atom
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Discovery of Atomic StructureLater Rutherford (1919) refined his model to include the positively charged particles in the nucleus called protons.
Chadwick (1932) noticed the dense nucleus had a particle that was not charged!
Discovered the neutron
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Rutherford and Chadwick Video
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Bohr (early 1900s)
proposed the “electron cloud” in which electrons orbit at a given distance from the nucleus.
Small orbits = low energy
Big orbits = high energy
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Modern Atomic Theory (quantum mechanical model)
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Nucleus: dense, central part of the atom.
– Protons and neutronsare found in the nucleus.
Electron cloud: large area outside of the nucleus.
– Electrons occupy the electron cloud in orbitals.
Structure of the Atom
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Parts of the Atom
Subatomic Particle
Location Charge Relative
Mass
Proton Nucleus 1++1.602 X 10-19 C
11.67x10-24 g
Neutron Nucleus 0 11.67x10-24 g
Electron e- cloud 1--1.602 X 10-19 C
1/18409.11x10-28 g
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What do all the numbers mean on the PT?
Atomic Number- # of protons in
an atom. It is also equal to the number of electrons in a neutral atom.
Atomic Mass – the decimal number on the periodic table. The average mass of all isotopes of that element.
The atomic number determines an
element’s identity!
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What about Neutrons?
The mass of the atom
comes from the nucleus.
Each proton has a mass
of 1 amu (atomic mass
unit)
Each neutron has a mass
of 1 amu.
Add the number of
protons and neutrons to
find the mass number of
the atom!
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Isotopes
What are they? atoms of the same element that
have different mass numbers.
This means the number of ________
is the same, and the number of
_________ is different.
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Carbon-12
Mass Number = 12
Atomic Number = 6
Neutrons = Mass – Protons
Carbon-14
Mass Number = 14
Atomic Number = 6
Neutrons = Mass - Protons
ISOTOPES OF CARBON
6 Neutrons 8 Neutrons
Protons
Neutrons
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2 ways to depict isotopes
Hyphen Notation: C-14
Super/Sub Script:
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Element Atomic
#
Mass # # p+ # n0 # e- Super/sub
method
Hyphen
method
Ruthenium
78 117
32 41
Scandium
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Practice:1. What is the atomic number of zinc? _______2. How many electrons does silver have? _______3. How many protons does iodine have? _______4. How many neutrons does Fe-53 have? _______5. What is the mass number of Chlorine-37? _______6. How many electrons does Chlorine-37 have? _______7. What is the mass number of tritium? _______8. How many neutrons does neon-20 have? _______9. How many neutrons does deuterium have? _________
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ATOMSDiffer by number of protons
ISOTOPESDiffer by number of neutrons
IONSDiffer by number of electrons
Differ by number of electrons
ISOTOPES
Differ by number of neutrons
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How is the mass of atoms measured?
Atoms can’t easily be measured in
grams because they are so small.
Scientists devised “atomic mass
units” (a carbon-12 isotope is
12.000000 amu’s)
What is the mass, in amu’s, of
Hydrogen-1?
You would think it was
1.000000… right?!
It’s actually a little different.
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Average atomic mass is a different kind of average – a “weighted” average. –
think about your six weeks average.
This means that we take into account the abundance of each isotope found in
nature.
Saying that Boron’s average mass is 10.5 would be misleading since there’s
so much more Boron-11.
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Average Atomic Mass
The average of the masses of the naturally
occurring isotopes weighted for their abundance
in nature. (weighted average)
(mass)(abundance/100)=
(mass)(abundance/100)= +
(mass)(abundance/100)= +
Is the AAM
on the
Periodic
Table?
Using this formula will ensure
the correct number of sig
figs…
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Calculating Average Atomic Mass1. Argon has three isotopes with the following percent
abundances: Ar-36 with a mass of 35.968 amu and an
abundance of 0.3337 %. Ar-38 with a mass of 37.963 amu
and an abundance of 0.063 %. Ar-40 with a mass of 39.962
amu and an abundance of 99.600 %. Calculate the
average atomic mass, but first write down what you expect
the average atomic mass to be close to!
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3. The atomic weight of gallium is 69.72 amu. The masses
of naturally occurring isotopes are 68.92 amu for Ga-69
and 70.92 amu for Ga-71. Calculate the percent
abundance of each isotope.
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The atomic weight of gallium is 69.72 amu. The masses of naturally
occurring isotopes are 68.92 amu for Ga-69 and 70.92 amu for Ga-71.
Calculate the percent abundance of each isotope.
[(68.92)(Ga-69 abundance) + (70.92)(Ga-71 abundance)]/100 =
69.72
We have 2 unknowns – we need another equation.
Can you think of a mathematical relationship between the two
abundances????
HINT – What should the abundances add up to?
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Answer – Abundance of Ga-71 is 40% and abundance of
Ga-69 is 60%.