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Unit Two-Atomic Theory Notes
Defining the Atom
The Greek philosopher ________________(460 B.C. – 370 B.C.) was among the first to
suggest the existence of ____________(from the Greek word “atomos”)
 He believed that atoms were indivisible and indestructible.
 His ideas did agree with later scientific theory, but did not explain______________
______________, and was not based on the _______________ _______________–
just philosophy-not experiment based
Dalton’s Atomic Theory (experiment based!)
1) All elements are composed of _______ ________________ ___________called atoms.
2) _________of the same element are ______________. Atoms of any one element are
different from those of any other element.
3) Atoms of different elements __________in simple ________-number ratios to form
_____________ compounds.
4) In chemical reactions, atoms are_________________, ________________, or
____________– but never changed into atoms of another element.
Sizing up the Atom
 ___________ are able to be ______________into smaller and smaller particles –
these are the atoms, and they still have_____________ of that element.
 If you could line up 100,000,000 copper atoms in a single file, they would be
approximately 1 cm long.
 Despite their small size, ______________atoms are ______________with
instruments such as scanning electron microscopes.
Discovery of the Proton
 1886-Eugene Goldstein observed what is now called the ___________-particles with a
__________ charge, and a relative mass of 1 (or 1840 times that of an electron).
Discovery of the Electron
In 1897, J.J. Thomson used a _______ _____ _______ to deduce the presence of a
negatively charged particle: the __________.
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Thomson’s Atomic Model
Thomson believed that the electrons were like plums embedded in a ____________
charged “pudding,” thus it was called the “plum pudding” model.
Plum Pudding
Discovery of the Neutron
 1932-James Chadwick confirmed the existence of the ___________ - a particle with
no charge, but a mass nearly equal to a proton.
Subatomic Particles
Particle
Charge
Mass
Location
Ernest Rutherford’s Gold Foil Experiment – 1911
 Alpha particles are ________ _________- The _________ particles were fired at a
thin sheet of ______ foil.
 Particles that hit on the detecting screen (film) are recorded.
Example Demo
Rutherford’s Findings
 Most of the particles passed right through.
 A few particles were ____________.
 VERY FEW were greatly deflected.
Conclusion
 The nucleus is __________.
 The nucleus is _________.
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
The nucleus is ____________ charged.
The Rutherford Atomic Model
 The atom is mostly ________space.
 All the positive _________, and almost all the _______is concentrated in a small area
in the center. He called this a “_______________”.
 The nucleus is composed of ___________ and ___________(they make the nucleus!).
 The __________ is held together by a _________ _________ ________.
 The electrons are distributed around the ____________, and occupy most of the
volume.
Atomic Number
 Atoms are composed of ______________ protons, neutrons, and electrons.
 How then are atoms of one element different from another element?
 Elements are different because they contain different numbers of ____________.
 The “_________ __________” of an element is the number of protons in the nucleus.
 # __________in an atom = # __________
___________ ___________of an element is the number of _________ in the _________.
of each atom of that element.
Element
# of Protons
Atomic #
Carbon
Phosphorus
Gold
Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope:
Nuclide
P+
N0
eMass #
Oxygen-18
Arsenic-75
Phosphorus-31
Isotopic Notation-Complete Symbols
Contain the symbol of the element, the mass number and the atomic number.
Example:
Find each of these:
 number of protons=
 number of neutrons=
 number of electrons=
 Atomic number=
 Mass Number=
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If an element has an atomic number of 34 and a mass number of 78, what is the:
 number of protons=
 number of neutrons=
 number of electrons=
 complete symbol=
If an element has 91 protons and 140 neutrons what is the
 Atomic number=
 Mass number=
 number of electrons=
 complete symbol=
If an element has 78 electrons and 117 neutrons what is the
 Atomic number=
 Mass number=
 number of protons=
 complete symbol=
Isotopes
 Frederick Soddy proposed the idea of isotopes in 1912.
 ____________ are atoms of the same element having different masses, due to the
varying numbers of neutrons.
 Atoms of the same element can have different numbers of _________.
 This is how the same element can have different mass numbers.
 We can also put the mass number after the name of the element:
o carbon-12
o carbon-14
o uranium-235
Elements occur in nature as _____________ of isotopes.
Isotope
Protons
Electrons
Neutrons
Nucleus
Diagram
0
1
2
Atomic Mass
 How heavy is an atom of oxygen?
It depends, because there are different isotopes of oxygen atoms.
 We are more concerned with the average atomic mass.
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This is based on the abundance (___________) of each isotope of that element in
nature.
Measuring Atomic Mass
Instead of grams, the unit we use is the _________ ________ ________(amu)
We don’t use grams for this mass because the numbers would be too small.
It is defined as one-twelfth the mass of a carbon-12 atom.
Each ____________has its own________ _______, thus we determine the average
from percent abundance.

Atomic mass is the ____________of all the naturally occurring isotopes of that element.
Isotope
Carbon-12
Carbon-13
Carbon-14
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Symbol
Composition of the
nucleus
% in nature
98.89%
1.11%