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ATOMIC STRUCTURE
The Theory of the Atom
 _______________, a famous Greek teacher who lived in the 4th Century B.C.,
first suggested the idea of the atom.
• Said that all matter is composed of tiny, _____________ particles
called __________ (atoms)
 In 1803, _______________ studied experiments and concluded that the
properties of matter could be explained in terms of __________.
 Dalton’s _________________ was based on the following ideas:
o Each __________ is composed of extremely small particles called atoms.
o All atoms of a given element are ___________, but they differ from
those of any other element.
o Atoms are neither __________ nor _________ in any chemical reaction.
o A given ____________ always has the same relative numbers and kinds
of atoms.
The Atom
 The smallest particle of an ___________ is an atom.
 The atom is made up of three ____________ particles: ___________,
_____________, and ___________
Proton
 Discovered in ________ by E. Goldstein
 Has a ____________ charge
Electron
 Discovered in 1897 by _________________
 Has a _____________ charge
 Its __________ is much smaller than the other 2 subatomic particles;
therefore its mass is usually _____________
Neutron
 Discovered in 1932 by __________________
 Has no charge (i.e. It is ____________)
 Has about the same __________ as a ____________
Nuclear Atomic Structure
 The atom is made up of 2 parts/sections
(1) The _____________ --- (in the center of the atom; contains protons
and neutrons)
(2) The ___________________ --- (surrounds the nucleus)
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The Nucleus
Discovered by Ernest ________________ in _______.
He shot a beam of positively charged “alpha particles”, which are
___________ nuclei, at a thin sheet of _______________.
99.9% of the particles went right on through to the _________. Some were
slightly deflected. Some even _________________ towards the source!
This would be like shooting a cannon ball at a piece of tissue paper and having
it bounce off.
Conclusions About the Nucleus
(1) Most of the atom is more or less ___________________.
(2) The nucleus is very _________. (Stadium Analogy)
(3) The nucleus is very _________. (Large Mass ÷ Small Volume)
(4) The nucleus is _____________ charged.
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Counting Subatomic Particles in an Atom
The atomic # of an element equals the number of ____________ in the
nucleus.
The mass # of an element equals the sum of the _____________ and
______________ in the nucleus.
In a neutral atom, the # of protons = # of ______________.
To calculate the # of neutrons in the nucleus, ______________ the
___________ # from the __________ #.
Practice Problems
(1) Find the # of e-, p+ and n0 for sodium. (mass # = 23)
2) Find the # of e-, p+ and n0 for uranium. (mass # = 238)
3) What is the atomic # and mass # for the following atom?
# e- = 15; # n0 = 16
Ions
• When an atom loses or gains one or more _________, it acquires a net
electrical charge and is called an _____.
• An ion that has more electrons than protons has a ________ charge. An ion
that has fewer electrons that protons has a _________ charge.
Charge of ion = # of protons - # of electrons
Practice Problems
1) A neutral magnesium atom (atomic number 12) has 12 protons and 12 electrons.
If the atom loses 2 electrons, what is the charge of the resulting ion?
2) How many electrons would be found in the ion O2-?
3) If an ion has 28 protons and 26 electrons, what is its charge? What is its
symbol (including charge)?
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Isotopes
An isotope refers to atoms that have the same # of ___________, but
they have a different # of ___________.
Because of this, they have different _________ #’s (or simply, different
___________.)
Isotopes are the same element, but the atoms weigh a different amount
because of the # of ______________.
Examples---> (1) Carbon-12 & Carbon-13
(2) Chlorine-35 & Chlorine-37
(The # shown after the name is the mass #.)
For each example, the elements have identical ___________ #’s, (# of p+)
but different _________ #’s, (# of n0).
Another way to write the isotopes in shorthand is as follows:
The top number is the ________ #, and the bottom # is the __________
number. Calculating the # n0 can be found by _____________ the #’s!
More Practice Problems
(1) Find the # e-, p+ and n0 for Xe-131.
2) Find the # e-, p+ and n0 for C
3) Write a shorthand way to represent the following isotope:
# e- = 1
# n0 = 0
# p+ = 1
Atomic Mass
• Based on the relative mass of Carbon-12 which is exactly _______.
• 1 p+ ≈ __ atomic mass unit (amu) 1 n0 ≈ __ amu 1e- ≈ __ amu
• The atomic masses listed in the Periodic Table are a “weighted average” of
all the isotopes of the element.
Practice Problems:
(1) Mrs. Smith’s geometry semester grades are calculated using a weighted
average of three category scores:
Tests/Quizzes= 60% of your grade
Homework/Classwork= 30% of your grade
Final Exam=10% of your grade
• If a student had the following scores, what would they receive for the
semester?
Tests/Quizzes = 80 (B-)
Homework/Classwork = 60 (D-)
Final Exam Exam=65 (D)
Step (1): Multiply each score by the % that it is weighted.
Step (2): Add these products up, and that is the weighted average!
A “normal average” would be calculated by simply adding the raw scores together
and dividing by 3…
80 + 60 + 65 = 205 ÷ 3 = 68.3 = D
(2) In chemistry, chlorine has 2 isotopes:
Cl-35 (75.8% abundance) Cl-37 (24.23 % abundance)
What is the weighted average atomic mass of chlorine?
(3) Oxygen has 3 isotopes:
O-16 (99.76%) O-17 (0.037%)
O-18 (0.2%)
Estimate oxygen’s average atomic mass.