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Atomic Structure
NC Competency Goal 2
A History of the Atom
Atomic Structure
• By the late 1800’s scientist were convinced that
atoms were the fundamental units of which all
matter is composed.
• They soon learned that the atom was not
indivisible.
• Atoms are composed are smaller (subatomic)
particles: proton, neutron, and electron
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Atoms
An atom is the smallest particle of an element that
retains the properties of that element.
Atoms are extremely small. One gram of Pb (lead) has
2.9 x 1021 atoms.
Compare that to the Earth’s population which is about
4 x 109 people.
So…….where did the idea of atoms come from?
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Atoms
The idea of atoms were first suggested by an ancient
Greek citizen named Democritus.
The word “atom” comes from the
Greek language:
Atomos = Indivisible
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Atoms
It wasn’t until the late 1700’s that chemist were able to
relate chemical changes to events at the level of
individual atoms.
At that time, an English
chemist and physicist named
John Dalton (1766-1844) first
stated his atomic theory.
NC Competency Goal 2
Atoms: Daltons Atomic Theory
1-All elements are composed of tiny indivisible particles
called atoms.
2-Atoms of the same element are identical. The atoms of
any one element are different from atoms of another
element.
3- Atoms of different elements can combine in simple
whole number ratios to form compounds.
4 – Chemical reactions occur when atoms are separated,
joined or rearranged. However, atoms of one
element are not changed into atoms of another
element by a chemical reaction.
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Atoms of “A”
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Goal 2
made
Atoms of “B”
Mixture of
“A” and “B”
Compound of
Molecules
Electrons, Protons, and Neutrons
*Electrons are negatively charged subatomic
particles.
*They were primarily the interest of electricians
rather than chemist.
*These scientists (i.e. electricians) studied the flow
of gases in an enclosed tube…at low pressure. The
tube had two metal plates at each end. One of the
plates held a positive charge and the other a
negative charge. When high voltage was applied, the
gas inside the tube glowed.
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The device they used is called a cathode ray tube.
The glowing beam inside the tube is called a cathode
ray.
Electrons (which are negatively charged are attracted
to the positive plate) hence electrons travel from the
negative cathode toward the positive anode.
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An English physicist named
Sir Joseph J. Thomson (18561940) experimented with
Cathode rays.
In 1897, he found that
cathode rays could be
deflected by a magnet or
an electrically charged
plate place within or near
the Cathode ray tube.
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Cathode rays are a collection of
very small negatively charged
particles.
He called these particles
Electrons.
He found out the following:
J.J. Thomson
• The mass of an electron is lighter than a hydrogen atom.
• No matter what gas was used in the cathode ray tube or
the type of metal used for electrodes within the
tube…cathode rays are ALWAYS made of electrons.
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J.J. Thomson believed
electrons exisited in
ALL atoms!
J.J. Thomson
His model of the atom
became known as the “plum
pudding” model of the
atom!
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Hydrogen Atom
Q: If atoms are electrically neutral, what would
happen to a hydrogen atom if it lost it’s only
electron?
Answer: It would exist as a bare proton.
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Hydrogen Atom
A single proton is 1840 x heavier than an
electron.
A proton carries a single positive charge.
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In 1932 the physicist, James
Chadwick (1891 – 1974)
confirmed the existence of yet
another subatomic particle:
the neutron.
* Neutrons are subatomic
particles with NO charge.
* They reside with protons in
the nucleus.
The mass of a neutron is nearly equal to the mass of a
proton!
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At this point, the
fundamental building blocks
of an atom were in
existence!
The fundamental
building blocks of an
atom are:
Protons, neutrons, and
electrons.
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In 1886, Eugen Goldstein observed rays
traveling in the opposite direction of the
cathode ray. This occurred when used a
cathode with holes in it.
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He later called these rays,
“Canal Rays”. Later, the
canal rays were found to
be made up of positively
charged particles.
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Eugen Goldstein
If the cathode ray tube
contained Hydrogen gas,
then the canal rays were
made up of bare protons!
Eugen Goldstein
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Properties of Subatomic Particles
Relative Approximate
Mass
Particle Symbol Electrical
(amu)
Charge
Actual
Mass
(grams)
Electron
e-
1-
1/1840
9.11 x 10-28
Proton
p+
1+
1
1.67 x 10-24
Neutron
n0
0
1
1.67 x 10-24
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The discovery of the
nucleus (which houses
protons and neutrons) is
credited to
Ernest Rutherford.
Click the link below to view
Rutherford’s now famous
“Gold Foil Experiment”.
Click Here to see Rutherford’s Gold Foil Experiment
NC Competency Goal 2
The Structure of the Atom
The nucleus of an atom is
protons
composed of ____________
neutrons
and _____________.
positive
The nucleus has a ___________
charge and occupies a very small
part of the volume of an atom.
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Atomic Number
The atomic number of an element
is the number of protons in the
nucleus of an atom for that element.
Since atoms are electrically neutral, the
number of protons in the nucleus of an atom
must equal the number of electrons orbiting
the nucleus
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Atomic Number
Q: Element Carbon is atomic number 6. How
many protons and electrons are in a carbon
atom?
+ and 6 e6
p
Answer: ______________________________
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Atomic Number
Q: The atomic number of an element is 11.
What is the element?
Sodium
Answer: ______________________________
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REVIEW
Who am I ?
I’m the first to develop the
concept of atoms. I am a Greek
philosopher.
Democritus
Answer: ______________________________
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REVIEW
Who am I ?
I developed an early atomic theory
that stated atoms can combine with
each other to form new compounds.
John Dalton
Answer: ______________________________
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REVIEW
Who am I ?
I “coined” the term “electrons”
and designed the “plum
pudding” model of the atom.
J.J. Thomson
Answer: ______________________________
NC Competency Goal 2
REVIEW
Who am I ?
I am an English physicist who
discovered the neutron.
James Chadwick
Answer: ______________________________
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REVIEW
Who am I ?
I designed a simple experiment known as
the gold foil experiment. I am credited
with the discovery of the nucleus.
Ernest Rutherford
Answer: ______________________________
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REVIEW
Who am I ?
I developed what is known
as the “planetary model”
of the atom.
Neils Bohr
Answer: ______________________________
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REVIEW
What is the term used
to describe the number
of protons in the nucleus
of an atom?
Atomic Number
Answer: ______________________________
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REVIEW
Which subatomic
particle is negatively
charged and orbits the
nucleus of an atom?
The electron
Answer: ______________________________
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REVIEW
Which subatomic
particle has no charge
and is usually found in
the nucleus of an atom?
The neutron
Answer: ______________________________
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REVIEW
Which subatomic
particle has a positive
charge and is found in the
nucleus of an atom?
The proton
Answer: ______________________________
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Mass Number
• Most of the mass of an
atom is concentrated in the
nucleus.
• The mass number is the total
number of protons and
neutrons in the nucleus.
NC Competency Goal 2
Using a periodic chart, complete the chart below:
Atomic
Mass
Number Number Number Symbol
number number
of
of
of
protons neutrons electrons
7
14
7
7
7
N
9
19
9
10
9
F
19
39
19
20
19
K
27
59
27
32
27
Co
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End of Review
• Great Job! Now let’s
continue our lesson…
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Isotopes
• Isotopes are atoms!
• They belong to the same element!
• Because they belong to the same element
they have the same number of protons…..BUT
they have a different number of neutrons
Greek: Isotopos = the same place
Atoms that have the same number of protons but a
different number of neutrons are called Isotopes.
NC Competency Goal 2
Isotopes
Different
Number
ofProtons
Neutrons
Atoms
Same of
Number
the same
of
element
6 p+
6 n0
6 p+
7 n0
6 p+
8 n0
6 e-
6 e-
6 e-
Carbon-12
Carbon-13
Carbon-14
98.89%
NC Competency Goal 2
1.10%
.01%
Symbols for Isotopes
p+
Mass
Number
6 e-
Atomic
Number
6
6 n0
Carbon-12
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12
6
Check your Understanding
Write the symbol for the following isotopes.
Oxygen – 16
Oxygen – 17
Oxygen – 18
16
17
18
8
8
8
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Check your Understanding
Determine the number of neutrons in Nitrogen-18.
18
7
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11 Neutrons
Atomic Mass
The atomic mass of an element is an
“average” of many numbers. That’s why it
doesn’t appear as a “whole number” on the
periodic chart.
Atomic Mass is defined as the weighted
average of the masses of the isotopes of that
element.
NC Competency Goal 2
Atomic Mass
Example Problem:
Element X has two natural isotopes. The isotope
with a mass number of 10 has a natural
abundance of 20%. The isotope with a mass
number of 11 has a relative abundance of 80%.
Estimate the average atomic mass for the element
from these figures. What is the true identity of
element X?
NC Competency Goal 2
Atomic Mass
Example Problem Solution:
10X
11X
10 amu x .20 = 2.0 amu
11 amu x .80 = 8.8 amu
Total 10.8 amu
The average atomic mass of element X is 10.8 amu.
Element X is Boron. Boron also has an
atomic number of 5
NC Competency Goal 2
Atomic Mass
Hydrogen has three naturally occurring isotopes.
1 p+
0 n0
1 eHydrogen-1
Since most of the
hydrogen atoms found in
+
+
1
p
1
p
nature
are Hydrogen-1
0
1n
2 n0
isotopes, the average
atomic- mass of Hydrogen
1 e1e
calculates to be
Hydrogen-2
Hydrogen-3
1.0079 amu.Tritium
Deuterium