Download Structure of the Atom

“Atomic Structure”
Essential Question: Describe how the
model of the atom has changed since
the Greek idea of “Atomos”
1
Defining the Atom
• Democritus
– He believed that atoms were indivisible and
indestructible “Atomos”
2
Dalton’s Atomic Theory (experiment based!)
1) All elements are composed of tiny
indivisible particles called atoms
2) Atoms of the same element are
identical.
John Dalton
3) Atoms combine in whole-number ratios
to form compounds
4) In chemical reactions, atoms are
combined, separated, or rearranged –
but never changed into atoms of
another element.
3
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce
the presence of a negatively charged particle: the
electron
4
Thomson’s Atomic Model
Thomson believed that the electrons were like plums
embedded in a positively charged “pudding,” thus it was
called the “plum pudding” model.
5
Gold Foil Experiment - 1911
6
The Rutherford Atomic Model
• Based on his experimental evidence:
– The atom is mostly empty space
– All the positive charge, and almost all the mass is
concentrated in a small area in the center. He
called this a “nucleus”
– The neutron was discovered
by Chadwick in 1932
7
Bohr Model
• Bohr refined Rutherford's model of the
atom by finding that electrons were found
in distinct energy levels around the
nucleus. When electrons move to different
energy levels energy is released.
8
Quantum Atomic Model
• This is the current view of atoms, it is a
mathematical model rather than a physical
model. It describes the electrons as being
found in regions of probability around the
nucleus, an “electron cloud”
9
Changes in Atomic model
10
Democritus/Dalton
Thompson(plum pudding)
Rutherford
Schrödinger (quantum Model)
Bohr
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Subatomic Particles
Particle
Charge
Mass (g)
Location
Electron
(e-)
-1
negligible
Electron
cloud
Proton
(p+)
+1
1
Nucleus
Neutron
(no)
0
1
Nucleus
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Atomic Number
Mass Number
The atomic number of
an element is the
number of protons in
the nucleus.
The protons determine
the identity of the
element
The mass number is the
number of protons and
neutrons in the nucleus of
an isotope.
Carbon-14 indicates the mass is 14
Carbon-13 indicates the mass is 13
These are isotopes of carbon
13
How to find number of protons
The number of protons is the same as the
atomic number.
Atomic number = number of protons
How to find number of electrons
The number of electrons is the same as the
number of protons.
Number of electrons = number of protons
How to find number of neutrons
1. Round mass number to the ones place.
35.453 = 35
24.805 = 25
2. Then subtract atomic number from mass
number.
Mass # - Atomic # = number of neutrons
Mass number
Complete Symbols
Superscript →
Subscript →
Mass
number
Atomic
number
X
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Symbols

Find each of these:
a) number of protons
b) number of
neutrons
c) number of
electrons
d) Atomic number
e) Mass Number
80
35
Br
18
p+
n0
e- Mass #
Oxygen - 18
8
10
8
18
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Nuclide
19
Isotopes and Atomic Mass Notes
Essential Question: Why is the mass listed on
the periodic table an average?
20
Isotopes are….
• Atoms of the same element that
have a different numbers of
neutrons.
• Thus, different mass numbers.
21
• To show different isotopes the
mass # is written after element:
–carbon-12
carbon-14
Elements occur in nature as
mixtures of isotopes.
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Isotope Examples
23
Examples of Isotopes:
Isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
Protons Electrons
Neutrons
1
1
0
1
1
1
1
1
2
Nucleus
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Average Atomic Mass
 This is based on the abundance
(percentage) of each isotope of that
element in nature.
 Instead of grams, the unit we use is the
Atomic Mass Unit (amu)
25
• It is defined as one-twelfth the
mass of a carbon-12 atom.
– Carbon-12 chosen because of its isotope purity.
• Each isotope has its own atomic
mass, thus we determine the
average from percent abundance.
26
To calculate the average:
• Multiply the atomic mass of
each isotope by it’s
abundance (expressed as a
decimal), then add the
results.
• If not told otherwise, the mass of the
isotope is expressed in atomic mass
units (amu)
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Atomic Masses
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Mass
% in nature
(mass ) x (% as a decimal)
Carbon-12
12
98.89%
12 x .9889 =11.87
Carbon-13
13
1.11%
13 x .0111 = 0.14
Carbon-14
14
0.01%
14 x .0001 =
.0014
Isotope
Carbon = 11.87 + 0.14 + .0014 =12.011
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The Periodic Table:
A Preview
 Each horizontal row (there are 7 of
them) is called a period
Each vertical column is called a
group, or family
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