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Atomic Structure
For some time, people thought that atoms were the
smallest particles and could not be broken into anything
smaller.
Scientists now know that atoms are actually made from
even smaller particles called protons, neutrons and
electrons.
proton
neutron
electron
How are these particles arranged inside the atom?
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Atomic Structure
The protons and neutrons are located in the
dense core at the center of the atom. This is
called the nucleus. The nucleus has a positive
charge (protons are positive and neutrons are
neutral and don’t have a charge).
out around the edge of the
atom. They orbit the
nucleus in layers called
shells. Electrons have a
negative charge.
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Labeling the atom
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Atomic Number
The atoms of any particular element always have the same
number of protons. For example:
 hydrogen atoms always contain 1 proton
 carbon atoms always contain 6 protons
 magnesium atoms always contain 12 protons.
The number of protons in an atom
is known as the atomic number
and is the smaller of the two
numbers shown in most periodic
tables.
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What is the atomic number?
What are the atomic numbers of these elements?
sodium
11
iron
26
tin
50
fluorine
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9
Atomic Number
On very rare
occasions, the number
of protons can change,
which means that it is
now a different
element. This happens
and nuclear
bombs/reactors.
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Mass Number
Electrons have a mass of almost zero, which means that the mass of
each atom results almost entirely from the number of protons and
neutrons in the nucleus.
The sum of the protons and neutrons
in an atom’s nucleus is the mass number.
It is the larger of the two numbers shown
in most periodic tables. To calculate how many
neutrons there are in an atom, simply subtract:
mass number minus atomic number (protons).
Atoms
Protons
Neutrons
Mass
number
hydrogen
1
0
1
lithium
3
4
7
aluminum
13
14
27
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What’s the mass number?
mass number = number of protons + number of neutrons
What is the mass number of these atoms?
Atoms
Protons
Neutrons
Mass
number
helium
2
2
4
copper
29
35
64
cobalt
27
32
59
iodine
53
74
127
germanium
32
41
73
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How many neutrons?
number of neutrons = mass number - number of protons
= mass number - atomic number
How many neutrons are there in these atoms?
Atoms
Mass
number
Atomic
number
Neutrons
helium
4
2
2
fluorine
19
9
10
strontium
88
38
50
zirconium
91
40
51
uranium
238
92
146
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Building a nucleus
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Calculating Electrons
Every atom has a neutral charge, so must have equal
numbers of protons and electrons. To calculate electrons,
just look at the atomic number (# of protons)!
Atoms
Protons Neutrons Electrons
helium
2
2
2
copper
29
35
29
iodine
53
74
53
Atomic number is the number of protons rather than the
number of electrons, because atoms can lose or gain
electrons but do not normally lose or gain protons.
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How many electrons per shell?
The arrangement of electrons in shells is called the electron
configuration. Each shell can hold a certain number of
electrons.
1st shell holds
a maximum of
2 electrons
2nd shell holds
a maximum of
8 electrons
3rd shell holds
a maximum of
8 electrons
This electron arrangement is written as 2,8,8.
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Making a Bohr Model Using Helium
1. Use the periodic table to determine how
many protons, neutrons and electrons are
in the atom.
P=____ N=_____ E=_____
2. Draw a circle and label the # of P and N in
the inside of the circle
P=
N=
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Making a Bohr Model Using Helium
3. Draw your 1st electron shell
(with up to 2 electrons).
P= 2
N= 2
P= 2
N= 2
4. If you need to add more electrons, you need to
add more electron shells! Remember…2, 8, 8!!!
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Calculate electron configurations
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Which element?
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What are the missing numbers?
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