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A Model of the Atom
Today we use a combination of the theories of past scientists and modern technology to develop our
own theory of what the atom looks like. We have, to date, discovered that an atom is composed of
smaller pieces called subatomic particles. There are 3 different subatomic particles: the proton,
the electron, and the neutron.
Subatomic Particle
Location in the Atom
Mass
Charge
Proton
Neutron
Electron
The modern day atom can be drawn as follows:
A)
Protons
The type of atom is determined entirely by the number of protons in its nucleus. For example:
C has __________ protons.
No other atom has this many protons. The number of protons an atom has is called its atomic
number. We can determine the atomic number of any atom by looking at the periodic table.
Examine the periodic table and answer the following questions:
An atom of chlorine has __________ protons.
An atom of tungsten has __________ protons.
An atom with 12 protons can only be __________.
An atom with 46 protons can only be __________.
B)
Neutrons
The nucleus of an atom is composed of protons and neutrons. The sum of protons and neutrons in
an atom is called the mass number. The mass number of an atom can also be found on the
periodic table. Examine the periodic table and answer the following questions:
A)
The mass number of oxygen is _____________
B)
The mass number of Aluminum is _____________
C)
What is the equation to determine the number of neutrons in an atom?
_______________________________________________________________
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Complete the following table:
Atomic Number
Atom
(# of p+)
Mass Number
(# of p+ + n0)
Number of
Neutrons (n0)
Nitrogen
Sodium
Potassium
C)
Electrons
If an atom is neutral (has no charge) then what do we know about the number of electrons?
Knowing this, fill in the following table:
Atom
# of p+
# of n0
# of e-
He
F
Ag
D)
Standard Atomic Notation
The number of subatomic particles in an atom can be represented using standard atomic
notation, as shown below:
16
8
O
Determine the number of protons, neutrons and electrons in each of the following atoms:
23
11
Na
# of p+
40
20
Ca
# of p+
# of n0
# of n0
# of e-
# of e-
Write the standard atomic notation for an atom of nitrogen with 7 protons and 8 neutrons:
Write the standard atomic notation for an atom of magnesium with 12 protons and 14 neutrons:
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E)
Bohr-Rutherford Diagrams
We can combine Rutherford’s nuclear model of the atom with Bohr’s planetary model of the atom
in diagrams that show the electronic structure of elements and their atoms. The following
guidelines should be used when drawing Bohr-Rutherford diagrams:
i. Determine the number of protons, neutrons and electrons the atom has.
ii. Draw a small circle representing the nucleus.
iii. Inside the nucleus, write the number of protons the atom has.
iv. Also inside the nucleus, write the number of neutrons the atom has.
v. Electrons are shown in increasingly larger circular orbits around the nucleus.

The first orbital contains a maximum of 2 electrons.

The second and subsequent orbitals contain a maximum of 8 electrons.
vi. draw a larger circle around the nucleus. This will represent the first orbital. Write the
number of electrons in that orbital by placing a number on the ring. Remember the first
orbital can only hold a maximum of 2 electrons. If you have more than 2 electrons you
place the first two in the first orbital and then place the rest in the second, third and
fourth orbitals respectively.
vii. When the first orbital fills up draw another circle to represent the next orbital and write
the number of electrons in this orbital, remember the maximum in the second orbital is
8 electrons. If you still have electrons “left over” start another orbital, continue in the
same way until all electrons are accounted for:
Draw Bohr-Rutherford diagrams for the following:
a)
23
11
Na
F)
b)
24
12
Mg
c)
12
6
C
Isotopes
An isotope is defined as:
For each of the following isotopes, determine the number of protons, electrons, and neutrons:
24
12
Mg
25
12
Mg
26
12
Mg
How are these atoms similar to one another?
How do these atoms differ from one another?
Draw the Bohr-Rutherford diagram for a neutral atom of Carbon and for its isotope C-14.
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Some isotopes are unstable – their nucleus breaks apart and ejects high energy particles (alpha
particles, beta particles and gamma rays). These isotopes are said to be radioactive and are called
radioisotopes. Give 3 uses of radioisotopes.
G)
Practise Problems:
1.
Complete the following table:
Atomic
Mass
Symbol
Number
Number
3
Li
Std Atomic
Notation
Protons
Neutrons
Electrons
12
11
7
16
18
8
27
13
Ar
P
19
2.
Write standard atomic notation for the following:
a. An atom of bromine with 35 protons and 80 neutrons:
b. An atom of sulphur with 16 protons and 18 neutrons
3.
An ion is defined as:
4.
Fill in the following table:
Atom
# of Protons
18
8
O
Ca 2 
38
20
23
11
Na
15
7
N 3
# of Neutrons
# of Electrons