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Atomic Structure
Topic 2
2.1 The atom
2.1.1 State the position of protons, neutrons and electrons in
the atom.
2.1.2 State the relative masses and relative charges of protons,
neutrons and electrons.
2.1.3 Define the terms mass number(A), atomic number(Z) and
isotopes of an element.
2.1.4 Deduce the symbol for an isotope given its mass number
and atomic number
2.1.5 Calculate the number of protons, neutrons and electrons
in atoms from the mass number atomic number and charge.
2.1.6 Compare the properties of the isotopes of an element.
2.1.7 Discuss the uses of radioisotopes.
• This part of the Topic is mostly a repeat of
what you learned at IGCSE.
• If you can’t remember what you did, then you
need to read through your notes or a
textbook.
• Make sure that you understand what ions are!
• You should already know quite a lot about
protons, neutrons and electrons.
• You should know:
 The relative charge on each of them.
 The relative mass of each of them.
 where in the atom each of them is located.
Proton
Relative Mass
Relative
Charge
Where it is
located
Neutron
Electron
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
Electron
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
1
Electron
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
1
Electron
0.0005
Relative Mass
Relative
Charge
Where it is
located
Proton
1
+1
Neutron
1
Electron
0.0005
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
1
+1
0
Electron
0.0005
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
1
Electron
0.0005
+1
0
-1
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
1
Electron
0.0005
+1
0
-1
In the nucleus
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
1
Electron
0.0005
+1
0
-1
In the nucleus
In the nucleus
Relative Mass
Relative
Charge
Where it is
located
Proton
1
Neutron
1
Electron
0.0005
+1
0
-1
In the nucleus
In the nucleus
Orbiting the
nucleus
• Some points to note:
• The mass of an electron is so small that we
often approximate it to zero.
• The size and the sign are both important for
the relative charges.
• The absolute values for mass and charge are
extremely small. We use the relative values
because they are more convenient.
This gives us a model of an atom:
We represent this:
• Notice we can use lots of different formats for
this
• But we can ALWAYS get the same information
• The smaller of the 2 numbers is called The Proton
Number or The Atomic Number, Z
• This tell us how many protons are in the nucleus
of an atom.
• The bigger number tells us the total number of
protons AND neutrons in the atom
• This may be called The Nucleon Number or The
Mass Number, A.
• Make sure you know both names!
You need to learn these definitions:
• The atomic number, Z, is the number of
protons in the nucleus.
• The mass number, A, is the number of
protons plus the number of neutrons in
an atom.
• Notice that electrons are not used in either of
these definitions!
• How can we use these numbers to find the
number of neutrons in a nucleus?
• Nucleon number – atomic number
• And how about the number of electrons?
• For an atom, the number of electrons must be
equal to the number of protons.
• Why?
• For positive ions, the atom loses one electron for
each positive charge it gains.
• For negative ions, the atom gains one electron for
each negative charge it gains.
• Note that the number of protons NEVER
CHANGES when we form ions.
• If a table only gives one number, it is may be the
mass number or the proton number. TAKE CARE!
• Notice that the mass number is sometimes
called the “atomic mass”
• Chemists are often very bad at language!
• We use lots of different words to mean the
same thing:
• Atomic mass; atomic weight; relative atomic
mass; RAM; Ar are all used to mean the same
thing!
• They are all number of protons + number of
neutrons
Isotopes
• How can the mass number be a fraction?
• There are often different forms of the same
element with different mass numbers.
• The value that is given for the mass number is
an average of the different forms that exist.
• These different forms are called isotopes.
• Learn the definition:
• Isotopes are forms of the same element
with different numbers of neutrons.
• Isotopes of the same element always have the
same number of protons. (The identity of an
element is defined by its atomic number)
• Isotopes are not all radioactive!
Some common isotopes
3 H
• 1 1H 21H
1
• Hydrogen, deuterium, tritium
•
35
17Cl
37
17Cl
• 235U 238U
• Notice it is not usually necessary to give the
number of protons – as this is the same for all
isotopes of an element.
Properties of Isotopes
• Because isotopes of the same element always
have the same number of protons, they also
have the same number of electrons.
• The chemical properties of an element are
determined by the number of electrons, so . . .
• Isotopes of the same element have the same
chemical properties.
• Many physical properties depend on the mass
of atoms.
• E.g.
• Melting point
• Boiling point
• Rate of diffusion
• Density
• These do differ between isotopes of the same
element.
• Make sure you know what is meant by
physical properties!
Radioisotopes
• The stability of a nucleus depends on the
balance between the numbers of neutrons
and protons.
• If there are either too many or not enough
neutrons, a nucleus changes by giving out
radiation.
• This radiation may be in different forms
• Already studied at IGCSE Physics!
• Alpha particles consist of 2 neutrons and 2
protons.
• They are sometimes referred to as “helium
nuclei” Why?
• Beta particles are high speed electrons
• Gamma radiation is a form of electromagnetic
radiation.
•
•
•
•
•
Radioisotopes can be used to:
Generate nuclear energy
Sterilize surgical instruments
Preserve food
Detect cracks in structural materials.
• It would also be a good idea to know the
specific isotopes and uses given below:
14C
is used in in radiocarbon dating.
• In this process use is made of the fact that living
organisms take up carbon throughout their lives. The
percentage of the isotope carbon 14 is fairly constant
in our atmosphere as it is produced in the upper
atmosphere by cosmic bombardment of naturally
occurring carbon dioxide.
• This means that the percentage of carbon 14 contained
by all living organisms is also constant. However, when
a living organism dies it stops taking up carbon 14. The
isotope decays naturally with a half life of about 5,600
years. So a simple procedure involving counting the
radioemissions due to carbon 14 from a sample of
material that was once alive, can be used to estimate
its date.
Cobalt 60 is used in hospitals as a gamma
emission source in the treatment of cancer
• Gamma rays are the most penetrating form of
radiation. They can be focussed onto
cancerous tissue to destroy it using a cobalt 60
source. This form of treatment is known as
radiotherapy.
131I
and 125I are used as medical tracers
• In several conditions the body can be scanned for
problems using iodine, which is easily taken up by
the body and transported through the lymphatic
system. The isotopes 131I and 125I are easy to
detect and short lived in the body.
• These isotopes emit both beta and gamma
radiation.
• They have short half lives (8 days for 131I, and 60
days for 125I)
• Questions:
• The calcium ion 40Ca2+ is essential for healthy
teeth and bones. Identify the numbers of
protons, neutrons and electrons present in it.
•
•
•
•
•
From the periodic table atomic number = 20
Hence number of protons = 20
No of neutrons = mass No – atomic No
Hence No neutrons = 20
As the ion has a charge of 2+ there are 2 fewer
electrons than protons
• Hence No electrons = 18
• Identify the species with 17 protons, 18
neutrons and 18 electrons.
• From the periodic table, the element with
Z=17 is Cl
• The mass number is No protons + No neutrons
• Mass No = 17 + 18 = 35
• As there is one more electron than protons,
charge is -1
• Hence the species is 3517Cl-
Complete the table
Species
No of protons
No of
neutrons
No of
electrons
18
22
18
19
20
18
7Li
1H
14C
19F56Fe3+
Complete the table
Species
No of protons
No of
neutrons
No of
electrons
7Li
3
1
6
9
26
18
19
4
0
8
10
30
22
20
3
1
6
10
23
18
18
1H
14C
19F56Fe3+
40 Ar
18
39 K+
19