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Topic 3 – Atomic Theory
The nucleus (centre of
an atom) contains
protons and neutrons
Protons have a one
positive charge
Neutrons have no
charge
Electrons have a
negative charge and
orbit the nucleus
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
The number of negative
electrons in an atom is the
same as the number of
positive protons.
So atoms are NEUTRAL
Protons
Electrons
Neutral Atoms
B Gilday – St Thomas Aquinas Secondary
They have no charge
Topic 3 – Atomic Theory
Na
Cl
2)8)1
2)8)7
First electron shell
Second electron shell
Third electron shell
Electron arrangements for all elements are in the
data book.
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Atomic Number and Mass Number may be given
in a question
Mass number
Atomic number
23
11
Na
Atomic Number = number of protons = number of electrons
Mass Number = number of protons + number of neutrons
Number of neutrons = Mass Number – Atomic Number
p = 11
e = 11
n = 23 –11 = 12
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Ions are formed when atoms gain or lose
electrons to achieve a full outer shell
•Metals form positive ions
•Non-metals form negative ions
Na
Loses one electron
2)8)1
+
Na
2)8)
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Ions are formed when atoms gain or lose
electrons to achieve a full outer shell
•Metals form positive ions
•Non-metals form negative ions
Cl
Gains one electron
2)8)7
Cl
2)8)8
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Ions are formed when atoms gain or lose
electrons to achieve a full outer shell
•Metals form positive ions
•Non-metals form negative ions
Mg
Loses two electrons
2)8)2
2+
Mg
2)8)
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Ions are formed when atoms gain or lose
electrons to achieve a full outer shell
•Metals form positive ions
•Non-metals form negative ions
N
Gains 3 electrons
2)5
3N
2)8)
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Ions are formed when atoms gain or lose
electrons to achieve a full outer shell
•Metals form positive ions
•Non-metals form negative ions
Al
Loses 3 electrons
2)8)3
3+
Al
2)8)
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Ions are formed when atoms gain or lose
electrons to achieve a full outer shell
•Metals form positive ions
•Non-metals form negative ions
S
Gains two electrons
2)8)6
2S
2)8)8
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Which ions have the same electron arrangement as argon?
A Li+
2)1
2)
D Cl2)8)7
2)8)8
B K+
2)8)8)1
2)8)8
E F2)8
2)7
C Mg2+
2)8)2
2)8
F O22)8
2)6
Argon has electron arrangement 2)8)8
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Isotopes are different types of the same element
35
17
37
17
Cl
Cl
Mass number is different
Atomic number is the same
p = 17
p = 17
e = 17
e = 17
n = 35-17 =18
n = 37-17 =20
They have different numbers of neutrons
B Gilday – St Thomas Aquinas Secondary
Topic 3 – Atomic Theory
Relative Atomic Mass
The relative atomic mass of an atom is the average mass of the atom.
It depends on the number of isotopes and how much of each isotope
there is (percentage abundance).
In chlorine 75% consists of the 35 isotope and 25% consists of the 37
isotope.
Mass of 100 Cl atoms = Mass of 35 isotopes + mass of 37 isotopes
= 75 x 35
+
25 x 37
= 2625
+
925
= 3550 atomic mass units
Average mass of one Cl atom
= 3550 / 100
= 35.5 amu
B Gilday – St Thomas Aquinas Secondary