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Matter
Proton
Neutron
Electron
Nucleus
Atomic Number
Mass Number
Isotope
Electron Shell
Ion

Atoms consist of:
◦ Positively charged, heavy protons in the nucleus.
◦ Heavy neutrons with no charge in the nucleus.
◦ Negatively charged, light electrons orbiting the
nucleus.

Atoms of different
elements have different
numbers of protons.
 He
 Li
 Pb
 Ag

To save time when describing elements
chemists use symbols instead of full words.
◦
◦
◦
◦
◦
Eg.
Sodium = Na
Potassium = K
Mercury = Hg
Lead = Pb
Hurry
up, I’m a
busy
man.
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We classify atoms by the number of protons
that they have in their nucleus.
In nature we find atoms with up to 92
protons, therefore there are 92 different
types of atom.
We call these ‘the elements’.
Atomic Number (symbol = Z)
Mass Number (symbol = A)
+
+
+
+
+
+
O
16
+
+
The number of protons that an atom has in its nucleus is
represented by its atomic number which may be written in
either of the ways above.
8

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The atomic number is the number of protons
an atom has (symbol Z).
The mass number is the number of protons
PLUS the number of neutrons (symbol A).
Helium
+ =Z=2
+
+ +
+
=A=4
+
+
+
+
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Sometimes an atom can have more or less
neutrons than another atom of the same
element.
This is known as an isotope (iso = same, tope
= place)
‘Where do
these go?’
+
+
‘Same
place.’
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To distinguish between isotopes of an
element, physicists often write the mass
number as part of the symbol.
Uranium-235

How would you write the isotopic symbol for
the following isotopes?
◦
◦
◦
◦
◦
◦
Carbon with seven neutrons
Carbon with eight neutrons
Deuterium (Hydrogen with one neutron)
Tritium (Hydrogen with two neutrons)
Uranium 238
Uranium 235

An atom suddenly lost two protons and two
neutrons?
◦ What would happen to the mass number?
◦ What would happen to the atomic number?

One of the neutrons in an atom suddenly
turned into a proton?
◦ What would happen to the mass number?
◦ What would happen to the atomic number?
+
+
+
+
+
+
O
16
+
+
8
Electron Shells
2, 8, 18, 32, 50
2n2
(n = shell number)
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An atom can have a MAXIMUM of:
◦
◦
◦
◦
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2 electrons in the first shell
8 in the second
18 in the third
32 in the fourth
HOWEVER, an outer shell is considered FULL if
it has EIGHT electrons.
The exceptions to this are hydrogen, helium,
lithium, beryllium and boron. Their outer
shell is full if it has TWO electrons.
F
-
S
2-
+
Na
+
K

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Atoms are most stable when they have a full
outer shell.
This means that atoms other than those in
group eight need to gain or lose electrons.

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The period (row) that the element is in gives
you the number of shells.
The group (column) that it is in gives you the
number of outer shell electrons.


Draw electron shell diagrams for these atoms.
How many electrons are in their outer shells?
H
O

Li
B
Cl
Ca
Al
K
What do they need to do to get a full outer
shell?
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‘Plum Pudding Model’ By Kurzon (Own work) available at
http://upload.wikimedia.org/wikipedia/commons/2/26/Plum_pudding_mod
el.svg via Wikimedia Commons
‘J.J. Thompson’ By Not Mentioned (First World War.com) available at
http://upload.wikimedia.org/wikipedia/commons/c/c1/J.J_Thomson.jpg, via
Wikimedia Commons
‘Thompson Cathode Ray Exp’ By Kurzon (Own work) available at
http://upload.wikimedia.org/wikipedia/commons/0/08/Thomson_cathode_r
ay_exp.gif, via Wikimedia Commons
‘John Dalton by Charles Turner’ by Charles Turner available at
http://upload.wikimedia.org/wikipedia/commons/d/d4/John_Dalton_by_Cha
rles_Turner.jpg, via Wikimedia Commons
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‘Ernest Rutherford LOC’ By George Grantham Bain Collection (Library of
Congress) available at
http://upload.wikimedia.org/wikipedia/commons/6/6e/Ernest_Rutherford_L
OC.jpg, via Wikimedia Commons
‘Geiger-Marsden Experiment Expectation and Result’ By Kurzon (Own work)
available at http://commons.wikimedia.org/wiki/File:GeigerMarsden_experiment_expectation_and_result.svg, via Wikimedia Commons
‘Atom Diagram’ By Fastfission at en.wikipedia available at
http://upload.wikimedia.org/wikipedia/commons/d/d8/Atom_diagram.png,
from Wikimedia Commons
‘Niels Bohr’ By The American Institute of Physics credits the photo [1] to AB
Lagrelius & Westphal, which is the Swedish company used by the Nobel
Foundation for most photos of its book series Les Prix Nobel. (Niels Bohr's
Nobel Prize biography, from 1922) available at
http://upload.wikimedia.org/wikipedia/commons/6/6d/Niels_Bohr.jpg, via
Wikimedia Commons
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‘Erwin Schrodinger’ available at
http://commons.wikimedia.org/wiki/File:Erwin_Schr%C3%B6dinger.jpg via
Wikimedia commons.
‘D3M0’ By Dhatfield (Own work) available at
http://upload.wikimedia.org/wikipedia/commons/5/57/D3M0.png, via
Wikimedia Commons
‘Periodic Table (polyatomic)’ By DePiep (Own work) available at
http://upload.wikimedia.org/wikipedia/commons/9/98/Periodic_table_%28p
olyatomic%29.svg, via Wikimedia Commons
‘Build an Atom’ by Kelly Lancaster et al available at
https://phet.colorado.edu/en/simulation/build-an-atom courtesy of PhET
Interactive Simulations at the University of Colorado.
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‘Blausen 0342 ElectronEnergyLevels’ By BruceBlaus (Own work) available at
http://upload.wikimedia.org/wikipedia/commons/2/2c/Blausen_0342_Electr
onEnergyLevels.png, via Wikimedia Commons
‘Blausen 0476 HeliumAtom’ By BruceBlaus (Own work) available at
http://upload.wikimedia.org/wikipedia/commons/d/de/Blausen_0476_Heliu
mAtom.png, via Wikimedia Commons