Download Lecture 1 - OoCities

YISHUN JC
The “Zen”
of tea-cups
YISHUN JC
Long long ago, a scholar
wanted to learn about Zen
from a learned master
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The learned master sat him
down, and started pouring
him a cup of tea……
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But he poured and poured
until the tea overflowed…..
And he still continued
pouring……
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Master! The cup is
already full! Stop
pouring tea into
the tea cup!
YISHUN JC
You are like this tea
cup…… You come forth
with you your own
knowledge and
thinking.
YISHUN JC
If you do not empty your
tea cup, how do you
expect me to teach you
about Zen?
….
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When someone brings
with them their own
thinking and knowledge,
sometimes they do not
hear the message that
other people is trying to
convey to them……
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When 2 people engage
in a conversation, often
they are anxious to
convey their own
thinking……
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But the result is that they
can only hear an echo of
what they have said, and
nothing else.
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In this topic, many of you will
bring with you a lot of your prior
knowledge. Sometimes, many of
these (whether you believe it or
not) ARE misconceptions.
YISHUN JC
I need you to compare / contrast
what you know with what I am
presenting to you, and clarify any
doubts – especially those you
think that I have made a mistake.
YISHUN JC
But in case you forget them, you
might want to jot it down on a
piece of paper… and remember
to ask them soon after the
lecture.
Atoms, Molecules, and
Stoichiometry
Introduction, Elementary particles,
and Relative masses
Some notes about this topic
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 The assessment objectives are given on the
first page of your notes
 Make sure you are able to achieve all of them
at the end of the topic – monitor your own
learning.
 The lecture notes is designed with a column for
your “notes”.
 It was found that students who write their own
notes / comments relate to their notes better
 “Notes” on the right margin are also part of your
notes.
What will I learn?
YISHUN JC
 What is an atom?
 What are the particles that make up an atom?
 How do I represent an atom in writing?
 How do I calculate the number of neutrons and
the charge of an atom?
 What is a molecule and formula unit?
 What is relative mass?
Introduction
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 Stoichiometry is the study of


the quantitative composition of chemical
substances
the quantitative changes that take place during
chemical reactions
Atoms
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 An atom is the smallest indivisible particle of
an element
 The “paper clip” analogy 
Atoms
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 An atom is the smallest indivisible particle of
an element
 An atom is made up of 3 sub-atomic particles
Particle
Symbol
Relative
mass
Relative
charge
proton
p
1
+1
neutron
n
1
0
electron
e-
1
0
1840
-1
Atoms
YISHUN JC
 The number of protons in an atom is called
atomic number (Z)
 The total number of protons and neutrons in an
atom is called mass / nucleon number (A)
 An atom can be represented as follows:
Mass
number
Atomic
number
A
Z
X
Chemical
symbol
Atoms
YISHUN JC
 The number of protons in an atom is called
atomic number (Z)
 The total number of protons and neutrons in an
atom is called mass / nucleon number (A)
 An atom can be represented as follows:
23
11
Na
9
4
Be
16
8
O
Atoms
YISHUN JC
 Atoms of the same element have the same
number of protons
 Same atomic number
 But can have different number of neutrons
 Can have different mass number
 Atoms of the same element having different
number of neutrons are called isotopes
3
Atoms
YISHUN JC
 Atoms of the same element have the same
number of protons
 Same atomic number
 But can have different number of neutrons
 Can have different mass number
 Atoms of the same element having different
mass numbers are called isotopes
Atoms
YISHUN JC
 Atoms of the same element having different
mass numbers are called isotopes
Element
Common isotopes
carbon
35
17
chlorine
hydrogen
C
13
6
Cl
37
17
12
6
1
1
H
2
1
C
H
Cl
3
1
H
Atoms
YISHUN JC
 Neutral atoms have equal number of protons
and electrons
 Hence, isotopes have the same number of
electrons  same chemical properties
 A particle have unequal number of protons and
electrons is called an ion.
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Atoms
Particle
Number of
protons
Number of
neutrons
Number of
electrons
35
17
Cl
17
18
17
37
17
Cl
17
20
17
40
19
K
19
21
19
39
19
K
19
20
19
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Atoms
Particle
238
92
U
16
8
23
11
O
2
Na

Number of
protons
Number of
neutrons
Number of
electrons
92
146
92
8
8
10
11
12
10
Molecules and Formula Units
YISHUN JC
 A molecule is a group of atoms covalently
bonded together in an element or a compound
 The smallest particle of simple covalent
compounds that can exist alone under
ordinary conditions
 E.g. N2, CO2, NH3 are covalent molecules
YISHUN JC
Structure of ice (H2O)
Molecules and Formula Units
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 A molecule is a group of atoms covalently
bonded together in an element or a compound
 The smallest particle of simple covalent
compounds that can exist alone under
ordinary conditions
 E.g. N2, CO2, NH3 are covalent molecules
 For an ionic compound,
 we define a formula unit as the smallest group
of atoms which the formula of the compound
can be established
 E.g. NaCl, Ca(NO3)2, and Al2O3
Ionic Compound
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 Example: NaCl
ClNa+
Relative Mass
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 The masses of atoms are very small (10-23g)
 chemists use a relative atomic mass scale
 Used to compare the mass of different atoms
 Relative atomic mass scale:
 Reference selected: 12C
 On this scale, the mass of a 12C atom is
assigned a value of 12.0
 The relative mass of other atoms are obtained
by comparing with the mass of 12C atom

4
Relative Mass
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 To calculate relative mass:
Relative mass
=
of a particle
mass of one particle
1
x mass of one
12
12C
atom
Relative Isotopic Mass
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 The relative isotopic mass
1
12
Mass of an isotope compared to
of a 12C atom
 To calculate relative isotopic mass:

Relative mass
=
of a particle
the mass
mass of one isotope
1
x mass of one
12
12C
atom
Relative Atomic Mass
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 Most elements consist of a mixture of isotopes
 relative atomic mass must be
representative of the mass and relative
amount of each isotope
 The relative Atomic mass
 Average mass of an atom compared to
mass of a 12C atom
 To calculate relative atomic mass:

1
12
average mass of one atom
Relative mass
= 1
of a particle
12C atom
x
mass
of
one
12
the
Relative “Mango” mass
YISHUN JC
 Imagine inside your fridge you’ve got
Mangos from 3 different countries:
Mango
Mass of mango / g
Number in the fridge
China
12
3
Malaysia
10
2
Philipines
15
5
 What is the average mass of the mangos?
Average mass
of a mango =
Total mass of mangos
Total number of mangos
Relative “Mango” mass
YISHUN JC
 Imagine inside your fridge you’ve got
Mangos from 3 different countries:
Mango
Mass of mango / g
Number in the fridge
China
12
3
Malaysia
10
2
Philipines
15
5
 What is the average mass of the mangos?
Average mass
of a mango =
(3 x 12) + (2 x 10) + (5 x 15)
3+2+5
Relative “Mango” mass
YISHUN JC
 Imagine inside your fridge you’ve got
Mangos from 3 different countries:
Mango
Mass of mango / g
Number in the fridge
China
12
3
Malaysia
10
2
Philipines
15
5
 What is the average mass of the mangos?
Average mass
of a mango =
13.1 g
Relative Atomic Mass
YISHUN JC
 An alternative equation:
Relative mass
of a particle =
(Relative abundance x relative isotopic mass)
Sum up all the
values contained
in the set
(relative abundance)
Relative Atomic Mass
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 Example 1:
Calculate the relative atomic mass of rhenium given that
rhenium consist of two isotopes 185Re and 187Re in the
ratio 2 : 3

2 185  3 187 
A (Re) 
23
r
= 186.2
(No units)
Relative Atomic Mass
YISHUN JC
 Example 1:
Calculate the relative atomic mass of rhenium given that
rhenium consist of two isotopes 185Re and 187Re in the
ratio 2 : 3

2 185  3 187 
A (Re) 
23
r
= 186.2
Meaning:
the average mass of one
rhenium atom is 186.2 times
1
heavier than 12 the mass of a
12C atom
Relative Atomic Mass
YISHUN JC
 Example 2:
The abundance of the isotopes of chlorine are:
Isotopes
Relative isotopic mass
% abundance
35Cl
34.97
75.53
37Cl
36.97
24.47
Calculate the relative atomic mass of chlorine

75.53  34.97   24.47  36.97 
A (Cl ) 
75.53  24.47
r
= 35.46
What have I learnt?
YISHUN JC
 What is an atom?
 What are the particles that make up an atom?
 How do I represent an atom in writing?
 How do I calculate the number of neutrons and
the charge of an atom?
 What is a molecule and formula unit?
 What is relative mass?
End of Lecture 1
Have a great weekend
(Do review your notes)