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The “Zen” of tea-cups
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CLong long ago, a scholar wanted to learn about Zen from a learned master
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CThe learned master sat him down, and started pouring him a cup of tea……
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CBut 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!
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CYou are like this tea cup…… You come forth with you your own knowledge and thinking.
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CIf 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.
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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.
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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
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Some notes about this topic 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.
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What will I learn? 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?
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Introduction Stoichiometry is the study of
the quantitative composition of chemical substances
the quantitative changes that take place during chemical reactions
2
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Atoms An atom is the smallest indivisible particle of
an element The “paper clip” analogy
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Atoms An atom is the smallest indivisible particle of
an element An atom is made up of 3 sub-atomic particles
-1e-
01n
+11p
electron
neutron
proton
Relative charge
Relative mass
SymbolParticle
01840
1
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Atoms 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:
XAZChemical symbol
Mass number
Atomic number
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Atoms 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:
Na2311 Be9
4 O168
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Atoms 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
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Atoms 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
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Atoms Atoms of the same element having different
mass numbers are called isotopes
Element Common isotopes
carbon
chlorine
hydrogen
C126 C13
6
Cl3517 Cl37
17
H11 H21 H31
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Atoms 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
192019
192119
172017
171817
Number of electrons
Number of neutrons
Number of protonsParticle
Cl3517
Cl3717
K4019
K3919
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Atoms
101211
1088
9214692
Number of electrons
Number of neutrons
Number of protonsParticle
U23892
2168O
Na2311
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Molecules and Formula Units 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
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CStructure of ice (H2O)
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Molecules and Formula Units 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
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Cl-
Na+
Ionic Compound Example: NaCl
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Relative Mass 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
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Relative Mass To calculate relative mass:
Relative mass of a particle
mass of one particle
x mass of one 12C atom 12
1=
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Relative Isotopic Mass The relative isotopic mass
Mass of an isotope compared to the mass of a 12C atom
To calculate relative isotopic mass:
Relative mass of a particle
mass of one isotope
x mass of one 12C atom 12
1=
12
1
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Relative Atomic Mass 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 the
mass of a 12C atom To calculate relative atomic mass:
12
1
Relative mass of a particle
average mass of one atom
x mass of one 12C atom 12
1=
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Relative “Mango” mass 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 =
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Relative “Mango” mass 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 =
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Relative “Mango” mass 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=
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Relative Atomic Mass
An alternative equation:
Relative mass of a particle
(Relative abundance x relative isotopic mass)
(relative abundance) =
Sum up all the values contained in the set
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Relative Atomic Mass Example 1:
Calculate the relative atomic mass of rhenium given that rhenium consist of two isotopes 185Re and 187Re in the ratio 2 : 3
32
18731852(Re)
rA
= 186.2 (No units)
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Relative Atomic Mass
32
18731852(Re)
rA
= 186.2 Meaning:the average mass of one rhenium atom is 186.2 times heavier than the mass of a 12C atom
12
1
Example 1:Calculate the relative atomic mass of rhenium given that rhenium consist of two isotopes 185Re and 187Re in the ratio 2 : 3
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Relative Atomic Mass Example 2:
The abundance of the isotopes of chlorine are:
Calculate the relative atomic mass of chlorine
47.2453.75
97.3647.2497.3453.75)(
ClAr
= 35.46
Isotopes Relative isotopic mass % abundance35Cl 34.97 75.5337Cl 36.97 24.47
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What have I learnt? 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)