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YISHUN JC The “Zen” of tea-cups YISHUN JC Long long ago, a scholar wanted to learn about Zen from a learned master YISHUN JC The learned master sat him down, and started pouring him a cup of tea…… YISHUN JC But he poured and poured until the tea overflowed….. And he still continued pouring…… YISHUN JC 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? …. YISHUN JC 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…… YISHUN JC When 2 people engage in a conversation, often they are anxious to convey their own thinking…… YISHUN JC But the result is that they can only hear an echo of what they have said, and nothing else. YISHUN JC 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 YISHUN JC 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 2 YISHUN JC Stoichiometry is the study of the quantitative composition of chemical substances the quantitative changes that take place during chemical reactions Atoms YISHUN JC An atom is the smallest indivisible particle of an element The “paper clip” analogy Atoms YISHUN JC 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. YISHUN JC 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 YISHUN JC 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 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 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 YISHUN JC Example: NaCl ClNa+ Relative Mass YISHUN JC 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 YISHUN JC To calculate relative mass: Relative mass = of a particle mass of one particle 1 x mass of one 12 12C atom Relative Isotopic Mass YISHUN JC 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 YISHUN JC 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 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) 23 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) 23 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)