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Published byJeffrey Woodard Modified over 2 years ago

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Key thing to remember: The proton number is what defines an element, if that changes it becomes a different element! Isotopes and Relative Atomic Mass

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Relative Atomic Mass Mass of atoms is incredibly small! So we have to use relative atomic mass. The symbol for relative atomic mass is Ar Carbon- 12 atom contains 6 protons, 6 neutrons and 6 elections. Its relative atomic mass is defined as exactly 12. Every other atom is compared to this. E.g Helium is one 3 rd of a carbon atom. So its relative atomic mass is 4.

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Defining Relative atomic mass The mean mass of an atom relative to the mass of an atom of carbon-12, which is assigned a mass of 12.

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Isotopes Isotopes are atoms of the same element with different numbers of neutrons. That is; Same Atomic Number Same no. of protons Different Mass Number Different no. of neutrons

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But if the neutron number has changed, is it a different element? NO!! It is the proton number that defines an element, if the neutron number has changed it is the same atom, just a different version (isotope).

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Isotopes of hydrogen IsotopeSymbolPNEUses Protium 101 Used as fuel, for hardening margarine. Deuterium 111 Used in heavy or deuterated water, solvent Tritium 121 Radioactive, Used in H-bombs, glow in the dark paints

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Isotopes of hydrogen ProtiumDeuterium Tritium 1 proton, 0 neutron, 1 electron 1 proton, 1 neutron, 1 electron 1 proton, 2 neutron, 1 electron

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Some to try Element Symbo l Mass No Atomic No ProtonsNeutrons Electron s Chlorine-35Cl3517 Chlorine-37Cl3717 Copper-63Cu6329 Copper-65Cu6529 Bromine-79Br7935 Bromine-81Br8135

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Answers Element Symbo l Mass No Atomic No ProtonsNeutrons Electron s Chlorine-35Cl3517 1817 Chlorine-37Cl3717 2017 Copper-63Cu6329 3429 Copper-65Cu6529 3629 Bromine-79Br7935 4435 Bromine-81Br8135 4635

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So if there’s different isotopes has atoms, then what is the actual mass of an atom? We have to calculate an average To do this we need a few pieces of information. The atomic mass of the isotope Relative abundance of the atom (this means how much there is of each isotope compared to the total in the world)

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How do we calculate the average mass? RAM = (mass no x %) + (mass no x %) 100

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Lets try Chlorine and Copper Chlorine has two isotopes Copper has two main isotopes Chlorine 3575% 3725% Copper 6369% 6531%

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Finding the RAM of Chlorine If you assume you have 100 atoms then the average is easier to find. 35 37 35 37 35 37 35

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Finding the RAM of Chlorine You have 75 atoms of chlorine-35 and 25 atoms of chlorine-37 35 37 35

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Finding the RAM of Chlorine That’s a mass of 75 x 35 and 25 x 37 To give the mass of the 100 atoms 35 37 35

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Using Weighted Averages So the average mass of a chlorine atom isn’t (35+37)/2 Its (35 x 75) + (37 x 25) 100 35 37 35

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From Weighted Averages to RAM Weighted = ( mass x number ) + ( mass x number ) ave total number So RAM = (35 x 75) + (37 x 25) 100 RAM = (2625 + 925)/100 RAM = 35.5

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Weighted Averages Use RAM = (mass no x %) + (mass no x %) 100 To work out the RAM of Copper

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Now you try copper Chlorine has two isotopes Copper has two main isotopes Chlorine 3575% 3725% Copper 6369% 6531%

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RAM of copper RAM = (mass no x %) + (mass no x %) 100 So RAM = (63 x 69) + (65 x 31) 100 RAM = (4347 + 2015)/100 RAM = 63.6

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