The arrangement of the electrons in an atom
2 marks · HardExplain why the atomic number of an element determines its position in the Periodic Table and not its mass number.
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Must show: Atomic number: number of protons, Position is based on atomic number, not mass/fixed properties
Also accept: Position in the Periodic Table is based on atomic number (number of protons), which is unique for each element, but mass number can vary due to isotopes.
Common mistakes: Saying mass number is unique for each element (it can vary).; Not mentioning protons.
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Marking points
B1 · 1 mark · States that atomic number is the number of protons and determines element's position.
B2 · 1 mark · States that mass number can vary for isotopes, so cannot be used to determine position.
Worked solution
The atomic number tells us the number of protons, which is unique for each element and places them in order in the Periodic Table.
The mass number can vary because of different isotopes, so it is not used to determine the position.