The periodic table
3 marks · NormalExplain, using an example, how you can determine both the number of electrons and the number of neutrons in an atom using information from the Periodic Table.
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Identify atomic number as the number of protons (equal to electrons in a neutral atom). Find mass number. Neutrons = mass number - atomic number. Example: Oxygen (atomic number 8, mass number 16) has 8 electrons and 8 neutrons.
Must show: Correct equations, Uses a valid example
Also accept: Example may be any valid element (not those in existing questions)
Common mistakes: Confusing mass number and atomic number; Incorrect example
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Marking points
M1 · 1 mark · Explains that the atomic number gives number of electrons in a neutral atom.
M2 · 1 mark · Describes that neutrons = mass number - atomic number.
B1 · 1 mark · Uses a clear example that matches the explanation.
Worked solution
To find the number of electrons in an atom, you look up the atomic number from the Periodic Table, as it tells you the number of protons, and in a neutral atom, protons equal electrons.
To find the number of neutrons, subtract the atomic number from the mass number.
For example, an atom of fluorine has atomic number 9 and mass number 19. It has 9 electrons and 19 - 9 = 10 neutrons.