The periodic table
3 marks · EasyState the number of electrons in a potassium atom.
State the number of neutrons in a potassium-39 atom.
State the number of protons in a potassium atom.
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(a) 19
(b) 20
(c) 19
Common mistakes: using mass number instead of atomic number; subtracting wrong way for neutrons
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Marking points
A1 (a) · 1 mark · Correctly states potassium has 19 electrons.
A2 (b) · 1 mark · Correctly calculates 20 neutrons (39 - 19).
A3 (c) · 1 mark · Correctly states potassium has 19 protons.
Worked solution
a. Potassium has atomic number 19, so 19 electrons.
b. Number of neutrons = mass number - atomic number = 39 - 19 = 20.
c. Number of protons is the atomic number, so 19.