Counting the number of electrons in an atom
3 marks · EasyComplete the following sentence: An atom is neutral because...
Potassium has atomic number 19. State the number of electrons in a neutral atom of potassium.
Explain your answer to part (b).
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(a) the number of protons and electrons are equal · must show must clearly state protons = electrons
(b) 19
(c) A neutral atom has equal numbers of protons and electrons. Potassium has 19 protons, so it must have 19 electrons. · must show relates number of electrons to atomic number/protons
Common mistakes: Incorrectly stating atom is neutral because protons = neutrons
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Marking points
B1 (a) · 1 mark · States atom is neutral because it has equal numbers of protons and electrons.
B2 (b) · 1 mark · Gives correct number of electrons in potassium: 19.
B3 (c) · 1 mark · Explains that in a neutral atom, the number of electrons equals the atomic number (or number of protons).
Worked solution
a) An atom is neutral because it has the same number of protons and electrons.
b) Number of electrons in potassium = 19
c) In a neutral atom, the number of electrons is the same as the number of protons. Potassium has atomic number 19, so it has 19 protons and 19 electrons.