The periodic table
5 marks · HardMolybdenum (Mo) is an element with an atomic number of 42. One isotope of molybdenum has a mass number of 98.
State the number of protons in an atom of this isotope.
Calculate the number of neutrons in this isotope.
Explain how you would determine the number of electrons in a neutral atom of molybdenum.
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(a) 42 protons
(b) 56 neutrons
(c) In a neutral atom, the number of electrons equals the number of protons (which is the atomic number); so a neutral molybdenum atom has 42 electrons.
Common mistakes: mass number for atomic number; atomic number for electron count
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Marking points
B1 (a) · 1 mark · States number of protons: 42 (must link to atomic number).
M1 (b) · 1 mark · Method: subtracts atomic number from mass number (98 - 42).
A1 (b) · 1 mark · Correct answer: 56 neutrons.
M2 (c) · 1 mark · States or explains the rule that in a neutral atom, number of electrons = number of protons.
A2 (c) · 1 mark · Links the answer specifically to molybdenum (states 42 electrons or refers to atomic number 42 for electrons).
Worked solution
a) The number of protons in an atom is given by the atomic number. So, protons = 42.
b) Number of neutrons = mass number - atomic number = 98 - 42 = 56.
c) In a neutral atom, the number of electrons equals the number of protons, which is the atomic number. Therefore, a neutral molybdenum atom has 42 electrons.