An oxygen atom is always made up of 8 protons.
The mass number of an atom is made up of both protons and neutrons.
So to find out the number of neutrons, you subtract the number of protons from the mass number.
Number of neutrons = Mass number - number of protons
So for oxygen with a mass number of 18:
There are: 18 - 8 = 10 neutrons
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Answer:
There are 10 neutrons in an atom of oxygen with a mass number of 18.
(option C)
