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An ion M2– contains 10 electrons and 10 neutrons. What are the atomic number and mass number of the element M? Name the element.

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Number of electrons in M2- ion = 10 

Number of neutrons = 10 

Number of electrons in M atom = 10 – 20 = 8 electrons 

Number of protons = atomic number = 8 

Mass number of the element = 8 + 10 = 18u 

The element is Oxygen and the ion is O2- oxide ion.

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