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Mendeleev · Isotope

Oxygen-18

Oxygen-18: atomic number 8, mass number 18, with 8 protons and 10 neutrons.

IsotopeO
188O
Protons8
Neutrons10
Electrons (neutral)8

The proton count fixes the element: 8 protons identify Oxygen. Mass number 18 is the total number of protons and neutrons.

A neutral Oxygen-18 atom also has 8 electrons. Changing neutrons changes the isotope; changing electrons changes the charge.