Isotope Notation (Nuclear Symbol)
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Isotope notation writes an element's mass number as a left superscript before its symbol, naming that specific isotope of the element.
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Magnesium-24, written 24Mg, has 12 protons like every magnesium atom, but exactly 12 neutrons, distinguishing it from 25Mg or 26Mg.
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The atomic number, sometimes shown as a left subscript, is often dropped since the element symbol already fixes it.