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Hydrogen-3 (tritium)

Hydrogen-3 (tritium): atomic number 1, mass number 3, with 1 protons and 2 neutrons.

IsotopeH
31H
Protons1
Neutrons2
Electrons (neutral)1

The proton count fixes the element: 1 protons identify Hydrogen. Mass number 3 is the total number of protons and neutrons.

A neutral Hydrogen-3 (tritium) atom also has 1 electrons. Changing neutrons changes the isotope; changing electrons changes the charge.