Chemistry › Isotope
Mendeleev · Isotope
Potassium-40
Potassium-40: atomic number 19, mass number 40, with 19 protons and 21 neutrons.
IsotopeK
4019K
Protons19
Neutrons21
Electrons (neutral)19
The proton count fixes the element: 19 protons identify Potassium. Mass number 40 is the total number of protons and neutrons.
A neutral Potassium-40 atom also has 19 electrons. Changing neutrons changes the isotope; changing electrons changes the charge.