GUIDE深度 · Deep dive
Counting Protons, Neutrons and Electrons
Atomic number = protons; a neutral atom has electrons = protons; mass number = protons + neutrons. Isotopes have the same protons but different neutrons. Decide whether it is an atom or ion before counting electrons.
Three numbers
Protons = atomic number.
Neutrons = mass number - protons.
Electrons = protons - charge.
Isotopes
Same protons, different mass numbers.
Similar chemistry, possibly different physical properties.
FAQ
Why subtract charge for electrons?
Positive charge means lost electrons; negative means gained.
Is mass number always an integer?
Yes; relative atomic mass is the one with decimals.